Site Security Handbook

Status of this Memo

This handbook is the product of the Site Security Policy Handbook Working Group (SSPHWG), a combined effort of the Security Area and User Services Area of the Internet Engineering Task Force (IETF). This FYI RFC provides information for the Internet community. It does not specify an Internet standard. Distribution of this memo is unlimited.

Contributing Authors

The following are the authors of the Site Security Handbook. Without their dedication, this handbook would not have been possible.

Dave Curry (Purdue University), Sean Kirkpatrick (Unisys), Tom Longstaff (LLNL), Greg Hollingsworth (Johns Hopkins University), Jeffrey Carpenter (University of Pittsburgh), Barbara Fraser (CERT), Fred Ostapik (SRI NISC), Allen Sturtevant (LLNL), Dan Long (BBN), Jim Duncan, Pennsylvania State University), and Frank Byrum (DEC).

Editors' Note

This FYI RFC is a first attempt at providing Internet users guidance on how to deal with security issues in the Internet. As such, this document is necessarily incomplete. There are some clear shortfalls; for example, this document focuses mostly on resources available in the United States. In the spirit of the Internet's "Request for Comments" series of notes, we encourage feedback from users of this handbook. In particular, those who utilize this document to craft their own policies and Table of Contents


Purpose of this Work

This handbook is a guide to setting computer security policies and procedures for sites that have systems on the Internet. This guide lists issues and factors that a site must consider when setting their own policies. It makes some recommendations and gives discussions of relevant areas.

This guide is only a framework for setting security policies and procedures. In order to have an effective set of policies and procedures, a site will have to make many decisions, gain agreement, and then communicate and implement the policies.


The audience procedures.

This handbook is meant to be a starting place for further research and should be viewed as a useful resource, but not the final authority. Different organizations and jurisdictions will have different resources and rules. Talk to your local organizations, consult an informed lawyer, or consult with local and national law enforcement. These groups can help fill in the gaps that this document cannot hope to cover.

Finally, we intend for this FYI RFC to grow and evolve. Please send comments and suggestions to:

for this work are system administrators and decision makers (who are more traditionally called "administrators" or "middle management") at sites. This document is not directed at programmers or those trying to create secure programs or systems. The focus of this document is on the policies and procedures that need to be in place to support any technical security features that a site may be implementing.

The primary audience for this work are sites that are members of the Internet community. However, this document should be useful to any site that allows communication with other sites. As a general guide to security policies, this document may also be useful to sites with isolated systems.


For the purposes of this guide, a "site" is any organization that owns computers or network-related resources. These resources may include host computers that users use, routers, terminal servers, PC's or other devices that have access to the Internet. A site may be a end user of Internet services or a service provider such as a regional network. However, most of the focus of this guide is on those end users of Internet services.

We assume that the site has the ability to set policies and procedures for itself with the concurrence and support from those who actually own the resources.

The "Internet" is those set of networks and machines that use the TCP/IP protocol suite, connected through gateways, and sharing a common name and address spaces [1].

The term "system administrator" is used to cover all those who are responsible for the day-to-day operation of resources. This may be a number of individuals or an organization.

The term "decision maker" refers to those people at a site who set or approve policy. These are often (but not always) the people who own the resources.

Related Work

The IETF Security Policy Working Group (SPWG) is working on a set of recommended security policy guidelines for the Internet [23]. These guidelines may be adopted as policy by regional networks or owners of other resources. This handbook should be a useful tool to help sites implement those policies as desired or required. However, even implementing the proposed policies isn't enough to secure a site. The proposed Internet policies deal only with network access security. It says nothing about how sites should deal with local security issues.


This document covers issues about what a computer security policy should contain, what kinds of procedures are need to enforce security, and some recommendations about how to deal with the problem. When developing a security policy, close attention should be made not only on the security needs and requirements of the local network, but also the security needs and requirements of the other interconnected networks.

This is not a cookbook for computer security. Each site has different needs; the security needs of a corporation might well be different than the security needs of an academic institution. Any security plan has to conform to the needs and culture of the site.

This handbook does not cover details of how to do risk assessment, contingency planning, or physical security. These things are essential in setting and implementing effective security policy, but this document leaves treatment of those issues to other documents. We will try to provide some pointers in that direction.

This document also doesn't talk about how to design or implement secure systems or programs.

Why Do We Need Security Policies and Procedures?

For most sites, the interest in computer security is proportional to the perception of risk and threats.

The world of computers has changed dramatically over the past twenty-five years. Twenty-five years ago, most computers were centralized and managed by data centers. Computers were kept in locked rooms and staffs of people made sure they were carefully managed and physically secured. Links outside a site were unusual. Computer security threats were rare, and were basically concerned with insiders: authorized users misusing accounts, theft and vandalism, and so forth. These threats were well understood and dealt with using standard techniques: computers behind locked doors, and accounting for all resources.

Computing in the 1990's is radically different. Many systems are in private offices and labs, often managed by individuals or persons employed outside a computer center. Many systems are connected into the Internet, and from there around the world: the United States, Europe, Asia, and Australia are all connected together.

Security threats are different today. The time honored advice says "don't write your password down and put it in your desk" lest someone find it. With world-wide Internet connections, someone could get into your system from the other side of the world and steal your password in the middle of the night when your building is locked up. Viruses and worms can be passed from machine to machine. The Internet allows the electronic equivalent of the thief who looks for open windows and doors; now a person can check hundreds of machines for vulnerabilities in a few hours. System administrators and decision makers have to understand the security threats that exist, what the risk and cost of a problem would be, and what kind of action they want to take (if any) to prevent and respond to security threats.

As an illustration of some of the issues that need to be dealt with in security problems, consider the following scenarios (thanks to Russell Brand [2, BRAND] for these):
· A system programmer gets a call reporting that a major underground cracker newsletter is being distributed from the administrative machine at his center to five thousand sites in the US and Western Europe.

Eight weeks later, the authorities call to inform you the information in one of these newsletters was used to disable "911" in a major city for five hours.
A user calls in to report that he can't login to his account at 3 o'clock in the morning on a Saturday. The system staffer can't login either. After rebooting to single user mode, he finds that password file is empty. By Monday morning, your staff determines that a number of privileged file transfers took place between this machine and a local university. Tuesday morning a copy of the deleted password file is found on the university machine along with password files for a dozen other machines. A week later you find that your system initialization files had been altered in a hostile fashion.
You receive a call saying that a breakin to a government lab occurred from one of your center's machines. You are requested to provide accounting files to help trackdown the attacker. A week later you are given a list of machines at your site that have been broken into. A reporter calls up asking about the breakin at your center. You haven't heard of any such breakin. Three days later, you learn that there was a breakin. The center director had his wife's name as a password.

A change in system binaries is detected. The day that it is corrected, they again are changed. This repeats itself for some weeks.
· If an intruder is found on your system, should you leave the system open to monitor the situation or should you close down the holes and open them up again later?
· If an intruder is using your site, should you call law enforcement? Who makes that decision? If law enforcement asks you to leave your site open, who makes that decision?
· What steps should be taken if another site calls you and says they see activity coming from an account on your system? What if the account is owned by a local manager?

Basic Approach

Setting security policies and procedures really means developing a plan for how to deal with computer security. One way to approach this task is suggested by Fites, et. al. [3, FITES]:
· Look at what you are trying to protect.
· Look at what you need to protect it from.
· Determine how likely the threats are.
· Implement measures which will protect your assets in a cost-effective manner.
· Review the process continuously, and improve things every time a weakness is found.

This handbook will concentrate mostly on the last two steps, but the first three are critically important to making effective decisions about security. One old truism in security is that the cost of protecting yourself against a threat should be less than the cost recovering if the threat were to strike you. Without reasonable knowledge of what you are protecting and what the likely threats are, following this rule could be difficult.

Organization of this Document

This document is organized into seven parts in addition to this introduction. The basic form of each section is to discuss issues that a site might want to consider in creating a computer security policy and setting procedures to implement that policy. In some cases, possible options are discussed along with the some of the ramifications of those choices. As far as possible, this document tries not to dictate the choices a site should make, since these depend on local circumstances. Some of the issues brought up may not apply to all sites. Nonetheless, all sites should at least consider the issues brought up here to ensure that they do not miss some important area.

The overall flow of the document is to discuss policy issues followed by the issues that come up in creating procedures to implement the policies.

Section 2 discusses setting official site policies for access to computing resources. It also goes into the issue of what happens when the policy is violated. The policies will drive the procedures that need to be created, so decision makers will need to make choices about policies before many of the procedural issues in following sections can be dealt with. A key part of creating policies is doing some kind of risk assessment to decide what really needs to be protected and the level of resources that should be applied to protect them.

Once policies are in place, procedures to prevent future security problems should be established. Section 3 defines and suggests actions to take when unauthorized activity is suspected. Resources to prevent secruity breaches are also discussed.

Section 4 discusses types of procedures to prevent security problems. Prevention is a key to security; as an example, the Computer Emergency Response Team/Coordination Center (CERT/CC) at Carnegie-Mellon University (CMU) estimates that 80% or more of the problems they see have to do with poorly chosen passwords.

Section 5 discusses incident handling: what kinds of issues does a site face when someone violates the security policy. Many decisions will have to made on the spot as the incident occurs, but many of the options and issues can be discussed in advance. At very least, responsibilities and methods of communication can be established before an incident. Again, the choices here are influenced by the policies discussed in section 2.

Section 6 deals with what happens after a security violation has been dealt with. Security planning is an on-going cycle; just after an incident has occurred is an excellent opportunity to improve policies and procedures.

The rest of the document provides references and an annotated bibliography.

Site Policy on Computer Security

Establishing Official Site Policy on Computer Security

Brief Overview

Organization Issues

The goal in developing an official site policy on computer security is to define the organization's expectations of proper computer and network use and to define procedures to prevent and respond to security incidents. In order to do this, aspects of the particular organization must be considered.

First, the goals and direction of the organization should be considered. For example, a military base may have very different security concerns from a those of a university.

Second, the site security policy developed must conform to existing policies, rules, regulations and laws that the organization is subject to. Therefore it will be necessary to identify these and take them into consideration while developing the policy.

Third, unless the local network is completely isolated and standalone, it is necessary to consider security implications in a more global context. The policy should address the issues when local security problems develop as a result of a remote site as well as when problems occur on remote systems as a result of a local host or user.

Who Makes the Policy?

Policy creation must be a joint effort by technical personnel, who understand the full ramifications of the proposed policy and the implementation of the policy, and by decision makers who have the power to enforce the policy. A policy which is neither implementable nor enforceable is useless.

Since a computer security policy can affect everyone in an organization, it is worth taking some care to make sure you have the right level of authority in on the policy decisions. Though a particular group (such as a campus information services group) may have responsibility for enforcing a policy, an even higher group may have to support and approve the policy.

Who is Involved?

Establishing a site policy has the potential for involving every computer user at the site in a variety of ways. Computer users may be responsible for personal password administration. Systems managers are obligated to fix security holes and to oversee the system.

It is critical to get the right set of people involved at the start of the process. There may already be groups concerned with security who would consider a computer security policy to be their area. Some of the types of groups that might be involved include auditing/control, organizations that deal with physical security, campus information systems groups, and so forth. Asking these types of groups to "buy in" from the start can help facilitate the acceptance of the policy.


A key element of a computer security policy is making sure everyone knows their own responsibility for maintaining security. A computer security policy cannot anticipate all possibilities; however, it can ensure that each kind of problem does have someone assigned to deal with it.

There may be levels of responsibility associated with a policy on computer security. At one level, each user of a computing resource may have a responsibility to protect his account. A user who allows his account to be compromised increases the chances of compromising other accounts or resources.

System managers may form another responsibility level: they must help to ensure the security of the computer system. Network managers may reside at yet another level.

Risk Assessment

General Discussion

One of the most important reasons for creating a computer security policy is to ensure that efforts spent on security yield cost effective benefits. Although this may seem obvious, it is possible to be mislead about where the effort is needed. As an example, there is a great deal of publicity about intruders on computers systems; yet most surveys of computer security show that for most organizations, the actual loss from "insiders" is much greater.

Risk analysis involves determining what you need to protect, what you need to protect it from, and how to protect it. Is is the process of examining all of your risks, and ranking those risks by level of severity. This process involves making cost-effective decisions on what you want to protect. The old security adage says that you should not spend more to protect something than it is actually worth.

A full treatment of risk analysis is outside the scope of this document. [3, FITES] and [16, PFLEEGER] provide introductions to this topic. However, there are two elements of a risk analysis that will be briefly covered in the next two sections:
1. Identifying the assets
2. Identifying the threats

For each asset, the basic goals of security are availability, confidentiality, and integrity. Each threat should be examined with an eye to how the threat could affect these areas.

Identifying the Assets

One step in a risk analysis is to identify all the things that need to be protected. Some things are obvious, like all the various pieces of hardware, but some are overlooked, such as the people who actually use the systems. The essential point is to list all things that could be affected by a security problem.

One list of categories is suggested by Pfleeger [16, PFLEEGER, page 459]; this list is adapted from that source:
1. Hardware: cpus, boards, keyboards, terminals, workstations, personal computers, printers, disk drives, communication lines, terminal servers, routers.
2. Software: source programs, object programs, utilities, diagnostic programs, operating systems, communication programs.
3. Data: during execution, stored on-line, archived off-line, backups, audit logs, databases, in transit over communication media.
4. People: users, people needed to run systems.
5. Documentation: on programs, hardware, systems, local administrative procedures.
6. Supplies: paper, forms, ribbons, magnetic media.

Identifying the Threats

Once the assets requiring protection are identified, it is necessary to identify threats to those assests. The threats can then be examined to determine what potential for loss exists. It helps to consider from what threats you are trying to protect your assets.

The following sections describe a few of the possible threats.

Unauthorized Access

A common threat that concerns many sites is unauthorized access to computing facilities. Unauthorized access takes many forms. One means of unauthorized access is the use of another user's account to gain access to a system. The use of any computer resource without prior permission may be considered unauthorized access to computing facilities.

The seriousness of an unauthorized access will vary from site to site. For some sites, the mere act of granting access to an unauthorized user may cause irreparable harm by negative media coverage. For other sites, an unauthorized access opens the door to other security threats. In addition, some sites may be more frequent targets than others; hence the risk from unauthorized access will vary from site to site. The Computer Emergency Response Team (CERT - see section has observed that well-known universities, government sites, and military sites seem to attract more intruders.

Disclosure of Information

Another common threat is disclosure of information. Determine the value or sensitivity of the information stored on your computers. Disclosure of a password file might allow for future unauthorized accesses. A glimpse of a proposal may give a competitor an unfair advantage. A technical paper may contain years of valuable research.

Denial of Service

Computers and networks provide valuable services to their users. Many people rely on these services in order to perform their jobs efficiently. When these services are not available when called upon, a loss in productivity results.

Denial of service comes in many forms and might affect users in a number of ways. A network may be rendered unusable by a rogue packet, jamming, or by a disabled network component. A virus might slow down or cripple a computer system. Each site should determine which services are essential, and for each of these services determine the affect to the site if that service were to become disabled.

Policy Issues

There are a number of issues that must be addressed when developing a security policy. These are:
1. Who is allowed to use the resources?
2. What is the proper use of the resources?
3. Who is authorized to grant access and approve usage?
4. Who may have system administration privileges?
5. What are the user's rights and responsibilities?
6. What are the rights and responsibilities of the system administrator vs. those of the user?
7. What do you do with sensitive information?

These issues will be discussed below. In addition you may wish to include a section in your policy concerning ethical use of computing resources. Parker, Swope and Baker [17, PARKER90] and Forester and Morrison [18, FORESTER] are two useful references that address ethical issues.

Who is Allowed to use the Resources?

One step you must take in developing your security policy is defining who is allowed to use your system and services. The policy should explicitly state who is authorized to use what resources.

What is the Proper Use of the Resources?

After determining who is allowed access to system resources it is necessary to provide guidelines for the acceptable use of the resources. You may have different guidelines for different types of users (i.e., students, faculty, external users). The policy should state what is acceptable use as well as unacceptable use. It should also include types of use that may be restricted.

Define limits to access and authority. You will need to consider the level of access various users will have and what resources will be available or restricted to various groups of people.

Your acceptable use policy should clearly state that individual users are responsible for their actions. Their responsibility exists regardless of the security mechanisms that are in place. It should be clearly stated that breaking into accounts or bypassing security is not permitted.

The following points should be covered when developing an acceptable use policy:
o Is breaking into accounts permitted?
o Is cracking passwords permitted?
o Is disrupting service permitted?

o Should users assume that a file being world-readable grants them the authorization to read it? o Should users be permitted to modify files that are not their own even if they happen to have write permission? o Should users share accounts?

The answer to most of these questions will be "no".

You may wish to incorporate a statement in your policies concerning copyrighted and licensed software. Licensing agreements with vendors may require some sort of effort on your part to ensure that the license is not violated. In addition, you may wish to inform users that the copying of copyrighted software may be a violation of the copyright laws, and is not permitted.

Specifically concerning copyrighted and/or licensed software, you may wish to include the following information:

o Copyrighted and licensed software may not be duplicated unless it is explicitly stated that you may do so. o Methods of conveying information on the copyright/licensed status of software. o When in doubt, DON'T COPY.

Your acceptable use policy is very important. A policy which does not clearly state what is not permitted may leave you unable to prove that a user violated policy.

There are exception cases like tiger teams and users or administrators wishing for "licenses to hack"-you may face the situation where users will want to "hack" on your services for security research purposes. You should develop a policy that will determine whether you will permit this type of research on your services and if so, what your guidelines for such research will be.

Points you may wish to cover in this area:

o Whether it is permitted at all. o What type of activity is permitted: breaking in, releasing worms, releasing viruses, etc.. o What type of controls must be in place to ensure that it does not get out of control (e.g., separate a segment of your network for these tests). o How you will protect other users from being victims of these activities, including external users and networks. o The process for obtaining permission to conduct these tests.

In cases where you do permit these activities, you should isolate the portions of the network that are being tested from your main network. Worms and viruses should never be released on a live network.

You may also wish to employ, contract, or otherwise solicit one or more people or organizations to evaluate the security of your services, of which may include "hacking". You may wish to provide for this in your policy.

Who Is Authorized to Grant Access and Approve Usage?

Your policy should state who is authorized to grant access to your services. Further, it must be determined what type of access they are permitted to give. If you do not have control over who is granted access to your system, you will not have control over who is using your system. Controlling who has the authorization to grant access will also enable you to know who was or was not granting access if problems develop later.

There are many schemes that can be developed to control the distribution of access to your services. The following are the factors that you must consider when determining who will distribute access to your services:

o Will you be distributing access from a centralized point or at various points?

You can have a centralized distribution point to a distributed system where various sites or departments independently authorize access. The trade off is between security and convenience. The more centralized, the easier to secure.

o What methods will you use for creating accounts and terminating access?

From a security standpoint, you need to examine the mechanism that you will be using to create accounts. In the least restrictive case, the people who are authorized to grant access would be able to go into the system directly and create an account by hand or through vendor supplied mechanisms. Generally, these mechanisms place a great deal of trust in the person running them, and the person running them usually has a large amount of privileges. If this is the choice you make, you need to select someone who is trustworthy to perform this task. The opposite solution is to have an integrated system that the people authorized to create accounts run, or the users themselves may actually run. Be aware that even in the restrictive case of having a mechanized facility to create accounts does not remove the potential for abuse.

You should have specific procedures developed for the creation of accounts. These procedures should be well documented to prevent confusion and reduce mistakes. A security vulnerability in the account authorization process is not only possible through abuse, but is also possible if a mistake is made. Having clear and well documented procedure will help ensure that these mistakes won't happen. You should also be sure that the people who will be following these procedures understand them.

The granting of access to users is one of the most vulnerable of times. You should ensure that the selection of an initial password cannot be easily guessed. You should avoid using an initial password that is a function of the username, is part of the user's name, or some algorithmically generated password that can easily be guessed. In addition, you should not permit users to continue to use the initial password indefinitely. If possible, you should force users to change the initial password the first time they login. Consider that some users may never even login, leaving their password vulnerable indefinitely. Some sites choose to disable accounts that have never been accessed, and force the owner to reauthorize opening the account.

Who May Have System Administration Privileges?

One security decision that needs to be made very carefully is who will have access to system administrator privileges and passwords for your services. Obviously, the system administrators will need access, but inevitably other users will request special privileges. The policy should address this issue. Restricting privileges is one way to deal with threats from local users. The challenge is to balance restricting access to these to protect security with giving people who need these privileges access so that they can perform their tasks. One approach that can be taken is to grant only enough privilege to accomplish the necessary tasks.

Additionally, people holding special privileges should be accountable to some authority and this should also be identified within the site's security policy. If the people you grant privileges to are not accountable, you run the risk of losing control of your system and will have difficulty managing a compromise in security.

What Are The Users' Rights and Responsibilities?

The policy should incorporate a statement on the users' rights and responsibilities concerning the use of the site's computer systems and services. It should be clearly stated that users are responsible for understanding and respecting the security rules of the systems they are using. The following is a list of topics that you may wish to cover in this area of the policy:

o What guidelines you have regarding resource consumption (whether users are restricted, and if so, what the restrictions are). o What might constitute abuse in terms of system performance. o Whether users are permitted to share accounts or let others use their accounts. o How "secret" users should keep their passwords. o How often users should change their passwords and any other password restrictions or requirements. o Whether you provide backups or expect the users to create their own. o Disclosure of information that may be proprietary. o Statement on Electronic Mail Privacy (Electronic Communications Privacy Act). o Your policy concerning controversial mail or postings to mailing lists or discussion groups (obscenity, harassment, etc.). o Policy on electronic communications: mail forging, etc.

The Electronic Mail Association sponsored a white paper on the privacy of electronic mail in companies [4]. Their basic recommendation is that every site should have a policy on the protection of employee privacy. They also recommend that organizations establish privacy policies that deal with all media, rather than singling out electronic mail.

They suggest five criteria for evaluating any policy:
1. Does the policy comply with law and with duties to third parties?

2. Does the policy unnecessarily compromise the interest of the employee, the employer

or third parties?
3. Is the policy workable as a practical matter and likely to be enforced?
4. Does the policy deal appropriately with all different forms of communications and record keeping with the office?
5. Has the policy been announced in advance and agreed to by all concerned?

2.3.6 What Are The Rights and Responsibilities of System Administrators Versus Rights of Users

There is a tradeoff between a user's right to absolute privacy and the need of system administrators to gather sufficient information to diagnose problems. There is also a distinction between a system administrator's need to gather information to diagnose problems and investigating security violations. The policy should specify to what degree system administrators can examine user files to diagnose problems or for other purposes, and what rights you grant to the users. You may also wish to make a statement concerning system administrators' obligation to maintaining the privacy of information viewed under these circumstances. A few questions that should be answered are:

o Can an administrator monitor or read a user's files for any reason? o What are the liabilities? o Do network administrators have the right to examine network or host traffic?

What To Do With Sensitive Information

Before granting users access to your services, you need to determine at what level you will provide for the security of data on your systems. By determining this, you are determining the level of sensitivity of data that users should store on your systems. You do not want users to store very sensitive information on a system that you are not going to secure very well. You need to tell users who might store sensitive information what services, if any, are appropriate for the storage of sensitive information. This part should include storing of data in different ways (disk, magnetic tape, file servers, etc.). Your policy in this area needs to be coordinated with the policy concerning the rights of system administrators versus users (see section 2.3.6).

What Happens When the Policy is Violated

It is obvious that when any type of official policy is defined, be it related to computer security or not, it will eventually be broken. The violation may occur due to an individual's negligence, accidental mistake, having not been properly informed of the current policy, or not understanding the current policy. It is equally possible that an individual (or group of individuals) may knowingly perform an act that is in direct violation of the defined policy.

When a policy violation has been detected, the immediate course of action should be pre-defined to ensure prompt and proper enforcement. An investigation should be performed to determine how and why the violation occurred. Then the appropriate corrective action should be executed. The type and severity of action taken varies depending on the type of violation that occurred.

Determining the Response to Policy Violations

Violations to policy may be committed by a wide variety of users. Some may be local users and others may be from outside the local environment. Sites may find it helpful to define what it considers "insiders" and "outsiders" based upon administrative, legal or political boundaries. These boundaries imply what type of action must be taken to correct the offending party; from a written reprimand to pressing legal charges. So, not only do you need to define actions based on the type of violation, you also need to have a clearly defined series of actions based on the kind of user violating your computer security policy. This all seems rather complicated, but should be addressed long before it becomes necessary as the result of a violation.

One point to remember about your policy is that proper education is your best defense. For the outsiders who are using your computer legally, it is your responsibility to verify that these individuals are aware of the policies that you have set forth. Having this proof may assist you in the future if legal action becomes necessary.

As for users who are using your computer illegally, the problem is basically the same. What type of user violated the policy and how and why did they do it? Depending on the results of your investigation, you may just prefer to "plug" the hole in your computer security and chalk it up to experience. Or if a significant amount of loss was incurred, you may wish to take more drastic action.

2.4.2 What to do When Local Users Violate the Policy of a Remote Site

In the event that a local user violates the security policy of a remote site, the local site should have a clearly defined set of administrative actions to take concerning that local user. The site should also be prepared to protect itself against possible actions by the remote site. These situations involve legal issues which should be addressed when forming the security policy.

Defining Contacts and Responsibilities to Outside Organizations

The local security policy should include procedures for interaction with outside organizations. These include law enforcement agencies, other sites, external response team organizations (e.g., the CERT, CIAC) and various press agencies. The procedure should state who is authorized to make such contact and how it should be handled. Some questions to be answered include:

o Who may talk to the press? o When do you contact law enforcement and investigative agencies? o If a connection is made from a remote site, is the system manager authorized to contact that site? o Can data be released? What kind?

Detailed contact information should be readily available along with clearly defined procedures to follow.

2.4.4 What are the Responsibilities to our Neighbors and Other Internet Sites?

The Security Policy Working Group within the IETF is working on a document entitled, "Policy Guidelines for the Secure Operation of the Internet" [23]. It addresses the issue that the Internet is a cooperative venture and that sites are expected to provide mutual security assistance. This should be addressed when developing a site's policy. The major issue to be determined is how much information should be released. This will vary from site to site according to the type of site (e.g., military, education, commercial) as well as the type of security violation that occurred.

Issues for Incident Handling Procedures

Along with statements of policy, the document being prepared should include procedures for incident handling. This is covered in detail in the next chapter. There should be procedures available that cover all facets of policy violation.

Locking In or Out

Whenever a site suffers an incident which may compromise computer security, the strategies for reacting may be influenced by two opposing pressures.

If management fears that the site is sufficiently vulnerable, it may choose a "Protect and Proceed" strategy. This approach will have as its primary goal the protection and preservation of the site facilities and to provide for normalcy for its users as quickly as possible. Attempts will be made to actively interfere with the intruder's processes, prevent further access and begin immediate damage assessment and recovery. This process may involve shutting down the facilities, closing off access to the network, or other drastic measures. The drawback is that unless the intruder is identified directly, they may come back into the site via a different path, or may attack another site.

The alternate approach, "Pursue and Prosecute", adopts the opposite philosophy and goals. The primary goal is to allow intruders to continue their activities at the site until the site can identify the responsible persons. This approach is endorsed by law enforcement agencies and prosecutors. The drawback is that the agencies cannot exempt a site from possible user lawsuits if damage is done to their systems and data.

Prosecution is not the only outcome possible if the intruder is identified. If the culprit is an employee or a student, the organization may choose to take disciplinary actions. The computer security policy needs to spell out the choices and how they will be selected if an intruder is caught.

Careful consideration must be made by site management regarding their approach to this issue before the problem occurs. The strategy adopted might depend upon each circumstance. Or there may be a global policy which mandates one approach in all circumstances. The pros and cons must be examined thoroughly and the users of the facilities must be made aware of the policy so that they understand their vulnerabilities no matter which approach is taken.

The following are checklists to help a site determine which strategy to adopt: "Protect and Proceed" or "Pursue and Prosecute".

Protect and Proceed

1. If assets are not well protected.
2. If continued penetration could result in great financial risk.
3. If the possibility or willingness to prosecute is not present.
4. If user base is unknown.
5. If users are unsophisticated and their work is vulnerable.
6. If the site is vulnerable to lawsuits from users, e.g., if their resources are undermined.

Pursue and Prosecute

1. If assets and systems are well protected.
2. If good backups are available.
3. If the risk to the assets is outweighed by the disruption caused by the present and possibly future penetrations.
4. If this is a concentrated attack occurring with great frequency and intensity.
5. If the site has a natural attraction to intruders, and consequently regularly attracts intruders.
6. If the site is willing to incur the financial (or other) risk to assets by allowing the penetrator continue.
7. If intruder access can be controlled.
8. If the monitoring tools are sufficiently well-developed to make the pursuit worthwhile.
9. If the support staff is sufficiently clever and knowledgable about the operating system, related utilities, and systems to make the pursuit worthwhile.
10. If there is willingness on the part of management to prosecute.
11. If the system adminitrators know in general what kind of evidence would lead to prosecution.
12. If there is established contact with knowledgeable law enforcement.
13. If there is a site representative versed in the relevant legal issues.
14. If the site is prepared for possible legal action from its own users if their data or systems become compromised during the pursuit.

Interpreting the Policy

It is important to define who will interpret the policy. This could be an individual or a committee. No matter how well written, the policy will require interpretation from time to time and this body would serve to review, interpret, and revise the policy as needed.

Publicizing the Policy

Once the site security policy has been written and established, a vigorous process should be engaged to ensure that the policy statement is widely and thoroughly disseminated and discussed. A mailing of the policy should not be considered sufficient. A period for comments should be allowed before the policy becomes effective to ensure that all affected users have a chance to state their reactions and discuss any unforeseen ramifications. Ideally, the policy should strike a balance between protection and productivity.

Meetings should be held to elicit these comments, and also to ensure that the policy is correctly understood. (Policy promulgators are not necessarily noted for their skill with the language.) These meetings should involve higher management as well as line employees. Security is a collective effort.

In addition to the initial efforts to publicize the policy, it is essential for the site to maintain a continual awareness of its computer security policy. Current users may need periodic reminders New users should have the policy included as part of their site introduction packet. As a condition for using the site facilities, it may be advisable to have them sign a statement that they have read and understood the policy. Should any of these users require legal action for serious policy violations, this signed statement might prove to be a valuable aid.

3. Establishing Procedures to Prevent Security Problems

The security policy defines what needs to be protected. This section discusses security procedures which specify what steps will be used to carry out the security policy.

3.1 Security Policy Defines What Needs to be Protected

The security policy defines the WHAT's: what needs to be protected, what is most important, what the priorities are, and what the general approach to dealing with security problems should be.

The security policy by itself doesn't say HOW things are protected. That is the role of security procedures, which this section discusses. The security policy should be a high level document, giving general strategy. The security procedures need to set out, in detail, the precise steps your site will take to protect itself.

The security policy should include a general risk assessment of the types of threats a site is mostly likely to face and the consequences of those threats (see section 2.2). Part of doing a risk assessment will include creating a general list of assets that should be protected (section 2.2.2). This information is critical in devising cost-effective procedures.

It is often tempting to start creating security procedures by deciding on different mechanisms first: "our site should have logging on all hosts, call-back modems, and smart cards for all users." This approach could lead to some areas that have too much protection for the risk they face, and other areas that aren't protected enough. Starting with the security policy and the risks it outlines should ensure that the procedures provide the right level of protect for all assets.

Identifing Possible Problems

To determine risk, vulnerabilities must be identified. Part of the purpose of the policy is to aid in shoring up the vulnerabilities and thus to decrease the risk in as many areas as possible. Several of the more popular problem areas are presented in sections below. This list is by no means complete. In addition, each site is likely to have a few unique vulnerabilities.

Access Points

Access points are typically used for entry by unauthorized users. Having many access points increases the risk of access to an organization's computer and network facilities.

Network links to networks outside the organization allow access into the organization for all others connected to that external network. A network link typically provides access to a large number of network services, and each service has a potential to be compromised.

Dialup lines, depending on their configuration, may provide access merely to a login port of a single system. If connected to a terminal server, the dialup line may give access to the entire network.

Terminal servers themselves can be a source of problem. Many terminal servers do not require any kind of authentication. Intruders often use terminal servers to disguise their actions, dialing in on a local phone and then using the terminal server to go out to the local network. Some terminal servers are configured so that intruders can TELNET [19] in from outside the network, and then TELNET back out again, again serving to make it difficult to trace them.

Misconfigured Systems

Misconfigured systems form a large percentage of security holes. Today's operating systems and their associated software have become so complex that understanding how the system works has become a full-time job. Often, systems managers will be non-specialists chosen from the current organization's staff.

Vendors are also partly responsible for misconfigured systems. To make the system installation process easier, vendors occasionally choose initial configurations that are not secure in all environments.

Software Bugs

Software will never be bug free. Publicly known security bugs are common methods of unauthorized entry. Part of the solution to this problem is to be aware of the security problems and to update the software when problems are detected. When bugs are found, they should be reported to the vendor so that a solution to the problem can be implemented and distributed.

"Insider" Threats

An insider to the organization may be a considerable threat to the security of the computer systems. Insiders often have direct access to the computer and network hardware components. The ability to access the components of a system makes most systems easier to compromise. Most desktop workstations can be easily manipulated so that they grant privileged access. Access to a local area network provides the ability to view possibly sensitive data traversing the network.

3.3 Choose Controls to Protect Assets in a Cost-Effective Way

After establishing what is to be protected, and assessing the risks these assets face, it is necessary to decide how to implement the controls which protect these assets. The controls and protection mechanisms should be selected in a way so as to adequately counter the threats found during risk assessment, and to implement those controls in a cost effective manner. It makes little sense to spend an exorbitant sum of money and overly constrict the user base if the risk of exposure is very small.

Choose the Right Set of Controls[]

The controls that are selected represent the physical embodiment of your security policy. They are the first and primary line of defense in the protection of your assets. It is therefore most important to ensure that the controls that you select are the right set of controls. If the major threat to your system is outside penetrators, it probably doesn't make much sense to use biometric devices to authenticate your regular system users. On the other hand, if the major threat is unauthorized use of computing resources by regular system users, you'll probably want to establish very rigorous automated accounting procedures.

Use Common Sense

Common sense is the most appropriate tool that can be used to establish your security policy. Elaborate security schemes and mechanisms are impressive, and they do have their place, yet there is little point in investing money and time on an elaborate implementation scheme if the simple controls are forgotten. For example, no matter how elaborate a system you put into place on top of existing security controls, a single user with a poor password can still leave your system open to attack.

Use Multiple Strategies to Protect Assets

Another method of protecting assets is to use multiple strategies. In this way, if one strategy fails or is circumvented, another strategy comes into play to continue protecting the asset. By using several simpler strategies, a system can often be made more secure than if one very sophisticated method were used in its place. For example, dial-back modems can be used in conjunction with traditional logon mechanisms. Many similar approaches could be devised that provide several levels of protection for assets. However, it's very easy to go overboard with extra mechanisms. One must keep in mind exactly what it is that needs to be protected.

Physical Security

It is a given in computer security if the system itself is not physically secure, nothing else about the system can be considered secure. With physical access to a machine, an intruder can halt the machine, bring it back up in privileged mode, replace or alter the disk, plant Trojan horse programs (see section, or take any number of other undesirable (and hard to prevent) actions.

Critical communications links, important servers, and other key machines should be located in physically secure areas. Some security systems (such as Kerberos) require that the machine be physically secure.

If you cannot physically secure machines, care should be taken about trusting those machines. Sites should consider limiting access from non-secure machines to more secure machines. In particular, allowing trusted access (e.g., the BSD Unix remote commands such as rsh) from these kinds of hosts is particularly risky.

For machines that seem or are intended to be physically secure, care should be taken about who has access to the machines. Remember that custodial and maintenance staff often have keys to rooms.

Procedures to Recognize Unauthorized Activity

Several simple procedures can be used to detect most unauthorized uses of a computer system. These procedures use tools provided with the operating system by the vendor, or tools publicly available from other sources.

Monitoring System Use

System monitoring can be done either by a system administrator, or by software written for the purpose. Monitoring a system involves looking at several parts of the system and searching for anything unusual. Some of the easier ways to do this are described in this section.

The most important thing about monitoring system use is that it be done on a regular basis. Picking one day out of the month to monitor the system is pointless, since a security breach can be isolated to a matter of hours. Only by maintaining a constant vigil can you expect to detect security violations in time to react to them.

Tools for Monitoring the System

This section describes tools and methods for monitoring a system against unauthorized access and use.


Most operating systems store numerous bits of information in log files. Examination of these log files on a regular basis is often the first line of defense in detecting unauthorized use of the system.
· Compare lists of currently logged in users and past login histories. Most users typically log in and out at roughly the same time each day. An account logged in outside the "normal" time for the account may be in use by an intruder.
· Many systems maintain accounting records for billing purposes. These records can also be used to determine usage patterns for the system; unusual accounting records may indicate unauthorized use of the system.
· System logging facilities, such as the UNIX "syslog" utility, should be checked for unusual error messages from system software. For example, a large number of failed login attempts in a short period of time may indicate someone trying to guess passwords.
· Operating system commands which list currently executing processes can be used to detect users running programs they are not authorized to use, as well as to detect unauthorized programs which have been started by an intruder.

Monitoring Software

Other monitoring tools can easily be constructed using standard operating system software, by using several, often unrelated, programs together. For example, checklists of file ownerships and permission settings can be constructed (for example, with "ls" and "find" on UNIX) and stored off-line. These lists can then be reconstructed periodically and compared against the master checklist (on UNIX, by using the "diff" utility).

Differences may indicate that unauthorized modifications have been made to the system.

Still other tools are available from third-party vendors and public software distribution sites. Section 3.9.9 lists several sources from which you can learn what tools are available and how to get them.

Other Tools

Other tools can also be used to monitor systems for security violations, although this is not their primary purpose. For example, network monitors can be used to detect and log connections from unknown sites.

Vary the Monitoring Schedule

The task of system monitoring is not as daunting as it may seem. System administrators can execute many of the commands used for monitoring periodically throughout the day during idle moments (e.g., while talking on the telephone), rather than spending fixed periods of each day monitoring the system. By executing the commands frequently, you will rapidly become used to seeing "normal" output, and will easily spot things which are out of the ordinary. In addition, by running various monitoring commands at different times throughout the day, you make it hard for an intruder to predict your actions. For example, if an intruder knows that each day at 5:00 p.m. the system is checked to see that everyone has logged off, he will simply wait until after the check has completed before logging in. But the intruder cannot guess when a system administrator might type a command to display all logged-in users, and thus he runs a much greater risk of detection.

Despite the advantages that regular system monitoring provides, some intruders will be aware of the standard logging mechanisms in use on systems they are attacking. They will actively pursue and attempt to disable monitoring mechanisms. Regular monitoring therefore is useful in detecting intruders, but does not provide any guarantee that your system is secure, nor should monitoring be considered an infallible method of detecting unauthorized use.

3.7 Define Actions to Take When Unauthorized Activity is Suspected

Sections 2.4 and 2.5 discussed the course of action a site should take when it suspects its systems are being abused. The computer security policy should state the general approach towards dealing with these problems.

The procedures for dealing with these types of problems should be written down. Who has authority to decide what actions will be taken? Should law enforcement be involved? Should your organization cooperate with other sites in trying to track down an intruder? Answers to all the questions in section 2.4 should be part of the incident handling procedures.

Whether you decide to lock out or pursue intruders, you should have tools and procedures ready to apply. It is best to work up these tools and procedures before you need them. Don't wait until an intruder is on your system to figure out how to track the intruder's actions; you will be busy enough if an intruder strikes.

Communicating Security Policy

Security policies, in order to be effective, must be communicated to both the users of the system and the system maintainers. This section describes what these people should be told, and how to tell them.

Educating the Users

Users should be made aware of how the computer systems are expected to be used, and how to protect themselves from unauthorized users.

Proper Account/Workstation Use

All users should be informed about what is considered the "proper" use of their account or workstation ("proper" use is discussed in section 2.3.2). This can most easily be done at the time a user receives their account, by giving them a policy statement. Proper use policies typically dictate things such as whether or not the account or workstation may be used for personal activities (such as checkbook balancing or letter writing), whether profit-making activities are allowed, whether game playing is permitted, and so on. These policy statements may also be used to summarize how the computer facility is licensed and what software licenses are held by the institution; for example, many universities have educational licenses which explicitly prohibit commercial uses of the system. A more complete list of items to consider when writing a policy statement is given in section 2.3.

Account/Workstation Management Procedures

Each user should be told how to properly manage their account and workstation. This includes explaining how to protect files stored on the system, how to log out or lock the terminal or workstation, and so on. Much of this information is typically covered in the "beginning user" documentation provided by the operating system vendor, although many sites elect to supplement this material with local information.

If your site offers dial-up modem access to the computer systems, special care must be taken to inform users of the security problems inherent in providing this access. Issues such as making sure to log out before hanging up the modem should be covered when the user is initially given dial-up access.

Likewise, access to the systems via local and wide-area networks presents its own set of security problems which users should be made aware of. Files which grant "trusted host" or "trusted user" status to remote systems and users should be carefully explained.

Determining Account Misuse

Users should be told how to detect unauthorized access to their account. If the system prints the last login time when a user logs in, he or she should be told to check that time and note whether or not it agrees with the last time he or she actually logged in.

Command interpreters on some systems (e.g., the UNIX C shell) maintain histories of the last several commands executed. Users should check these histories to be sure someone has not executed other commands with their account.

Problem Reporting Procedures

A procedure should be developed to enable users to report suspected misuse of their accounts or other misuse they may have noticed. This can be done either by providing the name and telephone number of a system administrator who manages security of the computer system, or by creating an electronic mail address (e.g., "security") to which users can address their problems.

Educating the Host Administrators

In many organizations, computer systems are administered by a wide variety of people. These administrators must know how to protect their own systems from attack and unauthorized use, as well as how to communicate successful penetration of their systems to other administrators as a warning.

Account Management Procedures

Care must be taken when installing accounts on the system in order to make them secure. When installing a system from distribution media, the password file should be examined for "standard" accounts provided by the vendor. Many vendors provide accounts for use by system services or field service personnel. These accounts typically have either no password or one which is common knowledge. These accounts should be given new passwords if they are needed, or disabled or deleted from the system if they are not.

Accounts without passwords are generally very dangerous since they allow anyone to access the system. Even accounts which do not execute a command interpreter (e.g., accounts which exist only to see who is logged in to the system) can be compromised if set up incorrectly. A related concept, that of "anonymous" file transfer (FTP) [20], allows users from all over the network to access your system to retrieve files from (usually) a protected disk area. You should carefully weigh the benefits that an account without a password provides against the security risks of providing such access to your system.

If the operating system provides a "shadow" password facility which stores passwords in a separate file accessible only to privileged users, this facility should be used. System V UNIX, SunOS 4.0 and above, and versions of Berkeley UNIX after 4.3BSD Tahoe, as well as others, provide this feature. It protects passwords by hiding their encrypted values from unprivileged users. This prevents an attacker from copying your password file to his or her machine and then attempting to break the passwords at his or her leisure.

Keep track of who has access to privileged user accounts (e.g., "root" on UNIX or "MAINT" on VMS). Whenever a privileged user leaves the organization or no longer has need of the privileged account, the passwords on all privileged accounts should be changed.

Configuration Management Procedures

When installing a system from the distribution media or when installing third-party software, it is important to check the installation carefully. Many installation procedures assume a"trusted" site, and hence will install files with world write permission enabled, or otherwise compromise the security of files.

Network services should also be examined carefully when first installed. Many vendors provide default network permission files which imply that all outside hosts are to be "trusted", which is rarely the case when connected to wide-area networks such as the Internet.

Many intruders collect information on the vulnerabilities of particular system versions. The older a system, the more likely it is that there are security problems in that version which have since been fixed by the vendor in a later release. For this reason, it is important to weigh the risks of not upgrading to a new operating system release (thus leaving security holes unplugged) against the cost of upgrading to the new software (possibly breaking third-party software, etc.). Bug fixes from the vendor should be weighed in a similar fashion, with the added note that "security" fixes from a vendor usually address fairly serious security problems.

Other bug fixes, received via network mailing lists and the like, should usually be installed, but not without careful examination. Never install a bug fix unless you're sure you know what the consequences of the fix are - there's always the possibility that an intruder has suggested a "fix" which actually gives him or her access to your system.

Recovery Procedures - Backups

It is impossible to overemphasize the need for a good backup strategy. File system backups not only protect you in the event of hardware failure or accidental deletions, but they also protect you against unauthorized changes made by an intruder. Without a copy of your data the way it's "supposed" to be, it can be difficult to undo something an attacker has done.

Backups, especially if run daily, can also be useful in providing a history of an intruder's activities. Looking through old backups can establish when your system was first penetrated. Intruders may leave files around which, although deleted later, are captured on the backup tapes. Backups can also be used to document an intruder's activities to law enforcement agencies if necessary.

A good backup strategy will dump the entire system to tape at least once a month. Partial (or "incremental") dumps should be done at least twice a week, and ideally they should be done daily. Commands specifically designed for performing file system backups (e.g., UNIX "dump" or VMS "BACKUP") should be used in preference to other file copying commands, since these tools are designed with the express intent of restoring a system to a known state.

Problem Reporting Procedures

As with users, system administrators should have a defined procedure for reporting security problems. In large installations, this is often done by creating an electronic mail alias which contains the names of all system administrators in the organization. Other methods include setting up some sort of response team similar to the CERT, or establishing a "hotline" serviced by an existing support group.

Resources to Prevent Security Breaches

This section discusses software, hardware, and procedural resources that can be used to support your site security policy.

Network Connections and Firewalls

A "firewall" is put in place in a building to provide a point of resistance to the entry of flames into another area. Similarly, a secretary's desk and reception area provides a point of controlling access to other office spaces. This same technique can be applied to a computer site, particularly as it pertains to network connections.

Some sites will be connected only to other sites within the same organization and will not have the ability to connect to other networks. Sites such as these are less susceptible to threats from outside their own organization, although intrusions may still occur via paths such as dial-up modems. On the other hand, many other organizations will be connected to other sites via much larger networks, such as the Internet. These sites are susceptible to the entire range of threats associated with a networked environment.

The risks of connecting to outside networks must be weighed against the benefits. It may be desirable to limit connection to outside networks to those hosts which do not store sensitive material, keeping "vital" machines (such as those which maintain company payroll or inventory systems) isolated. If there is a need to participate in a Wide Area Network (WAN), consider restricting all access to your local network through a single system. That is, all access to or from your own local network must be made through a single host computer that acts as a firewall between you and the outside world. This firewall system should be rigorously controlled and password protected, and external users accessing it should also be constrained by restricting the functionality available to remote users. By using this approach, your site could relax some of the internal security controls on your local net, but still be afforded the protection of a rigorously controlled host front end.

Note that even with a firewall system, compromise of the firewall could result in compromise of the network behind the firewall. Work has been done in some areas to construct a firewall which even when compromised, still protects the local network [6, CHESWICK].


Confidentiality, the act of keeping things hidden or secret, is one of the primary goals of computer security practitioners. Several mechanisms are provided by most modern operating systems to enable users to control the dissemination of information. Depending upon where you work, you may have a site where everything is protected, or a site where all information is usually regarded as public, or something in-between. Most sites lean toward the in-between, at least until some penetration has occurred.

Generally, there are three instances in which information is vulnerable to disclosure: when the information is stored on a computer system, when the information is in transit to another system (on the network), and when the information is stored on backup tapes.

The first of these cases is controlled by file permissions, access control lists, and other similar mechanisms. The last can be controlled by restricting access to the backup tapes (by locking them in a safe, for example). All three cases can be helped by using encryption mechanisms.

Encryption (hardware and software)

Encryption is the process of taking information that exists in some readable form and converting it into a non-readable form.

There are several types of commercially available encryption packages in both hardware and software forms. Hardware encryption engines have the advantage that they are much faster than the software equivalent, yet because they are faster, they are of greater potential benefit to an attacker who wants to execute a brute-force attack on your encrypted information.

The advantage of using encryption is that, even if other access control mechanisms (passwords, file permissions, etc.) are compromised by an intruder, the data is still unusable. Naturally, encryption keys and the like should be protected at least as well as account passwords.

Information in transit (over a network) may be vulnerable to interception as well. Several solutions to this exist, ranging from simply encrypting files before transferring them (end-to-end encryption) to special network hardware which encrypts everything it sends without user intervention (secure links). The Internet as a whole does not use secure links, thus end-to-end encryption must be used if encryption is desired across the Internet.

Data Encryption Standard (DES))

DES is perhaps the most widely used data encryption

mechanism today. Many hardware and software implementations exist, and some commercial computers are provided with a software version. DES transforms plain text information into encrypted data (or ciphertext) by means of a special algorithm and "seed" value called a key. So long as the key is retained (or remembered) by the original user, the ciphertext can be restored to the original plain text.

One of the pitfalls of all encryption systems is the need to remember the key under which a thing was encrypted (this is not unlike the password problem discussed elsewhere in this document). If the key is written down, it becomes less secure. If forgotten, there is little (if any) hope of recovering the original data.

Most UNIX systems provide a DES command that enables a user to encrypt data using the DES algorithm.


Similar to the DES command, the UNIX "crypt" command allows a user to encrypt data. Unfortunately, the algorithm used by "crypt" is very insecure (based on the World War II "Enigma" device), and files encrypted with this command can be decrypted easily in a matter of a few hours. Generally, use of the "crypt" command should be avoided for any but the most trivial encryption tasks.

Privacy Enhanced Mail

Electronic mail normally transits the network in the clear (i.e., anyone can read it). This is obviously not the optimal solution. Privacy enhanced mail provides a means to automatically encrypt electronic mail messages so that a person eavesdropping at a mail distribution node is not (easily) capable of reading them. Several privacy enhanced mail packages are currently being developed and deployed on the Internet.

The Internet Activities Board Privacy Task Force has defined a draft standard, elective protocol for use in implementing privacy enhanced mail. This protocol is defined in RFCs 1113, 1114, and 1115 [7,8,9]. Please refer to the current edition of the "IAB Official Protocol Standards" (currently, RFC 1200 [21]) for the standardization state and status of these protocols.

Origin Authentication

We mostly take it on faith that the header of an electronic mail message truly indicates the originator of a message. However, it iseasy to "spoof", or forge the source of a mail message. Origin authentication provides a means to be certain of the originator of a message or other object in the same way that a Notary Public assures a signature on a legal document. This is done by means of a "Public Key" cryptosystem.

A public key cryptosystem differs from a private key cryptosystem in several ways. First, a public key system uses two keys, a Public Key that anyone can use (hence the name) and a Private Key that only the originator of a message uses. The originator uses the private key to encrypt the message (as in DES). The receiver, who has obtained the public key for the originator, may then decrypt the message.

In this scheme, the public key is used to authenticate the originator's use of his or her private key, and hence the identity of the originator is more rigorously proven. The most widely known implementation of a public key cryptosystem is the RSA system [26]. The Internet standard for privacy enhanced mail makes use of the RSA system.

Information Integrity

Information integrity refers to the state of information such that it is complete, correct, and unchanged from the last time in which it was verified to be in an "integral" state. The value of information integrity to a site will vary. For example, it is more important for military and government installations to prevent the "disclosure" of classified information, whether it is right or wrong. A bank, on the other hand, is far more concerned with whether the account information maintained for its customers is complete and accurate.

Numerous computer system mechanisms, as well as procedural controls, have an influence on the integrity of system information. Traditional access control mechanisms maintain controls over who can access system information. These mechanisms alone are not sufficient in some cases to provide the degree of integrity required. Some other mechanisms are briefly discussed below.

It should be noted that there are other aspects to maintaining system integrity besides these mechanisms, such as two-person controls, and integrity validation procedures. These are beyond the scope of this document.


Easily the simplest mechanism, a simple checksum routine can compute a value for a system file and compare it with the last known value. If the two are equal, the file is probably unchanged. If not, the file has been changed by some unknown means.

Though it is the easiest to implement, the checksum scheme suffers from a serious failing in that it is not very sophisticated and a determined attacker could easily add enough characters to the file to eventually obtain the correct value.

A specific type of checksum, called a CRC checksum, is considerably more robust than a simple checksum. It is only slightly more difficult to implement and provides a better degree of catching errors. It too, however, suffers from the possibility of compromise by an attacker.

Checksums may be used to detect the altering of information.

However, they do not actively guard against changes being made. For this, other mechanisms such as access controls and encryption should be used.

Cryptographic Checksums

Cryptographic checksums (also called cryptosealing) involve breaking a file up into smaller chunks, calculating a (CRC) checksum for each chunk, and adding the CRCs together. Depending upon the exact algorithm used, this can result in a nearly unbreakable method of determining whether a file has been changed. This mechanism suffers from the fact that it is sometimes computationally intensive and may be prohibitive except in cases where the utmost integrity protection is desired.

Another related mechanism, called a one-way hash function (or a Manipulation Detection Code (MDC)) can also be used to uniquely identify a file. The idea behind these functions is that no two inputs can produce the same output, thus a modified file will not have the same hash value. One-way hash functions can be implemented efficiently on a wide variety of systems, making unbreakable integrity checks possible. (Snefru, a one-way hash function available via USENET as well as the Internet is just one example of an efficient one-way hash function.) [10]

Limiting Network Access

The dominant network protocols in use on the Internet, IP (RFC 791) [11], TCP (RFC 793) [12], and UDP (RFC 768) [13], carry certain control information which can be used to restrict access to certain hosts or networks within an organization.

The IP packet header contains the network addresses of both the sender and recipient of the packet. Further, the TCP and UDP protocols provide the notion of a "port", which identifies the endpoint (usually a network server) of a communications path. In some instances, it may be desirable to deny access to a specific TCP or UDP port, or even to certain hosts and networks altogether.

Gateway Routing Tables

One of the simplest approaches to preventing unwanted network connections is to simply remove certain networks from a gateway's routing tables. This makes it "impossible" for a host to send packets to these networks. (Most protocols require bidirectional packet flow even for unidirectional data flow, thus breaking one side of the route is usually sufficient.)

This approach is commonly taken in "firewall" systems by preventing the firewall from advertising local routes to the outside world. The approach is deficient in that it often prevents "too much" (e.g., in order to prevent access to one system on the network, access to all systems on the network is disabled).

Router Packet Filtering

Many commercially available gateway systems (more correctly called routers) provide the ability to filter packets based not only on sources or destinations, but also on source-destination combinations. This mechanism can be used to deny access to a specific host, network, or subnet from any other host, network, or subnet.

Gateway systems from some vendors (e.g., cisco Systems) support an even more complex scheme, allowing finer control over source and destination addresses. Via the use of address masks, one can deny access to all but one host on a particular network. The cisco Systems also allow packet screening based on IP protocol type and TCP or UDP port numbers [14].

This can also be circumvented by "source routing" packets destined for the "secret" network. Source routed packets may be filtered out by gateways, but this may restrict other legitimate activities, such as diagnosing routing problems.

Authentication Systems

Authentication refers to the process of proving a claimed identity to the satisfaction of some permission-granting authority. Authentication systems are hardware, software, or procedural mechanisms that enable a user to obtain access to computing resources. At the simplest level, the system administrator who adds new user accounts to the system is part of the system authentication mechanism. At the other end of the spectrum, fingerprint readers or retinal scanners provide a very high-tech solution to establishing a potential user's identity. Without establishing and proving a user's identity prior to establishing a session, your site's computers are vulnerable to any sort of attack.

Typically, a user authenticates himself or herself to the system by entering a password in response to a prompt. Challenge/Response mechanisms improve upon passwords by prompting the user for some piece of information shared by both the computer and the user (such as mother's maiden name, etc.).


Kerberos, named after the dog who in mythology is said to stand at the gates of Hades, is a collection of software used in a large network to establish a user's claimed identity. Developed at the Massachusetts Institute of Technology (MIT), it uses a combination of encryption and distributed databases so that a user at a campus facility can login and start a session from any computer located on the campus. This has clear advantages in certain environments where there are a large number of potential users who may establish a connection from any one of a large number of workstations. Some vendors are now incorporating Kerberos into their systems.

It should be noted that while Kerberos makes several advances in the area of authentication, some security weaknesses in the protocol still remain [15].

Smart Cards

Several systems use "smart cards" (a small calculator-like device) to help authenticate users. These systems depend on the user having an object in their possession. One such system involves a new password procedure that require a user to enter a value obtained from a "smart card" when asked for a password by the computer. Typically, the host machine will give the user some piece of information that is entered into the keyboard of the smart card. The smart card will display a response which must then be entered into the computer before the session will be established. Another such system involves a smart card which displays a number which changes over time, but which is synchronized with the authentication software on the computer.

This is a better way of dealing with authentication than with the traditional password approach. On the other hand, some say it's inconvenient to carry the smart card. Start-up costs are likely to be high as well.

Books, Lists, and Informational Sources

There are many good sources for information regarding computer security. The annotated bibliography at the end of this document can provide you with a good start. In addition, information can be obtained from a variety of other sources, some of which are described in this section.

Security Mailing Lists

The UNIX Security mailing list exists to notify system administrators of security problems before they become common knowledge, and to provide security enhancement information. It is a restricted-access list, open only to people who can be verified as being principal systems people at a site. Requests to join the list must be sent by either the site contact listed in the Defense Data Network's Network Information Center's (DDN NIC) WHOIS database, or from the "root" account on one of the major site machines. You must include the destination address you want on the list, an indication of whether you want to be on the mail reflector list or receive weekly digests, the electronic mail address and voice telephone number of the site contact if it isn't you, and the name, address, and telephone number of your organization. This information should be sent to SECURITY-REQUEST@CPD.COM.

The RISKS digest is a component of the ACM Committee on Computers and Public Policy, moderated by Peter G. Neumann. It is a discussion forum on risks to the public in computers and related systems, and along with discussing computer security and privacy issues, has discussed such subjects as the Stark incident, the shooting down of the Iranian airliner in the Persian Gulf (as it relates to the computerized weapons systems), problems in air and railroad traffic control systems, software engineering, and so on. To join the mailing list, send a message to RISKS-REQUEST@CSL.SRI.COM. This list is also available in the USENET newsgroup "comp.risks".

The VIRUS-L list is a forum for the discussion of computer virus experiences, protection software, and related topics. The list is open to the public, and is implemented as a moderated digest. Most of the information is related to personal computers, although some of it may be applicable to larger systems. To subscribe, send the line:

SUB VIRUS-L your full name to the address ISTSERV%LEHIIBM1.BITNET@MITVMA.MIT.EDU. This list is also available via the USENET newsgroup "comp.virus".

The Computer Underground Digest "is an open forum dedicated to sharing information among computerists and to the presentation and debate of diverse views." While not directly a security list, it does contain discussions about privacy and other security related topics. The list can be read on USENET as, or to join the mailing list, send mail to Gordon Myer (

Submissions may be mailed to:

Networking Mailing Lists

The TCP-IP mailing list is intended to act as a discussion forum for developers and maintainers of implementations of the TCP/IP protocol suite. It also discusses network-related security problems when they involve programs providing network services, such as "Sendmail". To join the TCP-IP list, send a message to TCP-IP-REQUEST@NISC.SRI.COM. This list is also available in the USENET newsgroup "comp.protocols.tcp-ip".

SUN-NETS is a discussion list for items pertaining to networking on Sun systems. Much of the discussion is related to NFS, NIS (formally Yellow Pages), and name servers. To subscribe, send a message to SUN-NETS-REQUEST@UMIACS.UMD.EDU.

The USENET groups and also discuss security issues. is a moderated group and also includes discussions of physical security and locks. is unmoderated.

Response Teams

Several organizations have formed special groups of people to deal with computer security problems. These teams collect information about possible security holes and disseminate it to the proper people, track intruders, and assist in recovery from security violations. The teams typically have both electronic mail distribution lists as well as a special telephone number which can be called for information or to report a problem. Many of these teams are members of the CERT System, which is coordinated by the National Institute of Standards and Technology (NIST), and exists to facilitate the exchange of information between the various teams.
DARPA Computer Emergency Response Team

The Computer Emergency Response Team/Coordination Center (CERT/CC) was established in December 1988 by the Defense Advanced Research Projects Agency (DARPA) to address computer security concerns of research users of the Internet. It is operated by the Software Engineering Institute (SEI) at Carnegie-Mellon University (CMU). The CERT can immediately confer with experts to diagnose and solve security problems, and also establish and maintain communications with the affected computer users and government authorities as appropriate.

The CERT/CC serves as a clearing house for the identification and repair of security vulnerabilities, informal assessments of existing systems, improvement of emergency response capability, and both vendor and user security awareness. In addition, the team works with vendors of various systems in order to coordinate the fixes for security problems.

The CERT/CC sends out security advisories to the CERT-ADVISORY mailing list whenever appropriate. They also operate a 24-hour hotline that can be called to report security problems (e.g., someone breaking into your system), as well as to obtain current (and accurate) information about rumored security problems.

To join the CERT-ADVISORY mailing list, send a message to CERT@CERT.SEI.CMU.EDU and ask to be added to the mailing list. The material sent to this list also appears in the USENET newsgroup "". Past advisories are available for anonymous FTP from the host CERT.SEI.CMU.EDU. The 24-hour hotline number is (412) 268-7090.

The CERT/CC also maintains a CERT-TOOLS list to encourage the exchange of information on tools and techniques that increase the secure operation of Internet systems. The CERT/CC does not review or endorse the tools described on the list. To subscribe, send a message to CERT-TOOLS-REQUEST@CERT.SEI.CMU.EDU and ask to be added to the mailing list.

The CERT/CC maintains other generally useful security information for anonymous FTP from CERT.SEI.CMU.EDU. Get the README file for a list of what is available.

For more information, contact:
CERT Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890

(412) 268-7090

DDN Security Coordination Center

For DDN users, the Security Coordination Center (SCC) serves a function similar to CERT. The SCC is the DDN's clearing-house for host/user security problems and fixes, and works with the DDN Network Security Officer. The SCC also distributes the DDN Security Bulletin, which communicates information on network and host security exposures, fixes, and concerns to security and management personnel at DDN facilities. It is available online, via kermit or anonymous FTP, from the host NIC.DDN.MIL, in SCC:DDN-SECURITY-yy-nn.TXT (where "yy" is the year and "nn" is the bulletin number). The SCC provides immediate assistance with DDN-related host security problems; call (800) 235-3155 (6:00 a.m. to 5:00 p.m. Pacific Time) or send email to SCC@NIC.DDN.MIL. For 24 hour coverage, call the MILNET Trouble Desk (800) 451-7413 or AUTOVON 231-1713. NIST Computer Security Resource and Response Center

The National Institute of Standards and Technology (NIST) has responsibility within the U.S. Federal Government for computer science and technology activities. NIST has played a strong role in organizing the CERT System and is now serving as the CERT System Secretariat. NIST also operates a Computer Security Resource and Response Center (CSRC) to provide help and information regarding computer security events and incidents, as well as to raise awareness about computer security vulnerabilities.

The CSRC team operates a 24-hour hotline, at (301) 975-5200. For individuals with access to the Internet, on-line publications and computer security information can be obtained via anonymous FTP from the host CSRC.NCSL.NIST.GOV ( NIST also operates a personal computer bulletin board that contains information regarding computer viruses as well as other aspects of computer security. To access this board, set your modem to 300/1200/2400 BPS, 1 stop bit, no parity, and 8-bit characters, and call (301) 948-5717. All users are given full access to the board immediately upon registering.

NIST has produced several special publications related to computer security and computer viruses in particular; some of these publications are downloadable. For further information, contact NIST at the following address:
Computer Security Resource and Response Center
A-216 Technology
Gaithersburg, MD 20899
Telephone: (301) 975-3359
Electronic Mail:

DOE Computer Incident Advisory Capability (CIAC)

CIAC is the Department of Energy's (DOE's) Computer Incident Advisory Capability. CIAC is a four-person team of computer scientists from Lawrence Livermore National Laboratory (LLNL) charged with the primary responsibility of assisting DOE sites faced with computer security incidents (e.g., intruder attacks, virus infections, worm attacks, etc.). This capability is available to DOE sites on a 24-hour-a-day basis.

CIAC was formed to provide a centralized response capability (including technical assistance), to keep sites informed of current events, to deal proactively with computer security issues, and to maintain liaisons with other response teams and agencies. CIAC's charter is to assist sites (through direct technical assistance, providing information, or referring inquiries to other technical experts), serve as a clearinghouse for information about threats/known incidents/vulnerabilities, develop guidelines for incident handling, develop software for responding to events/incidents, analyze events and trends, conduct training and awareness activities, and alert and advise sites about vulnerabilities and potential attacks.

CIAC's business hours phone number is (415) 422-8193 or FTS 532-8193. CIAC's e-mail address is CIAC@TIGER.LLNL.GOV.

NASA Ames Computer Network Security Response Team

The Computer Network Security Response Team (CNSRT) is NASA Ames Research Center's local version of the DARPA CERT. Formed in August of 1989, the team has a constituency that is primarily Ames users, but it is also involved in assisting other NASA Centers and federal agencies. CNSRT maintains liaisons with the DOE's CIAC team and the DARPA CERT. It is also a charter member of the CERT System. The team may be reached by 24 hour pager at (415) 694-0571, or by electronic mail to CNSRT@AMES.ARC.NASA.GOV.

DDN Management Bulletins

The DDN Management Bulletin is distributed electronically by the DDN NIC under contract to the Defense Communications Agency (DCA). It is a means of communicating official policy, procedures, and other information of concern to management personnel at DDN facilities.

The DDN Security Bulletin is distributed electronically by the DDN SCC, also under contract to DCA, as a means of communicating information on network and host security exposures, fixes, and concerns to security and management personnel at DDN facilities.

Anyone may join the mailing lists for these two bulletins by sending a message to NIC@NIC.DDN.MIL and asking to be placed on the mailing lists. These messages are also posted to the USENET newsgroup "ddn.mgt-bulletin". For additional information, see section 8.7.

System Administration List

The SYSADM-LIST is a list pertaining exclusively to UNIX system administration. Mail requests to be added to the list to SYSADM-LIST-REQUEST@SYSADMIN.COM.

Vendor Specific System Lists

The SUN-SPOTS and SUN-MANAGERS lists are discussion groups for users and administrators of systems supplied by Sun Microsystems. SUN-SPOTS is a fairly general list, discussing everything from hardware configurations to simple UNIX questions. To subscribe, send a message to SUN-SPOTS-REQUEST@RICE.EDU. This list is also available in the USENET newsgroup "comp.sys.sun". SUN-MANAGERS is a discussion list for Sun system administrators and covers all aspects of Sun system administration. To subscribe, send a message to SUN-MANAGERS-REQUEST@EECS.NWU.EDU.

The APOLLO list discusses the HP/Apollo system and its

software. To subscribe, send a message to APOLLO-

REQUEST@UMIX.CC.UMICH.EDU. APOLLO-L is a similar list which

can be subscribed to by sending


HPMINI-L pertains to the Hewlett-Packard 9000 series and HP/UX

operating system. To subscribe, send


INFO-IBMPC discusses IBM PCs and compatibles, as well as MS-DOS. To subscribe, send a note to INFO-IBMPC-REQUEST@WSMR-SIMTEL20.ARMY.MIL.

There are numerous other mailing lists for nearly every popular computer or workstation in use today. For a complete list, obtain the file "netinfo/interest-groups" via anonymous FTP from the host FTP.NISC.SRI.COM.

Professional Societies and Journals

The IEEE Technical Committee on Security & Privacy publishes a quarterly magazine, "CIPHER".

IEEE Computer Society,

1730 Massachusetts Ave. N.W.

Washington, DC 2036-1903

The ACM SigSAC (Special Interest Group on Security, Audit, and Controls) publishes a quarterly magazine, "SIGSAC Review".
Association for Computing Machinery
11 West 42nd St.
New York, N.Y. 10036

The Information Systems Security Association publishes a quarterly magazine called "ISSA Access".

Information Systems Security Association

P.O. Box 9457

Newport Beach, CA 92658

"Computers and Security" is an "international journal for the professional involved with computer security, audit and control, and data integrity."

$266/year, 8 issues (1990)

Elsevier Advanced Technology

Journal Information Center

655 Avenue of the Americas

New York, NY 10010

The "Data Security Letter" is published "to help data security professionals by providing inside information and knowledgable analysis of developments in computer and communications security."

$690/year, 9 issues (1990)

Data Security Letter

P.O. Box 1593

Palo Alto, CA 94302

Problem Reporting Tools


Auditing is an important tool that can be used to enhance the security of your installation. Not only does it give you a means of identifying who has accessed your system (and may have done something to it) but it also gives you an indication of how your system is being used (or abused) by authorized users and attackers alike. In addition, the audit trail traditionally kept by computer systems can become an invaluable piece of evidence should your system be penetrated.

Verify Security

An audit trail shows how the system is being used from day to day. Depending upon how your site audit log is configured, your log files should show a range of access attempts that can show what normal system usage should look like. Deviation from that normal usage could be the result of penetration from an outside source using an old or stale user account. Observing a deviation in logins, for example, could be your first indication that something unusual is happening.

Verify Software Configurations

One of the ruses used by attackers to gain access to a system is by the insertion of a so-called Trojan Horse program. A Trojan Horse program can be a program that does something useful, or merely something interesting. It always does something unexpected, like steal passwords or copy files without your knowledge [25]. Imagine a Trojan login program that prompts for username and password in the usual way, but also writes that information to a special file that the attacker can come back and read at will. Imagine a Trojan Editor program that, despite the file permissions you have given your files, makes copies of everything in your directory space without you knowing about it.

This points out the need for configuration management of the software that runs on a system, not as it is being developed, but as it is in actual operation. Techniques for doing this range from checking each command every time it is executed against some criterion (such as a cryptoseal, described above) or merely checking the date and time stamp of the executable. Another technique might be to check each command in batch mode at midnight.


COPS is a security tool for system administrators that checks for numerous common security problems on UNIX systems [27]. COPS is a collection of shell scripts and C programs that can easily be run on almost any UNIX variant. Among other things, it checks the following items and sends the results to the system administrator:
· Checks "/dev/kmem" and other devices for world read/writability.
· Checks special or important files and directories for "bad" modes (world writable, etc.).
· Checks for easily-guessed passwords.
· Checks for duplicate user ids, invalid fields in the password file, etc..
· Checks for duplicate group ids, invalid fields in the group file, etc..
· Checks all users' home directories and their ".cshrc", ".login", ".profile", and ".rhosts" files for security problems.

Checks all commands in the "/etc/rc" files and "cron" files for world writability.
· Checks for bad "root" paths, NFS file systems exported to the world, etc..
· Includes an expert system that checks to see if a given user (usually "root") can be compromised, given that certain rules are true.
· Checks for changes in the setuid status of programs on the system.

The COPS package is available from the "comp.sources.unix" archive on "", and also from the UNIX-SW repository on the MILNET host "".

Communication Among Administrators

Secure Operating Systems

The following list of products and vendors is adapted from the National Computer Security Center's (NCSC) Evaluated Products List. They represent those companies who have either received an evaluation from the NCSC or are in the process of a product evaluation. This list is not complete, but it is representative of those operating systems and add on components available in the commercial marketplace.

For a more detailed listing of the current products appearing in the NCSC EPL, contact the NCSC at:
National Computer Security Center
9800 Savage Road
Fort George G. Meade, MD 20755-6000
(301) 859-4458

Version Evaluation

Evaluated Product Vendor Evaluated Class


Secure Communications Honeywell Information 2.1 A1

Processor (SCOMP) Systems, Inc.

Multics Honeywell Information MR11.0 B2

Systems, Inc.

System V/MLS 1.1.2 on UNIX AT&T 1.1.2 B1

System V 3.1.1 on AT&T 3B2/500and 3B2/600

OS 1100 Unisys Corp. Security B1

Release 1

MPE V/E Hewlett-Packard Computer G.03.04 C2

Systems Division

AOS/VS on MV/ECLIPSE series Data General Corp. 7.60 C2

VM/SP or VM/SP HPO with CMS, IBM Corp. 5 C2

MVS/XA with RACF IBM Corp. 2.2,2.3 C2

AX/VMS Digital Equipment Corp. 4.3 C2

NOS Control Data Corp. NOS

Security C2

Eval Product

TOP SECRET CGA Software Products 3.0/163 C2

Group, Inc.

Access Control Facility 2 SKK, Inc. 3.1.3 C2

UTX/32S Gould, Inc. Computer 1.0 C2

Systems Division

A Series MCP/AS with Unisys Corp. 3.7 C2
InfoGuard Security

Primos Prime Computer, Inc. 21.0.1DODC2A C2

Resource Access Control IBM Corp. 1.5 C1

Facility (RACF)

Version Candidate

Candidate Product Vendor Evaluated Class


Boeing MLS LAN Boeing Aerospace A1 M1

Trusted XENIX Trusted Information

Systems, Inc. B2


System V/MLS AT&T B1

VM/SP with RACF IBM Corp. 5/1.8.2 C2

Wang SVS/OS with CAP Wang Laboratories, Inc. 1.0 C2

Obtaining Fixes for Known Problems

It goes without saying that computer systems have bugs. Even operating systems, upon which we depend for protection of our data, have bugs. And since there are bugs, things can be broken, both maliciously and accidentally. It is important that whenever bugs are discovered, a should fix be identified and implemented as soon as possible. This should minimize any exposure caused by the bug in the first place.

A corollary to the bug problem is: from whom do I obtain the fixes? Most systems have some support from the manufacturer or supplier. Fixes coming from that source tend to be implemented quickly after receipt. Fixes for some problems are often posted on the network and are left to the system administrators to incorporate as they can. The problem is that one wants to have faith that the fix will close the hole and not introduce any others. We will tend to trust that the manufacturer's fixes are better than those that are posted on the net.

Sun Customer Warning System

Sun Microsystems has established a Customer Warning System (CWS) for handling security incidents. This is a formal process which includes:
· Having a well advertised point of contact in Sun for reporting security problems.
· Pro-actively alerting customers of worms, viruses, or other security holes that could affect their systems.
· Distributing the patch (or work-around) as quickly as possible.

They have created an electronic mail address, SECURITY-ALERT@SUN.COM, which will enable customers to report security problems. A voice-mail backup is available at (415) 688-9081. A "Security Contact" can be designated by each customer site; this person will be contacted by Sun in case of any new security problems. For more information, contact your Sun representative.

Trusted Archive Servers

Several sites on the Internet maintain large repositories of public-domain and freely distributable software, and make this material available for anonymous FTP. This section describes some of the larger repositories. Note that none of these servers implements secure checksums or anything else guaranteeing the integrity of their data. Thus, the notion of "trust" should be taken as a somewhat limited definition.

Sun Fixes on UUNET

Sun Microsystems has contracted with UUNET Communications Services, Inc., to make fixes for bugs in Sun software available via anonymous FTP. You can access these fixes by using the "ftp" command to connect to the host FTP.UU.NET. Then change into the directory "sun-dist/security", and obtain a directory listing. The file "README" contains a brief description of what each file in this directory contains, and what is required to install the fix.

Berkeley Fixes

The University of California at Berkeley also makes fixes available via anonymous FTP; these fixes pertain primarily to the current release of BSD UNIX (currently, release 4.3). However, even if you are not running their software, these fixes are still important, since many vendors (Sun, DEC, Sequent, etc.) base their software on the Berkeley releases.

The Berkeley fixes are available for anonymous FTP from the host UCBARPA.BERKELEY.EDU in the directory "4.3/ucb-fixes". The file "INDEX" in this directory describes what each file contains. They are also available from UUNET (see section

Berkeley also distributes new versions of "sendmail" and "named" from this machine. New versions of these commands are stored in the "4.3" directory, usually in the files "sendmail.tar.Z" and "bind.tar.Z", respectively.

Simtel-20 and UUNET

The two largest general-purpose software repositories on the Internet are the hosts WSMR-SIMTEL20.ARMY.MIL and FTP.UU.NET.

WSMR-SIMTEL20.ARMY.MIL is a TOPS-20 machine operated by the U.S. Army at White Sands Missile Range (WSMR), New Mexico. The directory "pd2:<unix-c>" contains a large amount of UNIX software, primarily taken from the "comp.sources" newsgroups. The directories "pd1:<msdos>" and "pd2:<msdos2>" contains software for IBM PC systems, and "pd3:<macintosh>" contains software for the Apple Macintosh.

FTP.UU.NET is operated by UUNET Communications Services, Inc. in Falls Church, Virginia. This company sells Internet and USENET access to sites all over the country (and internationally). The software posted to the following USENET source newsgroups is stored here, in directories of the same name:

Numerous other distributions, such as all the freely distributable Berkeley UNIX source code, Internet Request for Comments (RFCs), and so on are also stored on this system.


Many vendors make fixes for bugs in their software available electronically, either via mailing lists or via anonymous FTP. You should contact your vendor to find out if they offer this service, and if so, how to access it. Some vendors that offer these services include Sun Microsystems (see above), Digital Equipment Corporation (DEC), the University of California at Berkeley (see above), and Apple Computer [5, CURRY].

Types of Security Procedures

System Security Audits

Most businesses undergo some sort of annual financial auditing as a regular part of their business life. Security audits are an important part of running any computing environment. Part of the security audit should be a review of any policies that concern system security, as well as the mechanisms that are put in place to enforce them.

Organize Scheduled Drills

Although not something that would be done each day or week, scheduled drills may be conducted to determine if the procedures defined are adequate for the threat to be countered. If your major threat is one of natural disaster, then a drill would be conducted to verify your backup and recovery mechanisms. On the other hand, if your greatest threat is from external intruders attempting to penetrate your system, a drill might be conducted to actually try a penetration to observe the effect of the policies.

Drills are a valuable way to test that your policies and procedures are effective. On the other hand, drills can be time-consuming and disruptive to normal operations. It is important to weigh the benefits of the drills against the possible time loss which may be associated with them.

Test Procedures

If the choice is made to not to use scheduled drills to examine your entire security procedure at one time, it is important to test individual procedures frequently. Examine your backup procedure to make sure you can recover data from the tapes. Check log files to be sure that information which is supposed to be logged to them is being logged to them, etc..

When a security audit is mandated, great care should be used in devising tests of the security policy. It is important to clearly identify what is being tested, how the test will be conducted, and results expected from the test. This should all be documented and included in or as an adjunct to the security policy document itself.

It is important to test all aspects of the security policy, both procedural and automated, with a particular emphasis on the automated mechanisms used to enforce the policy. Tests should be defined to ensure a comprehensive examination of policy features, that is, if a test is defined to examine the user logon process, it should be explicitly stated that both valid and invalid user names and passwords will be used to demonstrate proper operation of the logon program.

Keep in mind that there is a limit to the reasonableness of tests. The purpose of testing is to ensure confidence that the security policy is being correctly enforced, and not to "prove" the absoluteness of the system or policy. The goal should be to obtain some assurance that the reasonable and credible controls imposed by your security policy are adequate.

Account Management Procedures

Procedures to manage accounts are important in preventing unauthorized access to your system. It is necessary to decide several things: Who may have an account on the system? How long may someone have an account without renewing his or her request? How do old accounts get removed from the system? The answers to all these questions should be explicitly set out in the policy.

In addition to deciding who may use a system, it may be important to determine what each user may use the system for (is personal use allowed, for example). If you are connected to an outside network, your site or the network management may have rules about what the network may be used for. Therefore, it is important for any security policy to define an adequate account management procedure for both administrators and users. Typically, the system administrator would be responsible for creating and deleting user accounts and generally maintaining overall control of system use. To some degree, account management is also the responsibility of each system user in the sense that the user should observe any system messages and events that may be indicative of a policy violation. For example, a message at logon that indicates the date and time of the last logon should be reported by the user if it indicates an unreasonable time of last logon.

Password Management Procedures

A policy on password management may be important if your site wishes to enforce secure passwords. These procedures may range from asking or forcing users to change their passwords occasionally to actively attempting to break users' passwords and then informing the user of how easy it was to do. Another part of password management policy covers who may distribute passwords - can users give their passwords to other users?

Section 2.3 discusses some of the policy issues that need to be decided for proper password management. Regardless of the policies, password management procedures need to be carefully setup to avoid disclosing passwords. The choice of initial passwords for accounts is critical. In some cases, users may never login to activate an account; thus, the choice of the initial password should not be easily guessed. Default passwords should never be assigned to accounts: always create new passwords for each user. If there are any printed lists of passwords, these should be kept off-line in secure locations; better yet, don't list passwords.

Password Selection

Perhaps the most vulnerable part of any computer system is the account password. Any computer system, no matter how secure it is from network or dial-up attack, Trojan horse programs, and so on, can be fully exploited by an intruder if he or she can gain access via a poorly chosen password. It is important to define a good set of rules for password selection, and distribute these rules to all users. If possible, the software which sets user passwords should be modified to enforce as many of the rules as possible.

A sample set of guidelines for password selection is shown below:
· DON'T use your login name in any form (as-is, reversed, capitalized, doubled, etc.).
· DON'T use your first, middle, or last name in any form.
· DON'T use your spouse's or child's name.
· DON'T use other information easily obtained about you.
This includes license plate numbers, telephone numbers, social security numbers, the make of your automobile, the name of the street you live on, etc..
· DON'T use a password of all digits, or all the same letter.
· DON'T use a word contained in English or foreign language dictionaries, spelling lists, or other lists of words.
· DON'T use a password shorter than six characters.
· DO use a password with mixed-case alphabetics.
· DO use a password with non-alphabetic characters (digits or punctuation).
· DO use a password that is easy to remember, so you don't have to write it down.
· DO use a password that you can type quickly, without having to look at the keyboard.

Methods of selecting a password which adheres to these guidelines include:
· Choose a line or two from a song or poem, and use the first letter of each word.
· Alternate between one consonant and one or two vowels, up to seven or eight characters. This provides nonsense words which are usually pronounceable, and thus easily remembered.
· Choose two short words and concatenate them together with a punctuation character between them.

Users should also be told to change their password periodically, usually every three to six months. This makes sure that an intruder who has guessed a password will eventually lose access, as well as invalidating any list of passwords he/she may have obtained. Many systems enable the system administrator to force users to change their passwords after an expiration period; this software should be enabled if your system supports it [5, CURRY].

Some systems provide software which forces users to change their passwords on a regular basis. Many of these systems also include password generators which provide the user with a set of passwords to choose from. The user is not permitted to make up his or her own password. There are arguments both for and against systems such as these. On the one hand, by using generated passwords, users are prevented from selecting insecure passwords. On the other hand, unless the generator is good at making up easy to remember passwords, users will begin writing them down in order to remember them.

Procedures for Changing Passwords

How password changes are handled is important to keeping passwords secure. Ideally, users should be able to change their own passwords on-line. (Note that password changing programs are a favorite target of intruders. See section 4.4 on configuration management for further information.)

However, there are exception cases which must be handled carefully. Users may forget passwords and not be able to get onto the system. The standard procedure is to assign the user a new password. Care should be taken to make sure that the real person is requesting the change and gets the new password. One common trick used by intruders is to call or message to a system administrator and request a new password. Some external form of verification should be used before the password is assigned. At some sites, users are required to show up in person with ID.

There may also be times when many passwords need to be changed. If a system is compromised by an intruder, the intruder may be able to steal a password file and take it off the system. Under these circumstances, one course of action is to change all passwords on the system. Your site should have procedures for how this can be done quickly and efficiently. What course you choose may depend on the urgency of the problem. In the case of a known attack with damage, you may choose to forcibly disable all accounts and assign users new passwords before they come back onto the system. In some places, users are sent a message telling them that they should change their passwords, perhaps within a certain time period. If the password isn't changed before the time period expires, the account is locked.

Users should be aware of what the standard procedure is for passwords when a security event has occurred. One well-known spoof reported by the Computer Emergency Response Team (CERT) involved messages sent to users, supposedly from local system administrators, requesting them to immediately change their password to a new value provided in the message [24]. These messages were not from the administrators, but from intruders trying to steal accounts. Users should be warned to immediately report any suspicious requests such as this to site administrators.

Configuration Management Procedures

Configuration management is generally applied to the software development process. However, it is certainly applicable in a operational sense as well. Consider that the since many of the system level programs are intended to enforce the security policy, it is important that these be "known" as correct. That is, one should not allow system level programs (such as the operating system, etc.) to be changed arbitrarily. At very least, the procedures should state who is authorized to make changes to systems, under what circumstances, and how the changes should be documented.

In some environments, configuration management is also desirable as applied to physical configuration of equipment. Maintaining valid and authorized hardware configuration should be given due consideration in your security policy.

Non-Standard Configurations

Occasionally, it may be beneficial to have a slightly non-standard configuration in order to thwart the "standard" attacks used by some intruders. The non-standard parts of the configuration might include different password encryption algorithms, different configuration file locations, and rewritten or functionally limited system commands.

Non-standard configurations, however, also have their drawbacks. By changing the "standard" system, these modifications make software maintenance more difficult by requiring extra documentation to be written, software modification after operating system upgrades, and, usually, someone with special knowledge of the changes.

Because of the drawbacks of non-standard configurations, they are often only used in environments with a "firewall" machine (see section 3.9.1). The firewall machine is modified in non-standard ways since it is susceptible to attack, while internal systems behind the firewall are left in their standard configurations.

Incident Handling


This section of the document will supply some guidance to be applied when a computer security event is in progress on a machine, network, site, or multi-site environment. The operative philosophy in the event of a breach of computer security, whether it be an external intruder attack or a disgruntled employee, is to plan for adverse events in advance. There is no substitute for creating contingency plans for the types of events described above.

Traditional computer security, while quite important in the overall site security plan, usually falls heavily on protecting systems from attack, and perhaps monitoring systems to detect attacks. Little attention is usually paid for how to actually handle the attack when it occurs. The result is that when an attack is in progress, many decisions are made in haste and can be damaging to tracking down the source of the incident, collecting evidence to be used in prosecution efforts, preparing for the recovery of the system, and protecting the valuable data contained on the system.

Have a Plan to Follow in Case of an Incident

Part of handling an incident is being prepared to respond before the incident occurs. This includes establishing a suitable level of protections, so that if the incident becomes severe, the damage which can occur is limited. Protection includes preparing incident handling guidelines or a contingency response plan for your organization or site. Having written plans eliminates much of the ambiguity which occurs during an incident, and will lead to a more appropriate and thorough set of responses. Second, part of protection is preparing a method of notification, so you will know who to call and the relevant phone numbers. It is important, for example, to conduct "dry runs," in which your computer security personnel, system administrators, and managers simulate handling an incident.

Learning to respond efficiently to an incident is important for numerous reasons. The most important benefit is directly to human beings-preventing loss of human life. Some computing systems are life critical systems, systems on which human life depends (e.g., by controlling some aspect of life-support in a hospital or assisting air traffic controllers).

An important but often overlooked benefit is an economic one. Having both technical and managerial personnel respond to an incident requires considerable resources, resources which could be utilized more profitably if an incident did not require their services. If these personnel are trained to handle an incident efficiently, less of their time is required to deal with that incident.

A third benefit is protecting classified, sensitive, or proprietary information. One of the major dangers of a computer security incident is that information may be irrecoverable. Efficient incident handling minimizes this danger. When classified information is involved, other government regulations may apply and must be integrated into any plan for incident handling.

A fourth benefit is related to public relations. News about computer security incidents tends to be damaging to an organization's stature among current or potential clients. Efficient incident handling minimizes the potential for negative exposure.

A final benefit of efficient incident handling is related to legal issues. It is possible that in the near future organizations may be sued because one of their nodes was used to launch a network attack. In a similar vein, people who develop patches or workarounds may be sued if the patches or workarounds are ineffective, resulting in damage to systems, or if the patches or workarounds themselves damage systems. Knowing about operating system vulnerabilities and patterns of attacks and then taking appropriate measures is critical to circumventing possible legal problems.

5.1.2 Order of Discussion in this Session Suggests an Order for a Plan

This chapter is arranged such that a list may be generated from the Table of Contents to provide a starting point for creating a policy for handling ongoing incidents. The main points to be included in a policy for handling incidents are:

o Overview (what are the goals and objectives in handling the incident). o Evaluation (how serious is the incident). o Notification (who should be notified about the incident). o Response (what should the response to the incident be). o Legal/Investigative (what are the legal and prosecutorial implications of the incident). o Documentation Logs (what records should be kept from before, during, and after the incident).

Each of these points is important in an overall plan for handling incidents. The remainder of this chapter will detail the issues involved in each of these topics, and provide some guidance as to what should be included in a site policy for handling incidents.

5.1.3 Possible Goals and Incentives for Efficient Incident Handling

As in any set of pre-planned procedures, attention must be placed on a set of goals to be obtained in handling an incident. These goals will be placed in order of importance depending on the site, but one such set of goals might be:
Assure integrity of (life) critical systems.
Maintain and restore data.
Maintain and restore service.
Figure out how it happened.
Avoid escalation and further incidents.
Avoid negative publicity.
Find out who did it.
Punish the attackers.

It is important to prioritize actions to be taken during an incident well in advance of the time an incident occurs. Sometimes an incident may be so complex that it is impossible to do everything at once to respond to it; priorities are essential. Although priorities will vary from institution-to-institution, the following suggested priorities serve as a starting point for defining an organization's response:

o Priority one-protect human life and people's safety; human life always has precedence over all other considerations.

o Priority two-protect classified and/or sensitive data (as regulated by your site or by government regulations).

o Priority three-protect other data, including proprietary, scientific, managerial and other data, because loss of data is costly in terms of resources.

o Priority four-prevent damage to systems (e.g., loss or alteration of system files, damage to disk drives, etc.); damage to systems can result in costly down time and recovery.

o Priority five-minimize disruption of computing resources; it is better in many cases to shut a system down or disconnect from a network than to risk damage to data or systems.

An important implication for defining priorities is that once human life and national security considerations have been addressed, it is generally more important to save data than system software and hardware. Although it is undesirable to have any damage or loss during an incident, systems can be replaced; the loss or compromise of data (especially classified data), however, is usually not an acceptable outcome under any circumstances.

Part of handling an incident is being prepared to respond before the incident occurs. This includes establishing a suitable level of protections so that if the incident becomes severe, the damage which can occur is limited. Protection includes preparing incident handling guidelines or a contingency response plan for your organization or site. Written plans eliminate much of the ambiguity which occurs during an incident, and will lead to a more appropriate and thorough set of responses. Second, part of protection is preparing a method of notification so you will know who to call and how to contact them. For example, every member of the Department of Energy's CIAC Team carries a card with every other team member's work and home phone numbers, as well as pager numbers. Third, your organization or site should establish backup procedures for every machine and system. Having backups eliminates much of the threat of even a severe incident, since backups preclude serious data loss. Fourth, you should set up secure systems. This involves eliminating vulnerabilities, establishing an effective password policy, and other procedures, all of which will be explained later in this document. Finally, conducting training activities is part of protection. It is important, for example, to conduct "dry runs," in which your computer security personnel, system administrators, and managers simulate handling an incident.

Local Policies and Regulations Providing Guidance

Any plan for responding to security incidents should be guided by local policies and regulations. Government and private sites that deal with classified material have specific rules that they must follow.

The policies your site makes about how it responds to incidents (as discussed in sections 2.4 and 2.5) will shape your response. For example, it may make little sense to create mechanisms to monitor and trace intruders if your site does not plan to take action against the intruders if they are caught. Other organizations may have policies that affect your plans. Telephone companies often release information about telephone traces only to law enforcement agencies.

Section 5.5 also notes that if any legal action is planned, there are specific guidelines that must be followed to make sure that any information collected can be used as evidence.


Is It Real?

This stage involves determining the exact problem. Of course many, if not most, signs often associated with virus infections, system intrusions, etc., are simply anomalies such as hardware failures. To assist in identifying whether there really is an incident, it is usually helpful to obtain and use any detection software which may be available. For example, widely available software packages can greatly assist someone who thinks there may be a virus in a Macintosh computer. Audit information is also extremely useful, especially in determining whether there is a network attack. It is extremely important to obtain a system snapshot as soon as one suspects that something is wrong. Many incidents cause a dynamic chain of events to occur, and an initial system snapshot may do more good in identifying the problem and any source of attack than most other actions which can be taken at this stage. Finally, it is important to start a log book. Recording system events, telephone conversations, time stamps, etc., can lead to a more rapid and systematic identification of the problem, and is the basis for subsequent stages of incident handling.

There are certain indications or "symptoms" of an incident which deserve special attention:

o System crashes. o New user accounts (e.g., the account RUMPLESTILTSKIN has unexplainedly been created), or high activity on an account that has had virtually no activity for months. o New files (usually with novel or strange file names, such as data.xx or k). o Accounting discrepancies (e.g., in a UNIX system you might notice that the accounting file called /usr/admin/lastlog has shrunk, something that should make you very suspicious that there may be an intruder). o Changes in file lengths or dates (e.g., a user should be suspicious if he/she observes that the .EXE files in an MS DOS computer have unexplainedly grown by over 1800 bytes). o Attempts to write to system (e.g., a system manager notices that a privileged user in a VMS system is attempting to alter RIGHTSLIST.DAT). o Data modification or deletion (e.g., files start to disappear). o Denial of service (e.g., a system manager and all other users become locked out of a UNIX system, which has been changed to single user mode). o Unexplained, poor system performance (e.g., system response time becomes unusually slow). o Anomalies (e.g., "GOTCHA" is displayed on a display terminal or there are frequent unexplained "beeps"). o Suspicious probes (e.g., there are numerous unsuccessful login attempts from another node). o Suspicious browsing (e.g., someone becomes a root user on a UNIX system and accesses file after file in one user's account, then another's).

None of these indications is absolute "proof" that an incident is occurring, nor are all of these indications normally observed when an incident occurs. If you observe any of these indications, however, it is important to suspect that an incident might be occurring, and act accordingly. There is no formula for determining with 100 percent accuracy that an incident is occurring (possible exception: when a virus detection package indicates that your machine has the nVIR virus and you confirm this by examining contents of the nVIR resource in your Macintosh computer, you can be very certain that your machine is infected). It is best at this point to collaborate with other technical and computer security personnel to make a decision as a group about whether an incident is occurring.


Along with the identification of the incident is the evaluation of the scope and impact of the problem. It is important to correctly identify the boundaries of the incident in order to effectively deal with it. In addition, the impact of an incident will determine its priority in allocating resources to deal with the event. Without an indication of the scope and impact of the event, it is difficult to determine a correct response.

In order to identify the scope and impact, a set of criteria should be defined which is appropriate to the site and to the type of connections available. Some of the issues are:

o Is this a multi-site incident? o Are many computers at your site effected by this incident? o Is sensitive information involved? o What is the entry point of the incident (network, phone line, local terminal, etc.)? o Is the press involved? o What is the potential damage of the incident? o What is the estimated time to close out the incident?
o What resources could be required
to handle the incident?

Possible Types of Notification

When you have confirmed that an incident is occurring, the appropriate personnel must be notified. Who and how this notification is achieved is very important in keeping the event under control both from a technical and emotional standpoint.


First of all, any notification to either local or off-site personnel must be explicit. This requires that any statement (be it an electronic mail message, phone call, or fax) provides information about the incident that is clear, concise, and fully qualified. When you are notifying others that will help you to handle an event, a "smoke screen" will only divide the effort and create confusion. If a division of labor is suggested, it is helpful to provide information to each section about what is being accomplished in other efforts. This will not only reduce duplication of effort, but allow people working on parts of the problem to know where to obtain other information that would help them resolve a part of the incident.


Another important consideration when communicating about the incident is to be factual. Attempting to hide aspects of the incident by providing false or incomplete information may not only prevent a successful resolution to the incident, but may even worsen the situation. This is especially true when the press is involved. When an incident severe enough to gain press attention is ongoing, it is likely that any false information you provide will not be substantiated by other sources. This will reflect badly on the site and may create enough ill-will between the site and the press to damage the site's public relations.

Choice of Language

The choice of language used when notifying people about the incident can have a profound effect on the way that information is received. When you use emotional or inflammatory terms, you raise the expectations of damage and negative outcomes of the incident. It is important to remain calm both in written and spoken notifications.

Another issue associated with the choice of language is the notification to non-technical or off-site personnel. It is important to accurately describe the incident without undue alarm or confusing messages. While it is more difficult to describe the incident to a non-technical audience, it is often more important. A non-technical description may be required for upper-level management, the press, or law enforcement liaisons. The importance of these notifications cannot be underestimated and may make the difference between handling the incident properly and escalating to some higher level of damage.

Notification of Individuals

o Point of Contact (POC) people (Technical, Administrative, Response Teams, Investigative, Legal, Vendors, Service providers), and which POCs are visible to whom. o Wider community (users). o Other sites that might be affected.

Finally, there is the question of who should be notified during and after the incident. There are several classes of individuals that need to be considered for notification. These are the technical personnel, administration, appropriate response teams (such as CERT or CIAC), law enforcement, vendors, and other service providers. These issues are important for the central point of contact, since that is the person responsible for the actual notification of others (see section 5.3.6 for further information). A list of people in each of these categories is an important time saver for the POC during an incident. It is much more difficult to find an appropriate person during an incident when many urgent events are ongoing.

In addition to the people responsible for handling part of the incident, there may be other sites affected by the incident (or perhaps simply at risk from the incident). A wider community of users may also benefit from knowledge of the incident. Often, a report of the incident once it is closed out is appropriate for publication to the wider user community.

Public Relations - Press Releases

One of the most important issues to consider is when, who, and how much to release to the general public through the press. There are many issues to consider when deciding this particular issue. First and foremost, if a public relations office exists for the site, it is important to use this office as liaison to the press. The public relations office is trained in the type and wording of information released, and will help to assure that the image of the site is protected during and after the incident (if possible). A public relations office has the advantage that you can communicate candidly with them, and provide a buffer between the constant press attention and the need of the POC to maintain control over the incident.

If a public relations office is not available, the information released to the press must be carefully considered. If the information is sensitive, it may be advantageous to provide only minimal or overview information to the press. It is quite possible that any information provided to the press will be quickly reviewed by the perpetrator of the incident. As a contrast to this consideration, it was discussed above that misleading the press can often backfire and cause more damage than releasing sensitive information.

While it is difficult to determine in advance what level of detail to provide to the press, some guidelines to keep in mind are:

o Keep the technical level of detail low. Detailed information about the incident may provide enough information for copy-cat events or even damage the site's ability to prosecute once the event is over. o Keep the speculation out of press statements. Speculation of who is causing the incident or the motives are very likely to be in error and may cause an inflamed view of the incident. o Work with law enforcement professionals to assure that evidence is protected. If prosecution is involved, assure that the evidence collected is not divulged to the press. o Try not to be forced into a press interview before you are prepared. The popular press is famous for the "2am" interview, where the hope is to catch the interviewee off guard and obtain information otherwise not available. o Do not allow the press attention to detract from the handling of the event. Always remember that the successful closure of an incident is of primary importance.

Who Needs to Get Involved?

There now exists a number of incident response teams (IRTs) such as the CERT and the CIAC. (See sections and Teams exists for many major government agencies and large corporations. If such a team is available for your site, the notification of this team should be of primary importance during the early stages of an incident. These teams are responsible for coordinating computer security incidents over a range of sites and larger entities. Even if the incident is believed to be contained to a single site, it is possible that the information available through a response team could help in closing out the incident.

In setting up a site policy for incident handling, it may be desirable to create an incident handling team (IHT), much like those teams that already exist, that will be responsible for handling computer security incidents for the site (or organization). If such a team is created, it is essential that communication lines be opened between this team and other IHTs.

Once an incident is under way, it is difficult to open a trusted dialogue between other IHTs if none has existed before.


A major topic still untouched here is how to actually respond to an event. The response to an event will fall into the general categories of containment, eradication, recovery, and follow-up.


The purpose of containment is to limit the extent of an attack. For example, it is important to limit the spread of a worm attack on a network as quickly as possible. An essential part of containment is decision making (i.e., determining whether to shut a system down, to disconnect from a network, to monitor system or network activity, to set traps, to disable functions such as remote file transfer on a UNIX system, etc.). Sometimes this decision is trivial; shut the system down if the system is classified or sensitive, or if proprietary information is at risk! In other cases, it is worthwhile to risk having some damage to the system if keeping the system up might enable you to identify an intruder.

The third stage, containment, should involve carrying out predetermined procedures. Your organization or site should, for example, define acceptable risks in dealing with an incident, and should prescribe specific actions and strategies accordingly. Finally, notification of cognizant authorities should occur during this stage.


Once an incident has been detected, it is important to first think about containing the incident. Once the incident has been contained, it is now time to eradicate the cause. Software may be available to help you in this effort. For example, eradication software is available to eliminate most viruses which infect small systems. If any bogus files have been created, it is time to delete them at this point. In the case of virus infections, it is important to clean and reformat any disks containing infected files. Finally, ensure that all backups are clean. Many systems infected with viruses become periodically reinfected simply because people do not systematically eradicate the virus from backups.


Once the cause of an incident has been eradicated, the recovery is the next stage of action. The goal of recovery is to return the system to normal. In the case of a network-based attack, it is important to install patches for any operating system vulnerability which was exploited.


One of the most important stages of responding to incidents is also the most often omitted---the follow-up stage. This stage is important because it helps those involved in handling the incident develop a set of "lessons learned" (see section 6.3) to improve future performance in such situations. This stage also provides information which justifies an organization's computer security effort to management, and yields information which may be essential in legal proceedings.

The most important element of the follow-up stage is performing a postmortem analysis. Exactly what happened, and at what times? How well did the staff involved with the incident perform? What kind of information did the staff need quickly, and how could they have gotten that information as soon as possible? What would the staff do differently next time? A follow-up report is valuable because it provides a reference to be used in case of other similar incidents. Creating a formal chronology of events (including time stamps) is also important for legal reasons. Similarly, it is also important to as quickly obtain a monetary estimate of the amount of damage the incident caused in terms of any loss of software and files, hardware damage, and manpower costs to restore altered files, reconfigure affected systems, and so forth. This estimate may become the basis for subsequent prosecution activity by the FBI, the U.S. Attorney General's Office, etc..

What Will You Do?
o Restore control.
o Relation to policy.
o Which level of service is needed?
o Monitor activity.
o Constrain or shut down system.

Designating a "Single Point of Contact"

5.4.2 Consider Designating a "Single Point of Contact"

When an incident is under way, a major issue is deciding who is in charge of coordinating the activity of the multitude of players.

A major mistake that can be made is to have a number of "points of contact" (POC) that are not pulling their efforts together. This will only add to the confusion of the event, and will probably lead to additional confusion and wasted or ineffective effort.

The single point of contact may or may not be the person "in charge" of the incident. There are two distinct rolls to fill when deciding who shall be the point of contact and the person in charge of the incident. The person in charge will make decisions as to the interpretation of policy applied to the event. The responsibility for the handling of the event falls onto this person. In contrast, the point of contact must coordinate the effort of all the parties involved with handling the event.

The point of contact must be a person with the technical expertise to successfully coordinate the effort of the system managers and users involved in monitoring and reacting to the attack. Often the management structure of a site is such that the administrator of a set of resources is not a technically competent person with regard to handling the details of the operations of the computers, but is ultimately responsible for the use of these resources.

Another important function of the POC is to maintain contact with law enforcement and other external agencies (such as the CIA, DoD, U.S. Army, or others) to assure that multi-agency involvement occurs.

Finally, if legal action in the form of prosecution is involved, the POC may be able to speak for the site in court. The alternative is to have multiple witnesses that will be hard to coordinate in a legal sense, and will weaken any case against the attackers. A single POC may also be the single person in charge of evidence collected, which will keep the number of people accounting for evidence to a minimum. As a rule of thumb, the more people that touch a potential piece of evidence, the greater the possibility that it will be inadmissible in court. The section below (Legal/Investigative) will provide more details for consideration on this topic.


Establishing Contacts with Investigative Agencies

It is important to establish contacts with personnel from investigative agencies such as the FBI and Secret Service as soon as possible, for several reasons. Local law enforcement and local security offices or campus police organizations should also be informed when appropriate. A primary reason is that once a major attack is in progress, there is little time to call various personnel in these agencies to determine exactly who the correctpoint of contact is. Another reason is that it is important to cooperate with these agencies in a manner that will foster a good working relationship, and that will be in accordance with the working procedures of these agencies. Knowing the working procedures in advance and the expectations of your point of contact is a big step in this direction. For example, it is important to gather evidence that will be admissible in a court of law. If you don't know in advance how to gather admissible evidence, your efforts to collect evidence during an incident are likely to be of no value to the investigative agency with which you deal. A final reason for establishing contacts as soon as possible is that it is impossible to know the particular agency that will assume jurisdiction in any given incident. Making contacts and finding the proper channels early will make responding to an incident go considerably more smoothly.

If your organization or site has a legal counsel, you need to notify this office soon after you learn that an incident is in progress. At a minimum, your legal counsel needs to be involved to protect the legal and financial interests of your site or organization. There are many legal and practical issues, a few of which are:
1. Whether your site or organization is willing to risk negative publicity or exposure to cooperate with legal prosecution efforts.
2. Downstream liability-if you leave a compromised system as is so it can be monitored and another computer is damaged because the attack originated from your system, your site or organization may be liable for damages incurred.
3. Distribution of information-if your site or organization distributes information about an attack in which another site or organization may be involved or the vulnerability in a product that may affect ability to market that product, your site or organization may again be liable for any damages (including damage of reputation).
4. Liabilities due to monitoring-your site or organization may be sued if users at your site or elsewhere discover that your site is monitoring account activity without informing users.

Unfortunately, there are no clear precedents yet on the liabilities or responsibilities of organizations involved in a security incident or who might be involved in supporting an investigative effort. Investigators will often encourage organizations to help trace and monitor intruders-indeed, most investigators cannot pursue computer intrusions without extensive support from the organizations involved. However, investigators cannot provide protection from liability claims, and these kinds of efforts may drag out for months and may take lots of effort.

On the other side, an organization's legal council may advise extreme caution and suggest that tracing activities be halted and an intruder shut out of the system. This in itself may not provide protection from liability, and may prevent investigators from identifying anyone.

The balance between supporting investigative activity and limiting liability is tricky; you'll need to consider the advice of your council and the damage the intruder is causing (if any) in making your decision about what to do during any particular incident.

Your legal counsel should also be involved in any decision to contact investigative agencies when an incident occurs at your site. The decision to coordinate efforts with investigative agencies is most properly that of your site or organization. Involving your legal counsel will also foster the multi-level coordination between your site and the particular investigative agency involved which in turn results in an efficient division of labor. Another result is that you are likely to obtain guidance that will help you avoid future legal mistakes.

Finally, your legal counsel should evaluate your site's written procedures for responding to incidents. It is essential to obtain a "clean bill of health" from a legal perspective before you actually carry out these procedures.

Formal and Informal Legal Procedures

One of the most important considerations in dealing with investigative agencies is verifying that the person who calls asking for information is a legitimate representative from the agency in question. Unfortunately, many well intentioned people have unknowingly leaked sensitive information about incidents, allowed unauthorized people into their systems, etc., because a caller has masqueraded as an FBI or Secret Service agent. A similar consideration is using a secure means of communication. Because many network attackers can easily reroute electronic mail, avoid using electronic mail to communicate with other agencies (as well as others dealing with the incident at hand). Non-secured phone lines (e.g., the phones normally used in the business world) are also frequent targets for tapping by network intruders, so be careful!

There is no established set of rules for responding to an incident when the U.S. Federal Government becomes involved. Except by court order, no agency can force you to monitor, to disconnect from the network, to avoid telephone contact with the suspected attackers, etc.. As discussed in section 5.5.1, you should consult the matter with your legal counsel, especially before taking an action that your organization has never taken. The particular agency involved may ask you to leave an attacked machine on and to monitor activity on this machine, for example. Your complying with this request will ensure continued cooperation of the agency-usually the best route towards finding the source of the network attacks and, ultimately, terminating these attacks. Additionally, you may need some information or a favor from the agency involved in the incident. You are likely to get what you need only if you have been cooperative. Of particular importance is avoiding unnecessary or unauthorized disclosure of information about the incident, including any information furnished by the agency involved. The trust between your site and the agency hinges upon your ability to avoid compromising the case the agency will build; keeping "tight lipped" is imperative.

Sometimes your needs and the needs of an investigative agency will differ. Your site may want to get back to normal business by closing an attack route, but the investigative agency may want you to keep this route open. Similarly, your site may want to close a compromised system down to avoid the possibility of negativepublicity, but again the investigative agency may want you to continue monitoring. When there is such a conflict, there may be a complex set of tradeoffs (e.g., interests of your site's management, amount of resources you can devote to the problem, jurisdictional boundaries, etc.). An important guiding principle is related to what might be called "Internet citizenship" [22, IAB89, 23] and its responsibilities. Your site can shut a system down, and this will relieve you of the stress, resource demands, and danger of negative exposure. The attacker, however, is likely to simply move on to another system, temporarily leaving others blind to the attacker's intention and actions until another path of attack can be detected. Providing that there is no damage to your systems and others, the most responsible course of action is to cooperate with the participating agency by leaving your compromised system on. This will allow monitoring (and, ultimately, the possibility of terminating the source of the reat to systems just like yours). On the other hand, if there is damage to computers illegally accessed through your system, the choice is more complicated: shutting down the intruder may prevent further damage to systems, but might make it impossible to track down the intruder. If there has been damage, the decision about whether it is important to leave systems up to catch the intruder should involve all the organizations effected. Further complicating the issue of network responsibility is the consideration that if you do not cooperate with the agency involved, you will be less likely to receive help from that agency in the future.

Documentation Logs

When you respond to an incident, document all details related to the incident. This will provide valuable information to yourself and others as you try to unravel the course of events. Documenting all details will ultimately save you time. If you don't document every relevant phone call, for example, you are likely to forget a good portion of information you obtain, requiring you to contact the source of information once again. This wastes yours and others' time, something you can ill afford. At the same time, recording details will provide evidence for prosecution efforts, providing the case moves in this direction. Documenting an incident also will help you perform a final assessment of damage (something your management as well as law enforcement officers will want to know), and will provide the basis for a follow-up analysis in which you can engage in a valuable "lessons learned" exercise.

During the initial stages of an incident, it is often infeasible to determine whether prosecution is viable, so you should document as if you are gathering evidence for a court case. At a minimum, you should record:
o All system events (audit records).
o All actions you take (time tagged).
o All phone conversations (including the person with whom

you talked, the date and time, and the content of the conversation).

The most straightforward way to maintain documentation is keeping a log book. This allows you to go to a centralized, chronological source of information when you need it, instead of requiring you to page through individual sheets of paper. Much of this information is potential evidence in a court of law. Thus, when you initially suspect that an incident will result in prosecution or when an investigative agency becomes involved, you need to regularly (e.g., every day) turn in photocopied, signed copies of your logbook (as well as media you use to record system events) to a document custodian who can store these copied pages in a secure place (e.g., a safe). When you submit information for storage, you should in return receive a signed, dated receipt from the document custodian. Failure to observe these procedures can result in invalidation of any evidence you obtain in a court of law.

Establishing Post-Incident Procedures


In the wake of an incident, several actions should take place. These actions can be summarized as follows:
1. An inventory should be taken of the systems' assets, i.e., a careful examination should determine how the system was affected by the incident,
2. The lessons learned as a result of the incident should be included in revised security plan to prevent the incident from re-occurring,
3. A new risk analysis should be developed in light of the incident,
4. An investigation and prosecution of the individuals who caused the incident should commence, if it is deemed desirable.

All four steps should provide feedback to the site security policy committee, leading to prompt re-evaluation and amendment of the current policy.

Removing Vulnerabilities

Removing all vulnerabilities once an incident has occurred is difficult. The key to removing vulnerabilities is knowledge and understanding of the breach. In some cases, it is prudent to remove all access or functionality as soon as possible, and then restore normal operation in limited stages. Bear in mind that removing all access while an incident is in progress will obviously notify all users, including the alleged problem users, that the administrators are aware of a problem; this may have a deleterious effect on an investigation. However, allowing an incident to continue may also open the likelihood of greater damage, loss, aggravation, or liability (civil or criminal).

If it is determined that the breach occurred due to a flaw in the systems' hardware or software, the vendor (or supplier) and the CERT should be notified as soon as possible. Including relevant telephone numbers (also electronic mail addresses and fax numbers) in the site security policy is strongly recommended. To aid prompt acknowledgment and understanding of the problem, the flaw should be described in as much detail as possible, including details about how to exploit the flaw.

As soon as the breach has occurred, the entire system and all its components should be considered suspect. System software is the most probable target. Preparation is key to recovering from a possibly tainted system. This includes checksumming all tapes from the vendor using a checksum algorithm which (hopefully) is resistant to tampering [10]. (See sections, Assuming original vendor distribution tapes are available, an analysis of all system files should commence, and any irregularities should be noted and referred to all parties involved in handling the incident. It can be very difficult, in some cases, to decide which backup tapes to recover from; consider that the incident may have continued for months or years before discovery, and that the suspect may be an employee of the site, or otherwise have intimate knowledge or access to the systems. In all cases, the pre-incident preparation will determine what recovery is possible. At worst-case, restoration from the original manufactures' media and a re-installation of the systems will be the most prudent solution.

Review the lessons learned from the incident and always update the policy and procedures to reflect changes necessitated by the incident.

Assessing Damage

Before cleanup can begin, the actual system damage must be discerned. This can be quite time consuming, but should lead into some of the insight as to the nature of the incident, and aid investigation and prosecution. It is best to compare previous backups or original tapes when possible; advance preparation is the key. If the system supports centralized logging (most do), go back over the logs and look for abnormalities. If process accounting and connect time accounting is enabled, look for patterns of system usage. To a lesser extent, disk usage may shed light on the incident. Accounting can provide much helpful information in an analysis of an incident and subsequent prosecution.


Once the damage has been assessed, it is necessary to develop a plan for system cleanup. In general, bringing up services in the order of demand to allow a minimum of user inconvenience is the best practice. Understand that the proper recovery procedures for the system are extremely important and should be specific to the site.

It may be necessary to go back to the original distributed tapes and recustomize the system. To facilitate this worst case scenario, a record of the original systems setup and each customization change should be kept current with each change to the system.

Follow up

Once you believe that a system has been restored to a "safe" state, it is still possible that holes and even traps could be lurking in the system. In the follow-up stage, the system should be monitored for items that may have been missed during the cleanup stage. It would be prudent to utilize some of the tools mentioned in section (e.g., COPS) as a start. Remember, these tools don't replace continual system monitoring and good systems administration procedures.

Keep a Security Log

As discussed in section 5.6, a security log can be most valuable during this phase of removing vulnerabilities. There are two considerations here; the first is to keep logs of the procedures that have been used to make the system secure again. This should include command procedures (e.g., shell scripts) that can be run on a periodic basis to recheck the security. Second, keep logs of important system events. These can be referenced when trying to determine the extent of the damage of a given incident.

Capturing Lessons Learned

Understand the Lesson

After an incident, it is prudent to write a report describing the incident, method of discovery, correction procedure, monitoring procedure, and a summary of lesson learned. This will aid in the clear understanding of the problem. Remember, it is difficult to learn from an incident if you don't understand the source.


Other Security Devices, Methods

Security is a dynamic, not static process. Sites are dependent on the nature of security available at each site, and the array of devices and methods that will help promote security. Keeping up with the security area of the computer industry and their methods will assure a security manager of taking advantage of the latest technology.

Repository of Books, Lists, Information Sources

Keep an on site collection of books, lists, information sources, etc., as guides and references for securing the system. Keep this collection up to date. Remember, as systems change, so do security methods and problems.

Form a Subgroup

Form a subgroup of system administration personnel that will be the core security staff. This will allow discussions of security problems and multiple views of the site's security issues. This subgroup can also act to develop the site security policy and make suggested changes as necessary to ensure site security.

Upgrading Policies and Procedures

Updating Policies, Procedures, and Tools

Establish Mechanisms for Updating Policies, Procedures, and Tools

If an incident is based on poor policy, and unless the policy is changed, then one is doomed to repeat the past. Once a site has recovered from and incident, site policy and procedures should be reviewed to encompass changes to prevent similar incidents. Even without an incident, it would be prudent to review policies and procedures on a regular basis. Reviews are imperative due to today's changing computing environments.

Problem Reporting Procedures

A problem reporting procedure should be implemented to describe, in detail, the incident and the solutions to the incident. Each incident should be reviewed by the site security subgroup to allow understanding of the incident with possible suggestions to the site policy and procedures.


[1] Quarterman, J., "The Matrix: Computer Networks and Conferencing Systems Worldwide", Pg. 278, Digital Press, Bedford, MA, 1990.

[2] Brand, R., "Coping with the Threat of Computer Security Incidents: A Primer from Prevention through Recovery", R. Brand, available on-line from:, 8 June 1990.

[3] Fites, M., Kratz, P. and A. Brebner, "Control and Security of Computer Information Systems", Computer Science Press, 1989.

[4] Johnson, D., and J. Podesta, "Formulating a Company Policy on Access to and Use and Disclosure of Electronic Mail on Company Computer Systems", Available from: The Electronic Mail Association (EMA) 1555 Wilson Blvd, Suite 555, Arlington VA 22209, (703) 522-7111, 22 October 1990.

[5] Curry, D., "Improving the Security of Your UNIX System", SRI International Report ITSTD-721-FR-90-21, April 1990.

[6] Cheswick, B., "The Design of a Secure Internet Gateway", Proceedings of the Summer Usenix Conference, Anaheim, CA, June 1990.

[7] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part I-Message Encipherment and Authentication Procedures", RFC 1113, IAB Privacy Task Force, August 1989.

[8] Kent, S., and J. Linn, "Privacy Enhancement for Internet Electronic Mail: Part II-Certificate-Based Key Management", RFC 1114, IAB Privacy Task Force, August 1989.

[9] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part III-Algorithms, Modes, and Identifiers", RFC 1115, IAB Privacy Task Force, August 1989.

[10] Merkle, R., "A Fast Software One Way Hash Function", Journal of

Cryptology, Vol. 3, No. 1.

[11] Postel, J., "Internet Protocol - DARPA Internet Program Protocol

Specification", RFC 791, DARPA, September 1981.

[12] Postel, J., "Transmission Control Protocol - DARPA Internet

Program Protocol Specification", RFC 793, DARPA, September 1981.

[13] Postel, J., "User Datagram Protocol", RFC 768, USC/Information

Sciences Institute, 28 August 1980.

[14] Mogul, J., "Simple and Flexible Datagram Access Controls for

UNIX-based Gateways", Digital Western Research Laboratory Research Report 89/4, March 1989.

[15] Bellovin, S., and M. Merritt, "Limitations of the Kerberos

Authentication System", Computer Communications Review, October 1990.

[16] Pfleeger, C., "Security in Computing", Prentice-Hall, Englewood Cliffs, N.J., 1989.

[17] Parker, D., Swope, S., and B. Baker, "Ethical Conflicts:

Information and Computer Science, Technology and Business", QED Information Sciences, Inc., Wellesley, MA.

[18] Forester, T., and P. Morrison, "Computer Ethics: Tales and

Ethical Dilemmas in Computing", MIT Press, Cambridge, MA, 1990.

[19] Postel, J., and J. Reynolds, "Telnet Protocol Specification", RFC

854, USC/Information Sciences Institute, May 1983.

[20] Postel, J., and J. Reynolds, "File Transfer Protocol", RFC 959,

USC/Information Sciences Institute, October 1985.

[21] Postel, J., Editor, "IAB Official Protocol Standards", RFC 1200,

IAB, April 1991.

[22] Internet Activities Board, "Ethics and the Internet", RFC 1087,

Internet Activities Board, January 1989.

[23] Pethia, R., Crocker, S., and B. Fraser, "Policy Guidelines for

the Secure Operation of the Internet", CERT, TIS, CERT, RFC in preparation.

[24] Computer Emergency Response Team (CERT/CC), "Unauthorized

Password Change Requests", CERT Advisory CA-91:03, April 1991.

[25] Computer Emergency Response Team (CERT/CC), "TELNET Breakin

Warning", CERT Advisory CA-89:03, August 1989.

[26] CCITT, Recommendation X.509, "The Directory: Authentication

Framework", Annex C.

[27] Farmer, D., and E. Spafford, "The COPS Security Checker System",

Proceedings of the Summer 1990 USENIX Conference, Anaheim, CA, Pgs. 165-170, June 1990.

Annotated Bibliography

The intent of this annotated bibliography is to offer a representative collection of resources of information that will help the user of this handbook. It is meant provide a starting point for further research in the security area. Included are references to other sources of information for those who wish to pursue issues of the computer security environment.

Computer Law


American Bar Association, Section of Science and Technology, "Guide to the Prosecution of Telecommunication Fraud by the Use of Computer Crime Statutes", American Bar Association, 1989.


Bender, D., "Computer Law: Evidence and Procedure", M. Bender, New York, NY, 1978-present.

Kept up to date with supplements.

Years covering 1978-1984 focuses on: Computer law, evidence and procedures. The years 1984 to the current focus on general computer law. Bibliographical references and index included.


Bloombecker, B., "Spectacular Computer Crimes", Dow Jones-Irwin, Homewood, IL. 1990.


Commerce Clearing House, "Guide to Computer Law", (Topical Law Reports), Chicago, IL., 1989.

Court cases and decisions rendered by federal and state courts throughout the United States on federal and state computer law. Includes Case Table and Topical Index.


Conly, C., "Organizing for Computer Crime Investigation and Prosecution", U.S. Dept. of Justice, Office of Justice Programs, Under Contract Number OJP-86-C-002, National Institute of Justice, Washington, DC, July 1989.


Fenwick, W., Chair, "Computer Litigation, 1985: Trial Tactics and Techniques", Litigation Course Handbook Series No. 280, Prepared for distribution at the Computer Litigation, 1985: Trial Tactics and Techniques Program, February-March 1985.


Gemignani, M., "Viruses and Criminal Law", Communications of the ACM, Vol. 32, No. 6, Pgs. 669-671, June 1989.


Huband, F., and R. Shelton, Editors, "Protection of

Computer Systems and Software: New Approaches for Combating Theft of Software and Unauthorized Intrusion", Papers presented at a workshop sponsored by the National Science Foundation, 1986.


McEwen, J., "Dedicated Computer Crime Units", Report Contributors: D. Fester and H. Nugent, Prepared for the National Institute of Justice, U.S. Department of Justice, by Institute for Law and Justice, Inc., under contract number OJP-85-C-006, Washington, DC, 1989.


Parker, D., "Computer Crime: Criminal Justice Resource Manual", U.S. Dept. of Justice, National Institute of Justice, Office of Justice Programs, Under Contract Number OJP-86-C-002, Washington, D.C., August 1989.


Shaw, E., Jr., "Computer Fraud and Abuse Act of 1986, Congressional Record (3 June 1986), Washington, D.C., 3 June 1986.


Trible, P., "The Computer Fraud and Abuse Act of 1986", U.S. Senate Committee on the Judiciary, 1986.

Computer Security


Caelli, W., Editor, "Computer Security in the Age of Information", Proceedings of the Fifth IFIP International Conference on Computer Security, IFIP/Sec '88.


Carroll, J., "Computer Security", 2nd Edition, Butterworth Publishers, Stoneham, MA, 1987.


Cooper, J., "Computer and Communications Security:

Strategies for the 1990s", McGraw-Hill, 1989.


Brand, R., "Coping with the Threat of Computer Security

Incidents: A Primer from Prevention through Recovery",

R. Brand, 8 June 1990.

As computer security becomes a more important issue in modern society, it begins to warrant a systematic approach. The vast majority of the computer security problems and the costs associated with them can be prevented with simple inexpensive measures. The most important and cost effective of these measures are available in the prevention and planning phases. These methods are presented in this paper, followed by a simplified guide to incident handling and recovery. Available on-line from:


Cheswick, B., "The Design of a Secure Internet Gateway", Proceedings of the Summer Usenix Conference, Anaheim, CA, June 1990.

Brief abstract (slight paraphrase from the original abstract): AT&T maintains a large internal Internet that needs to be protected from outside attacks, while providing useful services between the two. This paper describes AT&T's Internet gateway. This gateway passes mail and many of the common Internet services between AT&T internal machines and the Internet. This is accomplished without IP connectivity using a pair of machines: a trusted internal machine and an untrusted external gateway. These are connected by a private link. The internal machine provides a few carefully-guarded services to the external gateway. This configuration helps protect the internal internet even if the external machine is fully compromised.

This is a very useful and interesting design. Most firewall gateway systems rely on a system that, if compromised, could allow access to the machines behind the firewall. Also, most firewall systems require users who want access to Internet services to have accounts on the firewall machine. AT&T's design allows AT&T internal internet users access to the standard services of TELNET and FTP from their own workstations without accounts on the firewall machine. A very useful paper that shows how to maintain some of the benefits of Internet connectivity while still maintaining strong security.


Curry, D., "Improving the Security of Your UNIX System", SRI International Report ITSTD-721-FR-90-21, April 1990.

This paper describes measures that you, as a system administrator can take to make your UNIX system(s) more secure. Oriented primarily at SunOS 4.x, most of the information covered applies equally well to any Berkeley UNIX system with or without NFS and/or Yellow Pages (NIS). Some of the information can also be applied to System V, although this is not a primary focus of the paper. A very useful reference, this is also available on the Internet in various locations, including the directory


Fites, M., Kratz, P. and A. Brebner, "Control and

Security of Computer Information Systems", Computer Science Press, 1989.

This book serves as a good guide to the issues encountered in forming computer security policies and procedures. The book is designed as a textbook for an introductory course in information systems security.

The book is divided into five sections: Risk Management (I), Safeguards: security and control measures, organizational and administrative (II), Safeguards: Security and Control Measures, Technical (III), Legal Environment and Professionalism (IV), and CICA Computer Control Guidelines (V).

The book is particularly notable for its straight-forward approach to security, emphasizing that common sense is the first consideration in designing a security program. The authors note that there is a tendency to look to more technical solutions to security problems while overlooking organizational controls which are often cheaper and much more effective. 298 pages, including references and index.


Garfinkel, S, and E. Spafford, "Practical Unix Security", O'Reilly & Associates, ISBN 0-937175-72-2, May 1991.

Approx 450 pages, $29.95. Orders: 1-800-338-6887

(US & Canada), 1-707-829-0515 (Europe), email:

This is one of the most useful books available on Unix security. The first part of the book covers standard Unix and Unix security basics, with particular emphasis on passwords. The second section covers enforcing security on the system. Of particular interest to the Internet user are the sections on network security, which address many of the common security problems that afflict Internet Unix users. Four chapters deal with handling security incidents, and the book concludes with discussions of encryption, physical security, and useful checklists and lists of resources. The book lives up to its name; it is filled with specific references to possible security holes, files to check, and things to do to improve security. This book is an excellent complement to this handbook.


Greenia, M., "Computer Security Information Sourcebook", Lexikon Services, Sacramento, CA, 1989.

A manager's guide to computer security. Contains a sourcebook of key reference materials including access control and computer crimes bibliographies.


Hoffman, L., "Rogue Programs: Viruses, Worms, and Trojan Horses", Van Nostrand Reinhold, NY, 1990. (384 pages, includes bibliographical references and index.)


Johnson, D., and J. Podesta, "Formulating A Company Policy on Access to and Use and Disclosure of Electronic Mail on Company Computer Systems".

A white paper prepared for the EMA, written by two experts in privacy law. Gives background on the issues, and presents some policy options.

Available from: The Electronic Mail Association (EMA)

1555 Wilson Blvd, Suite 555, Arlington, VA, 22209.

(703) 522-7111.


Kent, Stephen, "E-Mail Privacy for the Internet: New Software and Strict Registration Procedures will be Implemented this Year", Business Communications Review, Vol. 20, No. 1, Pg. 55, 1 January 1990.


Lu, W., and M. Sundareshan, "Secure Communication in Internet Environments: A Hierachical Key Management Scheme for End-to-End Encryption", IEEE Transactions on Communications, Vol. 37, No. 10, Pg. 1014, 1 October 1989.


Lu, W., and M. Sundareshan, "A Model for Multilevel Security in Computer Networks", IEEE Transactions on Software Engineering, Vol. 16, No. 6, Page 647, 1 June 1990.


National Security Agency, "Information Systems Security Products and Services Catalog", NSA, Quarterly Publication.

NSA's catalogue contains chapter on: Endorsed Cryptographic Products List; NSA Endorsed Data Encryption Standard (DES) Products List; Protected Services List; Evaluated Products List; Preferred Products List; and Endorsed Tools List.

The catalogue is available from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. One may place telephone orders by calling:

(202) 783-3238.


United States Congress, Office of Technology Assessment, "Defending Secrets, Sharing Data: New Locks and Keys for Electronic Information", OTA-CIT-310, October 1987.

This report, prepared for congressional committee considering Federal policy on the protection of electronic information, is interesting because of the issues it raises regarding the impact of technology used to protect information. It also serves as a reasonable introduction to the various encryption and information protection mechanisms. 185 pages. Available from the U.S. Government Printing Office.


Palmer, I., and G. Potter, "Computer Security Risk Management", Van Nostrand Reinhold, NY, 1989.


Pfleeger, C., "Security in Computing", Prentice-Hall, Englewood Cliffs, NJ, 1989.

A general textbook in computer security, this book provides an excellent and very readable introduction to classic computer security problems and solutions, with a particular emphasis on encryption. The encryption coverage serves as a good introduction to the subject. Other topics covered include building secure programs and systems, security of database, personal computer security, network and communications security, physical security, risk analysis and security planning, and legal and ethical issues. 538 pages including index and bibliography.


Shirey, R., "Defense Data Network Security Architecture", Computer Communication Review, Vol. 20, No. 2, Page 66, 1 April 1990.


Spafford, E., Heaphy, K., and D. Ferbrache, "Computer Viruses: Dealing with Electronic Vandalism and Programmed Threats", ADAPSO, 1989. (109 pages.)

This is a good general reference on computer viruses and related concerns. In addition to describing viruses in some detail, it also covers more general security issues, legal recourse in case of security problems, and includes lists of laws, journals focused on computers security, and other security-related resources.

Available from: ADAPSO, 1300 N. 17th St, Suite 300, Arlington VA 22209. (703) 522-5055.


Stoll, C., "Stalking the Wily Hacker", Communications of the ACM, Vol. 31, No. 5, Pgs. 484-497, ACM, New York, NY, May 1988.

This article describes some of the technical means used to trace the intruder that was later chronicled in "Cuckoo's Egg" (see below).


Stoll, C., "The Cuckoo's Egg", ISBN 00385-24946-2, Doubleday, 1989.

Clifford Stoll, an astronomer turned UNIX System Administrator, recounts an exciting, true story of how he tracked a computer intruder through the maze of American military and research networks. This book is easy to understand and can serve as an interesting introduction to the world of networking. Jon Postel says in a book review, "[this book] ... is absolutely essential reading for anyone that uses or operates any computer connected to the Internet or any other computer network."


Vallabhaneni, S., "Auditing Computer Security: A Manual with Case Studies", Wiley, New York, NY, 1989.



Computer Professionals for Social Responsibility, "CPSR Statement on the Computer Virus", CPSR, Communications of the ACM, Vol. 32, No. 6, Pg. 699, June 1989.

This memo is a statement on the Internet Computer Virus by the Computer Professionals for Social Responsibility (CPSR).


Denning, Peter J., Editor, "Computers Under Attack:

Intruders, Worms, and Viruses", ACM Press, 1990.

A collection of 40 pieces divided into six sections: the emergence of worldwide computer networks, electronic breakins, worms, viruses, counterculture (articles examining the world of the "hacker"), and finally a section discussing social, legal, and ethical considerations.

A thoughtful collection that addresses the phenomenon of attacks on computers. This includes a number of previously published articles and some new ones. The previously published ones are well chosen, and include some references that might be otherwise hard to obtain. This book is a key reference to computer security threats that have generated much of the concern over computer security in recent years.


Ermann, D., Williams, M., and C. Gutierrez, Editors, "Computers, Ethics, and Society", Oxford University Press, NY, 1990. (376 pages, includes bibliographical references).


Forester, T., and P. Morrison, "Computer Ethics: Tales and Ethical Dilemmas in Computing", MIT Press, Cambridge, MA, 1990. (192 pages including index.) From the preface: "The aim of this book is two-fold: (1) to describe some of the problems created by society by computers, and (2) to show how these problems present ethical dilemmas for computers professionals and computer users.

The problems created by computers arise, in turn, from two main sources: from hardware and software malfunctions and from misuse by human beings. We argue that computer systems by their very nature are insecure, unreliable, and unpredictable-and that society has yet to come to terms with the consequences. We also seek to show how society has become newly vulnerable to human misuse of computers in the form of computer crime, software theft, hacking, the creation of viruses, invasions of privacy, and so on."

The eight chapters include "Computer Crime", "Software Theft", "Hacking and Viruses", "Unreliable Computers", "The Invasion of Privacy", "AI and Expert Systems", and "Computerizing the Workplace." Includes extensive notes on sources and an index.


Gould, C., Editor, "The Information Web: Ethical and Social Implications of Computer Networking", Westview Press, Boulder, CO, 1989.


Internet Activities Board, "Ethics and the Internet", RFC 1087, IAB, January 1989. Also appears in the Communications of the ACM, Vol. 32, No. 6, Pg. 710, June 1989.

This memo is a statement of policy by the Internet Activities Board (IAB) concerning the proper use of the resources of the Internet. Available on-line on host, directory rfc, filename rfc1087.txt. Also available on host, directory RFC, filename RFC1087.TXT-1.


Martin, M., and R. Schinzinger, "Ethics in Engineering", McGraw Hill, 2nd Edition, 1989.


Massachusetts Institute of Technology, "Teaching Students About Responsible Use of Computers", MIT, 1985-1986. Also reprinted in the Communications of the ACM, Vol. 32, No. 6, Pg. 704, Athena Project, MIT, June 1989. This memo is a statement of policy by the Massachusetts Institute of Technology (MIT) on the responsible use of computers.


National Institute of Standards and Technology, "Computer Viruses and Related Threats: A Management Guide", NIST Special Publication 500-166, August 1989.


National Science Foundation, "NSF Poses Code of Networking Ethics", Communications of the ACM, Vol. 32, No. 6, Pg. 688, June 1989. Also appears in the minutes of the regular meeting of the Division Advisory Panel for Networking and Communications Research and Infrastructure, Dave Farber, Chair, November 29-30, 1988.

This memo is a statement of policy by the National Science Foundation (NSF) concerning the ethical use of the Internet.


Parker, D., Swope, S., and B. Baker, "Ethical Conflicts:

Information and Computer Science, Technology and Business", QED Information Sciences, Inc., Wellesley, MA. (245 pages).

Additional publications on Ethics:

The University of New Mexico (UNM)

The UNM has a collection of ethics documents. Included are legislation from several states and policies from many institutions.

Access is via FTP, IP address Look in the directory /ethics.

The Internet Worm


Brock, J., "November 1988 Internet Computer Virus and the Vulnerability of National Telecommunications Networks to Computer Viruses", GAO/T-IMTEC-89-10, Washington, DC, 20 July 1989.

Testimonial statement of Jack L. Brock, Director, U. S.

Government Information before the Subcommittee on Telecommunications and Finance, Committee on Energy and Commerce, House of Representatives.


Eichin, M., and J. Rochlis, "With Microscope and Tweezers:

An Analysis of the Internet Virus of November 1988", Massachusetts Institute of Technology, February 1989.

Provides a detailed dissection of the worm program. The paper discusses the major points of the worm program then reviews strategies, chronology, lessons and open issues, Acknowledgments; also included are a detailed appendix on the worm program subroutine by subroutine, an appendix on the cast of characters, and a reference section.


Eisenberg, T., D. Gries, J. Hartmanis, D. Holcomb, M. Lynn, and T. Santoro, "The Computer Worm", Cornell University, 6 February 1989.

A Cornell University Report presented to the Provost of the University on 6 February 1989 on the Internet Worm.


U.S. General Accounting Office, "Computer Security - Virus Highlights Need for Improved Internet Management", United States General Accounting Office, Washington, DC, 1989.

This 36 page report (GAO/IMTEC-89-57), by the U.S. Government Accounting Office, describes the Internet worm and its effects. It gives a good overview of the various U.S. agencies involved in the Internet today and their concerns vis-a-vis computer security and networking.

Available on-line on host, directory pub, filename GAO_RPT; and on, directory nsfnet, filename GAO_RPT.TXT.


The Helminthiasis of the Internet, RFC 1135, USC/Information Sciences Institute, Marina del Rey, CA, December 1989.

This report looks back at the helminthiasis (infestation with, or disease caused by parasitic worms) of the Internet that was unleashed the evening of 2 November 1988.

This document provides a glimpse at the infection,its festering, and cure. The impact of the worm on the Internet community, ethics statements, the role of the news media, crime in the computer world, and future prevention is discussed. A documentation review presents four publications that describe in detail this particular parasitic computer program. Reference and bibliography sections are also included. Available on-line on host directory rfc, filename rfc1135.txt. Also available on host, directory RFC, filename RFC1135.TXT-1.


Seeley, D., "A Tour of the Worm", Proceedings of 1989 Winter USENIX Conference, Usenix Association, San Diego, CA, February 1989.

Details are presented as a "walk thru" of this particular worm program. The paper opened with an abstract, introduction, detailed chronology of events upon the discovery of the worm, an overview, the internals of the worm, personal opinions, and conclusion.


Spafford, E., "The Internet Worm Program: An Analysis", Computer Communication Review, Vol. 19, No. 1, ACM SIGCOM, January 1989. Also issued as Purdue CS Technical Report CSD-TR-823, 28 November 1988.

Describes the infection of the Internet as a worm program that exploited flaws in utility programs in UNIX based systems. The report gives a detailed description of the components of the worm program: data and functions. Spafford focuses his study on two completely independent reverse-compilations of the worm and a version disassembled to VAX assembly language.


Spafford, G., "An Analysis of the Internet Worm", Proceedings of the European Software Engineering Conference 1989, Warwick England, September 1989. Proceedings published by Springer-Verlag as: Lecture Notes in Computer Science #387. Also issued as Purdue Technical Report #CSD-TR-933.

National Computer Security Center (NCSC)

All NCSC publications, approved for public release, are available from the NCSC Superintendent of Documents.

NCSC = National Computer Security Center

9800 Savage Road

Ft Meade, MD 20755-6000

CSC = Computer Security Center: an earlier name for the NCSC

NTISS = National Telecommunications and Information Systems Security

NTISS Committee, National Security Agency Ft Meade, MD 20755-6000


Department of Defense, "Password Management Guideline", CSC-STD-002-85, 12 April 1985, 31 pages.

The security provided by a password system depends on the passwords being kept secret at all times. Thus, a password is vulnerable to compromise whenever it is used, stored, or even known. In a password-based authentication mechanism implemented on an ADP system, passwords are vulnerable to compromise due to five essential aspects of the password system: 1) a password must be initially assigned to a user when enrolled on the ADP system;
2) a user's password must be changed periodically;
3) the ADP system must maintain a 'password database'; 4) users must remember their passwords; and
5) users must enter their passwords into the ADP system at authentication time. This guideline prescribes steps to be taken to minimize the vulnerability of passwords in each of these circumstances.


NCSC, "A Guide to Understanding AUDIT in Trusted Systems", NCSC-TG-001, Version-2, 1 June 1988, 25 pages.

Audit trails are used to detect and deter penetration of a computer system and to reveal usage that identifies misuse. At the discretion of the auditor, audit trails may be limited to specific events or may encompass all of the activities on a system. Although not required by the criteria, it should be possible for the target of the audit mechanism to be either a subject or an object. That is to say, the audit mechanism should be capable of monitoring every time John accessed the system as well as every time the nuclear reactor file was accessed; and likewise every time John accessed the nuclear reactor file.


NCSC, "A Guide to Understanding DISCRETIONARY ACCESS CONTROL in Trusted Systems", NCSC-TG-003, Version-1, 30 September 1987, 29 pages.

Discretionary control is the most common type of access control mechanism implemented in computer systems today. The basis of this kind of security is that an individual user, or program operating on the user's behalf, is allowed to specify explicitly the types of access other users (or programs executing on their behalf) may have to information under the user's control. [...] Discretionary controls are not a replacement for mandatory controls. In any environment in which information is protected, discretionary security provides for a finer granularity of control within the overall constraints of the mandatory policy.


NCSC, "A Guide to Understanding CONFIGURATION MANAGEMENT in Trusted Systems", NCSC-TG-006, Version-1, 28 March 1988, 31 pages.

Configuration management consists of four separate tasks:

identification, control, status accounting, and auditing. For every change that is made to an automated data processing (ADP) system, the design and requirements of the changed version of the system should be identified. The control task of configuration management is performed by subjecting every change to documentation, hardware, and software/firmware to review and approval by an authorized authority. Configuration status accounting is responsible for recording and reporting on the configuration of the product throughout the change. Finally, though the process of a configuration audit, the completed change can be verified to be functionally correct, and for trusted systems, consistent with the security policy of the system.


NTISS, "Advisory Memorandum on Office Automation Security Guideline", NTISSAM CONPUSEC/1-87, 16 January 1987, 58 pages.

This document provides guidance to users, managers, security officers, and procurement officers of Office Automation Systems. Areas addressed include: physical security, personnel security, procedural security, hardware/software security, emanations security (TEMPEST), and communications security for stand-alone OA Systems, OA Systems used as terminals connected to mainframe computer systems, and OA Systems used as hosts in a Local Area Network (LAN). Differentiation is made between those Office Automation Systems equipped with removable storage media only (e.g., floppy disks, cassette tapes, removable hard disks) and those Office Automation Systems equipped with fixed media (e.g., Winchester disks).

Additional NCSC Publications:


National Computer Security Center, "Glossary of Computer Security Terms", NCSC-TG-004, NCSC, 21 October 1988.


National Computer Security Center, "Trusted

Computer System Evaluation Criteria", DoD 5200.28-STD, CSC-STD-001-83, NCSC, December 1985.


National Computer Security Center, "Guidance for Applying the Department of Defense Trusted Computer System Evaluation Criteria in Specific Environments", CSC-STD-003-85, NCSC, 25 June 1985.


National Computer Security Center, "Technical Rationale Behind CSC-STD-003-85: Computer Security Requirements", CSC-STD-004-85, NCSC, 25 June 85.


National Computer Security Center, "Magnetic Remanence Security Guideline", CSC-STD-005-85, NCSC, 15 November 1985.

This guideline is tagged as a "For Official Use Only" exemption under Section 6, Public Law 86-36 (50 U.S. Code 402). Distribution authorized of U.S. Government agencies and their contractors to protect unclassified technical, operational, or administrative data relating to operations of the National Security Agency.


National Computer Security Center, "Guidelines for Formal Verification Systems", Shipping list no.: 89-660-P, The Center, Fort George G. Meade, MD, 1 April 1990.


National Computer Security Center, "Glossary of Computer Security Terms", Shipping list no.: 89-254-P, The Center, Fort George G. Meade, MD, 21 October 1988.


National Computer Security Center, "Trusted UNIX Working Group (TRUSIX) rationale for selecting access control list features for the UNIX system", Shipping list no.:

90-076-P, The Center, Fort George G. Meade, MD, 1990.


National Computer Security Center, "Trusted Network Interpretation", NCSC-TG-005, NCSC, 31 July 1987.


Tinto, M., "Computer Viruses: Prevention, Detection, and Treatment", National Computer Security Center C1 Technical Report C1-001-89, June 1989.


National Computer Security Conference, "12th National Computer Security Conference: Baltimore Convention Center, Baltimore, MD, 10-13 October, 1989: Information Systems Security, Solutions for Today - Concepts for Tomorrow", National Institute of Standards and National Computer Security Center, 1989.

Security Checklists


Aucoin, R., "Computer Viruses: Checklist for Recovery", Computers in Libraries, Vol. 9, No. 2, Pg. 4, 1 February 1989.


Wood, C., Banks, W., Guarro, S., Garcia, A., Hampel, V., and H. Sartorio, "Computer Security: A Comprehensive Controls Checklist", John Wiley and Sons, Interscience Publication, 1987.

Additional Publications

Defense Data Network's Network Information Center (DDN NIC)

The DDN NIC maintains DDN Security bulletins and DDN Management bulletins online on the machine: NIC.DDN.MIL. They are available via anonymous FTP. The DDN Security bulletins are in the directory: SCC, and the DDN Management bulletins are in the directory: DDN-NEWS.

For additional information, you may send a message to:

NIC@NIC.DDN.MIL, or call the DDN NIC at: 1-800-235-3155.


Defense Data Network, "BSD 4.2 and 4.3 Software Problem Resolution", DDN MGT Bulletin #43, DDN Network Information Center, 3 November 1988.

A Defense Data Network Management Bulletin announcement on the 4.2bsd and 4.3bsd software fixes to the Internet worm.


DCA DDN Defense Communications System, "DDN Security Bulletin 03", DDN Security Coordination Center, 17 October 1989.

IEEE Proceedings


"Proceedings of the IEEE Symposium on Security and Privacy", published annually.

IEEE Proceedings are available from:
Computer Society of the IEEE
P.O. Box 80452
Worldway Postal Center
Los Angeles, CA 90080

Other Publications:
Computer Law and Tax Report
Computers and Security
Security Management Magazine
Journal of Information Systems Management
Data Processing & Communications Security
SIG Security, Audit & Control Review


Thanks to the SSPHWG's illustrious "Outline Squad", who assembled at USC/Information Sciences Institute on 12-June-90: Ray Bates (ISI), Frank Byrum (DEC), Michael A. Contino (PSU), Dave Dalva (Trusted Information Systems, Inc.), Jim Duncan (Penn State Math Department), Bruce Hamilton (Xerox), Sean Kirkpatrick (Unisys), Tom Longstaff (CIAC/LLNL), Fred Ostapik (SRI/NIC), Keith Pilotti (SAIC), and Bjorn Satdeva (/sys/admin, inc.).

Many thanks to Rich Pethia and the Computer Emergency Response Team (CERT); much of the work by Paul Holbrook was done while he was working for CERT. Rich also provided a very thorough review of this document. Thanks also to Jon Postel and USC/Information Sciences Institute for contributing facilities and moral support to this effort.

Last, but NOT least, we would like to thank members of the SSPHWG and Friends for their additional contributions: Vint Cerf (CNRI), Dave Grisham (UNM), Nancy Lee Kirkpatrick (Typist Extraordinaire), Chris McDonald (WSMR), H. Craig McKee (Mitre), Gene Spafford (Purdue), and Aileen Yuan (Mitre).

Security Considerations

If security considerations had not been so widely ignored in the Internet, this memo would not have been possible.

Authors' Addresses

J. Paul Holbrook

CICNet, Inc.

2901 Hubbard

Ann Arbor, MI 48105
Phone: (313) 998-7680

Joyce K. Reynolds

University of Southern California

Information Sciences Institute

4676 Admiralty Way

Marina del Rey, CA 90292
Phone: (213) 822-1511