Site Security Handbook

General points of security
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[Free Dmitry Sklyarov] [Advertisement] [Free Dmitry Sklyarov] Network Working Group P. Holbrook Request for Comments: 1244 CICNet FYI: 8 J. Reynolds ISI Editors July 1991 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 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. Site Security Policy Handbook Working Group [Page 1] RFC 1244 Site Security Handbook July 1991 Finally, we intend for this FYI RFC to grow and evolve. Please send comments and suggestions to: ssphwg@cert.sei.cmu.edu. Table of Contents 1. Introduction..................................................... 3 1.1 Purpose of this Work............................................ 3 1.2 Audience........................................................ 3 1.3 Definitions..................................................... 4 1.4 Related Work.................................................... 4 1.5 Scope........................................................... 4 1.6 Why Do We Need Security Policies and Procedures?................ 5 1.7 Basic Approach.................................................. 7 1.8 Organization of this Document................................... 7 2. Establishing Official Site Policy on Computer Security........... 9 2.1 Brief Overview.................................................. 9 2.2 Risk Assessment................................................. 10 2.3 Policy Issues................................................... 13 2.4 What Happens When the Policy Is Violated........................ 19 2.5 Locking In or Out............................................... 21 2.6 Interpreting the Policy......................................... 23 2.7 Publicizing the Policy.......................................... 23 3. Establishing Procedures to Prevent Security Problems............. 24 3.1 Security Policy Defines What Needs to be Protected.............. 24 3.2 Identifing Possible Problems.................................... 24 3.3 Choose Controls to Protect Assets in a Cost-Effective Way....... 26 3.4 Use Multiple Strategies to Protect Assets....................... 26 3.5 Physical Security............................................... 27 3.6 Procedures to Recognize Unauthorized Activity................... 27 3.7 Define Actions to Take When Unauthorized Activity is Suspected.. 29 3.8 Communicating Security Policy................................... 30 3.9 Resources to Prevent Security Breaches.......................... 34 4. Types of Security Procedures..................................... 56 4.1 System Security Audits.......................................... 56 4.2 Account Management Procedures................................... 57 4.3 Password Management Procedures.................................. 57 4.4 Configuration Management Procedures............................. 60 5. Incident Handling................................................ 61 5.1 Overview........................................................ 61 5.2 Evaluation...................................................... 65 5.3 Possible Types of Notification.................................. 67 5.4 Response........................................................ 71 5.5 Legal/Investigative............................................. 73 5.6 Documentation Logs.............................................. 77 6. Establishing Post-Incident Procedures............................ 78 6.1 Overview........................................................ 78 6.2 Removing Vulnerabilities........................................ 78 6.3 Capturing Lessons Learned....................................... 80 Site Security Policy Handbook Working Group [Page 2] RFC 1244 Site Security Handbook July 1991 6.4 Upgrading Policies and Procedures............................... 81 7. References....................................................... 81 8. Annotated Bibliography........................................... 83 8.1 Computer Law.................................................... 84 8.2 Computer Security............................................... 85 8.3 Ethics.......................................................... 91 8.4 The Internet Worm............................................... 93 8.5 National Computer Security Center (NCSC)........................ 95 8.6 Security Checklists............................................. 99 8.7 Additional Publications......................................... 99 9. Acknlowledgements................................................101 10. Security Considerations.........................................101 11. Authors' Addresses..............................................101 1. Introduction 1.1 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. 1.2 Audience The audience 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. Site Security Policy Handbook Working Group [Page 3] RFC 1244 Site Security Handbook July 1991 1.3 Definitions 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. 1.4 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. 1.5 Scope 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. Site Security Policy Handbook Working Group [Page 4] RFC 1244 Site Security Handbook July 1991 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. 1.6 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 Site Security Policy Handbook Working Group [Page 5] RFC 1244 Site Security Handbook July 1991 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. Site Security Policy Handbook Working Group [Page 6] RFC 1244 Site Security Handbook July 1991 - 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? 1.7 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. 1.8 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 Site Security Policy Handbook Working Group [Page 7] RFC 1244 Site Security Handbook July 1991 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 Security Policy Handbook Working Group [Page 8] RFC 1244 Site Security Handbook July 1991 2. Establishing Official Site Policy on Computer Security 2.1 Brief Overview 2.1.1 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. 2.1.2 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. 2.1.3 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 Site Security Policy Handbook Working Group [Page 9] RFC 1244 Site Security Handbook July 1991 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. 2.1.4 Responsibilities 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. 2.2 Risk Assessment 2.2.1 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 Site Security Policy Handbook Working Group [Page 10] RFC 1244 Site Security Handbook July 1991 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. 2.2.2 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. Site Security Policy Handbook Working Group [Page 11] RFC 1244 Site Security Handbook July 1991 2.2.3 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. 2.2.3.1 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 3.9.7.3.1) has observed that well-known universities, government sites, and military sites seem to attract more intruders. 2.2.3.2 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. 2.2.3.3 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 Site Security Policy Handbook Working Group [Page 12] RFC 1244 Site Security Handbook July 1991 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. 2.3 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. 2.3.1 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. 2.3.2 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 Site Security Policy Handbook Working Group [Page 13] RFC 1244 Site Security Handbook July 1991 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: Site Security Policy Handbook Working Group [Page 14] RFC 1244 Site Security Handbook July 1991 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. 2.3.3 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 Site Security Policy Handbook Working Group [Page 15] RFC 1244 Site Security Handbook July 1991 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. 2.3.4 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. Site Security Policy Handbook Working Group [Page 16] RFC 1244 Site Security Handbook July 1991 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. 2.3.5 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 Site Security Policy Handbook Working Group [Page 17] RFC 1244 Site Security Handbook July 1991 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? 2.3.7 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). Site Security Policy Handbook Working Group [Page 18] RFC 1244 Site Security Handbook July 1991 2.4 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. 2.4.1 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. Site Security Policy Handbook Working Group [Page 19] RFC 1244 Site Security Handbook July 1991 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. 2.4.3 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. 2.4.5 Issues for Incident Handling Procedures Along with statements of policy, the document being prepared should include procedures for incident handling. This is covered Site Security Policy Handbook Working Group [Page 20] RFC 1244 Site Security Handbook July 1991 in detail in the next chapter. There should be procedures available that cover all facets of policy violation. 2.5 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". Site Security Policy Handbook Working Group [Page 21] RFC 1244 Site Security Handbook July 1991 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. Site Security Policy Handbook Working Group [Page 22] RFC 1244 Site Security Handbook July 1991 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. 2.6 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. 2.7 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. Site Security Policy Handbook Working Group [Page 23] RFC 1244 Site Security Handbook July 1991 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. 3.2 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. 3.2.1 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. Site Security Policy Handbook Working Group [Page 24] RFC 1244 Site Security Handbook July 1991 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. 3.2.2 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. 3.2.3 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. 3.2.4 "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 Site Security Policy Handbook Working Group [Page 25] RFC 1244 Site Security Handbook July 1991 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. 3.3.1 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. 3.3.2 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. 3.4 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 Site Security Policy Handbook Working Group [Page 26] RFC 1244 Site Security Handbook July 1991 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. 3.5 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 2.13.9.2), 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. 3.6 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. 3.6.1 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 Site Security Policy Handbook Working Group [Page 27] RFC 1244 Site Security Handbook July 1991 vigil can you expect to detect security violations in time to react to them. 3.6.2 Tools for Monitoring the System This section describes tools and methods for monitoring a system against unauthorized access and use. 3.6.2.1 Logging 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. 3.6.2.2 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 Site Security Policy Handbook Working Group [Page 28] RFC 1244 Site Security Handbook July 1991 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. 3.6.2.3 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. 3.6.3 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. Site Security Policy Handbook Working Group [Page 29] RFC 1244 Site Security Handbook July 1991 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. 3.8 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. 3.8.1 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. 3.8.1.1 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. 3.8.1.2 Account/Workstation Management Procedures Each user should be told how to properly manage their account Site Security Policy Handbook Working Group [Page 30] RFC 1244 Site Security Handbook July 1991 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. 3.8.1.3 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. 3.8.1.4 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. 3.8.2 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 Site Security Policy Handbook Working Group [Page 31] RFC 1244 Site Security Handbook July 1991 to communicate successful penetration of their systems to other administrators as a warning. 3.8.2.1 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. 3.8.2.2 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 Site Security Policy Handbook Working Group [Page 32] RFC 1244 Site Security Handbook July 1991 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. 3.8.2.3 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 Site Security Policy Handbook Working Group [Page 33] RFC 1244 Site Security Handbook July 1991 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. 3.8.2.4 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. 3.9 Resources to Prevent Security Breaches This section discusses software, hardware, and procedural resources that can be used to support your site security policy. 3.9.1 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 Site Security Policy Handbook Working Group [Page 34] RFC 1244 Site Security Handbook July 1991 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]. 3.9.2 Confidentiality 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. 3.9.2.1 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 Site Security Policy Handbook Working Group [Page 35] RFC 1244 Site Security Handbook July 1991 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. 3.9.2.1.1 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. 3.9.2.1.2 Crypt 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. Site Security Policy Handbook Working Group [Page 36] RFC 1244 Site Security Handbook July 1991 3.9.2.2 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. 3.9.3 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. 3.9.4 Information Integrity Information integrity refers to the state of information such that it is complete, correct, and unchanged from the last time in which Site Security Policy Handbook Working Group [Page 37] RFC 1244 Site Security Handbook July 1991 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. 3.9.4.1 Checksums 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. Site Security Policy Handbook Working Group [Page 38] RFC 1244 Site Security Handbook July 1991 3.9.4.2 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] 3.9.5 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. 3.9.5.1 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 Site Security Policy Handbook Working Group [Page 39] RFC 1244 Site Security Handbook July 1991 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). 3.9.5.2 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 th