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Special Publication 800-12: An Introduction to Computer Security - The NIST Handbook
Like other aspects of information processing systems, security is most effective and efficient if planned and managed throughout a computer system's life cycle, from initial planning, through design, implementation, and operation, to disposal.65 Many security-relevant events and analyses occur during a system's life. This chapter explains the relationship among them and how they fit together.66 It also discusses the important role of security planning in helping to ensure that security issues are addressed comprehensively.
This chapter examines:
Planning is used to help ensure that security is addressed in a comprehensive manner throughout a system's life cycle. For federal systems, the Computer Security Act of 1987 set forth a statuary requirement for the preparation of computer security plans for all sensitive systems.67 The intent and spirit of the Act is to improve computer security in the federal government, not to create paperwork. In keeping with this intent, the Office of Management and Budget (OMB) and NIST have guided agencies toward a planning process that emphasizes good planning and management of computer security within an agency and for each computer system. As emphasized in this chapter, computer security management should be a part of computer systems management. The benefit of having a distinct computer security plan is to ensure that computer security is not overlooked.
"The purpose of the system security plan is to provide a basic overview of the security and privacy requirements of the subject system and the agency's plan for meeting those requirements. The system security plan may also be viewed as documentation of the structured process of planning adequate, cost-effective security protection for a system."
-OMB Bulletin 90-08
The act required the submission of plans to NIST and the National Security Agency (NSA) for review and comment, a process which has been complemented. Current guidance on implementing the Act requires agencies to obtain independent review of computer security plans. This review may be internal or external, as deemed appropriate by the agency.
A "typical" plan briefly describes the important security considerations for the system and provides references to more detailed documents, such as system security plans, contingency plans, training programs, accreditation statements, incident handling plans, or audit results. This enables the plan to be used as a management tool without requiring repetition of existing documents. For smaller systems, the addresses specific vulnerabilities or other information that could compromise the system, it should be kept private. It also has to be kept up-to-date.
|Different people can provide security input throughout the life cycle of a system, including the accrediting official, data users, systems users, and system technical staff.|
Although a computer security plan can be developed for a system at any point in the life cycle, the recommended approach is to draw up the plan at the beginning of the computer system life cycle. Security, like other aspects of a computer system, is best managed if planned for throughout the computer system life cycle. It has been a tenet of the computer community that it costs ten times more to add a feature in a system after it has been designed than to include the feature in the system at the initial design phase. The principal reason for implementing security during a system's development is that it is more difficult to implement it later (as is usually reflected in the higher cost of doing so). It also tends to disrupt ongoing operations.
Security also needs to be incorporated into the later phases of the computer system life cycle to help ensure that security keeps up with changes in the system's environment, technology, procedures, and personnel. It also ensures that security is considered in system upgrades, including the purchase of new components or the design of new modules. Adding new security controls to a system after a security breach, mishap, or audit can lead to haphazard security that can be more expensive and less effective that security that is already integrated into the system. It can also significantly degrade system performance. Of course, it is virtually impossible to anticipate the whole array of problems that may arise during a system's lifetime. Therefore, it is generally useful to update the computer security plan at least at the end of each phase in the life cycle and after each re-accreditation. For many systems, it may be useful to update the plan more often.
Life cycle management also helps document security-relevant decisions, in addition to helping assure management that security is fully considered in all phases. This documentation benefits system management officials as well as oversight and independent audit groups. System management personnel use documentation as a self-check reminder of why decisions were made so that the impact of changes in the environment can be more easily assessed. Oversight and independent audit groups use the documentation in their reviews to verify that system management has done an adequate job and to highlight areas where security may have been overlooked. This includes examining whether the documentation accurately reflects how the system is actually being operated.
Within the federal government, the Computer Security Act of 1987 and its implementing instructions provide specific requirements for computer security plans. These plans are a form of documentation that helps ensure that security is considered not only during system design and development but also throughout the rest of the life cycle. Plans can also be used to be sure that requirements of Appendix III to OMB Circular A-130, as well as other applicable requirements, have been addressed.
There are many models for the computer system life cycle but most contain five basic phases, as pictured in Figure 8.1.
|Many different "life cycles" are associated with computer systems, including the system development, acquisition, and information life cycles.|
Each phase can apply to an entire system, a new component or module, or a system upgrade. As with other aspects of systems management, the level of detail and analysis for each activity described here is determined by many factors including size, complexity, system cost, and sensitivity.
Many people find the concept of a computer system life cycle confusing because many cycles occur within the broad framework of the entire computer system life cycle. For example, an organization could develop a system, using a system development life cycle. During the system's life, the organization might purchase new components, using the acquisition life cycle.
Moreover, the computer system life cycle itself is merely one component of other life cycles. For example, consider the information life cycle. Normally information, such as personnel data, is used much longer than the life of one computer system. If an employee works for an organization for thirty years and collects retirement for another twenty, the employee's automated personnel record will probably pass through many different organizational computer systems owned by the company. In addition, parts of the information will also be used in other computer systems, such as those of the Internal Revenue Service and the Social Security Administration.
This section reviews the security activities that arise in each stage of the computer system life cycle. (See Figure 8.1.)
The conceptual and early design process of a system involves the discovery of a need for a new system or enhancements to an existing system; early ideas as to system characteristics and proposed functionality; brainstorming sessions on architectural, performance, or functional system aspects; and environmental, financial, political, or other constraints. At the same time, the basic security aspects of a system should be developed along with the early system design. This can be done through a sensitivity assessment.
Security in the System Life Cycle
|The definition of sensitive is often misconstrued. Sensitive is synonymous with important or valuable. Some data is sensitive because it must be kept confidential. Much more data, however, is sensitive because its integrity or availability must be assured. The Computer Security Act and OMB Circular A-130 clearly state that information is sensitive if its unauthorized disclosure, modification (i.e., loss of integrity), or unavailability would harm the agency. In general, the more important a system is to the mission of the agency, the more sensitive it is.|
A sensitivity assessment looks at the sensitivity of both the information to be processed and the system itself. The assessment should consider legal implications, organization policy (including federal and agency policy if a federal system), and the functional needs of the system. Sensitivity is normally expressed in terms of integrity, availability, and confidentiality. Such factors as the importance of the system to the organization's mission and the consequences of unauthorized modification, unauthorized disclosure, or unavailability of the system or data need to be examined when assessing sensitivity. To address these types of issues, the people who use or own the system or information should participate in the assessment.
A sensitivity assessment should answer the following questions:
The sensitivity assessment starts an analysis of security that continues throughout the life cycle. The assessment helps determine if the project needs special security oversight, if further analysis is needed before committing to begin system development (to ensure feasibility at a reasonable cost), or in rare instances, whether the security requirements are so strenuous and costly that system development or acquisition will not be pursued. The sensitivity assessment can be included with the system initiation documentation either a separate document or as a section of another planning document. The development of security features, procedures, and assurances, described in the next section, builds on the sensitivity assessment.
A sensitivity assessment can also be performed during the planning stagers of system upgrades (for either upgrades being procured or developed in house). In this case, the assessment focuses on the affected areas. If the upgrade significantly affects the original assessment, steps can be taken to analyze the impact on the rest of the system. For example, are new controls needed? Will some controls become necessary?
For most systems, the development/acquisition phase is more complicated than the initiation phase. Security activities can be divided into three parts:
These divisions apply to systems that are designed and built in house, to systems that are purchased, and to systems developed using a hybrid approach.
During the phase, technical staff and system sponsors should actively work together to ensure that the technical designs reflect the system's security needs. As with development and incorporation of other system requirements, this process requires an open dialogue between technical staff and system sponsors. It is important to address security requirements effectively in synchronization with development of the overall system.
During the first part of the development / acquisition phase, system planners define the requirements of the system. Security requirements should be developed at the same time. These requirements can be expressed as technical features (e.g., access controls), assurances (e.g., background checks for system developers), or operational practices (e.g., awareness and training). System security requirements, like other system requirements, are derived from a number of sources including law, policy, applicable standards and guidelines, functional needs of the system, and cost-benefit tradeoffs.
Law. Besides specific laws that place security requirements on information, such as the Privacy Act of 1974, there are laws, court cases, legal options, and other similar legal material that may affect security directly or indirectly.
Policy. As discussed in Chapter 5, management officials issue several different types of policy. System security requirements are often derived from issue-specific policy.
Standards and Guidelines. International, national, and organizational standards and guidelines are another source for determining security features, assurances, and operational practices. Standards and guidelines are often written in an "if then" manner (e.g., if the system is encrypting data, then a particular cryptographic algorithm should be used). Many organizations specify baseline controls for different types of systems, such as administrative, mission- or business- critical, or proprietary. As required, special care should be given to interoperability standards.
Functional Needs of the System. The purpose of security is to support the function of the system, not to undermine it. Therefore, many aspects of the function of the system will produce related security requirements.
Cost-Benefit Analysis. When considering security, cost-benefit analysis is done through risk assessment, which examines the assets, threats, and vulnerabilities of the system in order to determine the most appropriate, cost-effective safeguards (that comply with applicable laws, policy, standards, and the functional needs of the system). Appropriate safeguards are normally those whose anticipated benefits outweigh their costs. Benefits and cost include monetary and nonmonetary issues, such as prevented losses, maintaining an organization's reputation, decreased user friendliness, or increased system administration.
Risk assessment, like cost-benefit analysis, is used to support decision-making. It helps managers select cost-effective safeguards. The extent of the risk assessment, like that of other cost-benefit analyses, should be commensurate with the complexity and cost (normally an indicator of complexity) of the system and the expected benefits of the assessment. Risk assessment is further discussed n Chapter 7.
Risk assessment can be performed during the requirements analysis phase of a procurement or the design phase of a system development cycle. Risk should also normally be assessed during the development/acquisition phase of a system upgrade. The risk assessment may be performed once or multiple times, depending upon the projects methodology.
Care should be taken in differentiating between security risk assessment and project risk analysis. Many system development and acquisition projects analyze the risk of failing to successfully complete the project - a different activity from security risk assessment.
Determining security features, assurances, and operational practices can yield significant security information and often voluminous requirements. This information needs to be validated, updated, and organized into the detailed security protection requirements and specifications used by systems designers or purchasers. Specifications can take on quite different forms, depending on the methodology used for to develop the system, or whether the system, or parts of the system, are being purchased off the shelf.
|Developing testing specifications early can be critical to being able to cost-effectively test security features.|
As specifications are developed, it may be necessary to update initial risk assessments. A safeguard recommended by the risk assessment could be incompatible with other requirements or a control may be difficult to implement. For example, a security requirement that prohibits dial-in access could prevent employees from checking their e-mail while away from the office.69
Besides the technical and operational controls of a system, assurance also should be addressed. The degree to which assurance (that the security features and practices can and do work correctly and effectively) is needed should be determined early. Once the desired level of assurance is determined, it is necessary to figure out how the system will be tested or reviewed to determine whether the specifications have been satisfied (to obtain the desired assurance). This applies to both system developments and acquisitions. For example, if rigorous assurance is needed, the ability to test the system or to provide another form of initial and ongoing assurance needs to be designed into the system or otherwise provided for. See Chapter 9 for more information.
During this phase, the system is actually built or bought. If the system is being built, security activities may include developing the system's security aspects, monitoring the development process itself for security problems, responding to changes, and monitoring threat. Threats or vulnerabilities that may arise during the development phase include Trojan horses, incorrect code, poorly functioning development tools, manipulation of code, and malicious insiders.
If the system is being acquired off the shelf, security activities may include monitoring to ensure security is a part of market surveys, contract solicitation documents, and evaluation of proposed systems. Many systems use a combination of development and acquisition. In this case, security activities include both sets.
|In federal government contracting, it is often useful if personnel with security expertise participate as members of the source selection board to help evaluate the security aspects of proposals.|
As the system is built or bought, choices are made about the system, which can affect security. These choices include selection of specific off-the-shelf products, finalizing an architecture, or selecting a processing site or platform. Additional security analysis will probably be necessary.
In addition to obtaining the system, operational practices need to be developed. These refer to human activities that take place around the system such as contingency planning, awareness and training, and preparing documentation. The chapters in the Operational Controls section of this handbook discuss these areas. These areas, like technical specifications, should be considered from the beginning of the development and acquisition phase.
A separate implementation phase is not always specified in some life cycle planning efforts. (It is often incorporated into the end of development and acquisition or the beginning of operation and maintenance.) However, from a security point of view, a critical security activity, accreditation, occurs between development and the start of system operation. The other activities described in this section, turning on the controls and testing, are often incorporated at the end of the development/acquisition phase.
While obvious, this activity is often overlooked. When acquired, a system often comes with security features disabled. These need to be enabled and configured. For many systems this is a complex task requiring significant skills. Custom-developed systems may also require similar work.
System security testing includes both the testing of the particular parts of the system that have been developed or acquired and the testing of the entire system. Security management, physical facilities, personnel, procedures, the use of commercial or in-house services (such as networking services), and contingency planning are examples of areas that affect the security of the entire system, but may be specified outside of the development or acquisition cycle. Since only items within the development of acquisition cycle will have been tested during system acceptance testing, separate tests or reviews may need to be performed for these additional security elements.
Security certification is a formal testing of the security safeguards implemented in the computer system to determine whether they meet applicable requirements and specifications.70 To provide more reliable technical information, certification is often performed by an independent reviewer, rather than by the people who designed the system.
System security accreditation is the formal authorization by the accrediting (management) official for system operation and an explicit acceptance of risk. It is usually supported by a review of the system, including its management, operational, and technical controls. This review may include a detailed technical evaluation (such as a Federal Information Processing Standard 102 certification, particularly for complex, critical, or high-risk systems), security evaluation, risk assessment, audit, or other such review. If the life cycle process is being used to manage a project (such as a system upgrade), it is important to recognize that the accreditation is for the entire system, not just for the new addition.
Sample Accreditation Statement
In accordance with (Organization Directive), I hereby issue an accreditation for (name of system). This accreditation is my formal declaration that a satisfactory level of operational security is present and that the system can operate under reasonable risk. This accreditation is valid for three years. The system will be re-evaluated annually to determine if changes have occurred affecting its security.
The best way to view computer security accreditation is as a form of quality control. It forces managers and technical staff to work together to find the best fit for security, given technical constraints, operational constraints, and mission requirements. The accreditation process obliges managers to make critical decisions regarding the adequacy of security safeguards. A decision based on reliable information about the effectiveness of technical and non-technical safeguards and the residual risk is more likely to be a sound decision.
After deciding on the acceptability of security safeguards and residual risks, the accrediting official should issue a formal accreditation statement. While most flaws in system security are not severe enough to remove an operational system from service or to prevent a new system from becoming operational, the flaws may require some restrictions on operation (e.g., limitations on dial-in access or electronic connections to other organizations). In some cases, an interim accreditation may be granted, allowing the system to operate requiring review at the end of the interim period, presumably after security upgrades have been made.
Many security activities take place during the operational phase of a system's life. In general these fall into three areas: (1) security operations and administration; (2) operational assurance; and (3) periodic re-analysis of the security. Figure 8.2 diagrams the flow of security activities during the operational phase.
Operation of a system involves many security activities discussed throughout this handbook. Performing backups, holding training classes, managing cryptographic keys, keeping up with user administration and access privileges, and updating security software are some examples.
|Operational assurance examines whether a system is operated according to its current security requirements. This includes both the actions of people who operate or use the system and the functioning of technical controls.|
Security is never perfect when a system is implemented. In addition, system users and operators discover new ways to intentionally or unintentionally bypass or subvert security. Changes in the system or the environment can create new vulnerabilities. Strict adherence to procedures is rare over time, and procedures become outdated. Thinking risk is minimal, users may tend to bypass security measures and procedures.
As shown in Figure 8.2, changes occur. Operational assurance is one way of becoming aware of these changes whether they are new vulnerabilities (or old vulnerabilities that have not been corrected), system changes, or environmental changes. Operational assurance is the process of reviewing an operational system to see that security controls, both automated and manual, are functioning correctly and effectively.
To maintain operational assurance, organizations use two basic methods: system audits and monitoring. These terms are used loosely within the computer security community and often overlap. A system audit is a one-time or periodic event to evaluate security. Monitoring refers to an ongoing activity that examines either the system or the users. In general, the more "real-time" an activity is, the more it falls into the category of monitoring. (See Chapter 9.)
|Security change management helps develop new security requirements.|
Computer systems and the environments in which they operate change continually. In response to various events such as user complaints, availability of new features and services, or the discovery of new threats and vulnerabilities, system managers and users modify the system and incorporate new features, new procedures, and software updates.
The environment in which the system operates also changes. Networking and interconnections tend to increase. A new user group may be added, possibly external groups or anonymous groups. New threats may emerge, such as increases in network intrusions or the spread of personal computer viruses. If the system has a configuration control board or other structure to manage technical system changes, a security specialist can be assigned to the board to make determinations about whether (and if so, how) changes will affect security.
Security should also be considered during system upgrades (and other planned changes) and in determining the impact of unplanned changes. As shown in Figure 8.2, when a change occurs or is planned, a determination is made whether the change is major or minor. A major change, such as reengineering the structure of the system, significantly affects the system. Major changes often involve the purchase of new hardware, software, or services or the development of new software modules.
An organization does not need to have a specific cutoff for major-minor change decisions. A sliding scale between the two can be implemented by using a combination of the following methods:
Periodically, it is useful to formally reexamine the security of a system from a wider perspective. The analysis, which leads to reaccredidation, should address such questions as: Is the security still sufficient? Are major changes needed?
|It is important to consider legal requirements for records retention when disposing of computer systems. For federal systems, system management officials should consult with their agency office responsible for retaining and archiving federal records.|
The reaccredidation should address high-level security and management concerns as well as the implementation of the security. It is not always necessary to perform a new risk assessment or certification in conjunction with the re-accreditation, but the activities support each other (and both need be performed periodically). The more extensive system changes have been, the more extensive the analyses should be (e.g., a risk assessment or re-certification). A risk assessment is likely to uncover security concerns that result in system changes. After the system has been changed, it may need testing (including certification). Management then reaccredits the system for continued operation if the risk is acceptable.
Since electronic information
is easy to copy and transmit, information that is sensitive to disclosure
often needs to be controlled throughout the computer system life cycle
so that managers can ensure its proper disposition. The removal of information
from a storage medium (such as a hard disk or tape) is called sanitization.
Different kinds of sanitization provide different levels of protection.
A distinction can be made between clearing information (rendering it unrecoverable
by keyboard attack) and purging (rendering information unrecoverable against
laboratory attack). There are three general methods of purging media:
overwriting, degaussing (for magnetic media only), and destruction.
The disposal phase of the computer
system life cycle involves the disposition of information, hardware, and software.
Information may be moved to another system, archived, discarded, or destroyed.
When archiving information, consider the method for retrieving the information
in the future. The technology used to create the records may not be readily
available in the future.
Hardware and software can be sold, given away, or discarded. There is rarely a need to destroy hardware, except for some storage media containing confidential information that cannot be sanitized without destruction. The disposition of software needs to be in keeping with its license or other agreements with the developer, if applicable. Some licenses are site-specific or contain other agreements that prevent the software from being transferred.
Measures may also have to be taken for the future use of data that has been encrypted, such as taking appropriate steps to ensure the secure long-term storage of cryptographic keys.
Like many management controls, life cycle planning relies upon other controls. Three closely linked control areas are policy, assurance, and risk management.
Policy. The development of system-specific policy is an integral part of determining the security requirements.
Assurance. Good life cycle management provides assurance that security is appropriately considered in system design and operation.
Risk Management. The maintenance of security throughout the operational phase of a system is a process of risk management: analyzing risk, reducing risk, and monitoring safeguards. Risk assessment is a critical element in designing the security of systems and in reaccreditations.
Security is a factor throughout the life cycle of a system. Sometimes security choices are made by default, without anyone analyzing why choices are made; sometimes security choices are made carefully, based on analysis. The first case is likely to result in a system with poor security that is susceptible to many types of loss. In the second case, the cost of life cycle management should be much smaller than the losses avoided. The major cost considerations for life cycle management are personnel costs and some delays as the system progresses through the life cycle for completing analyses and reviews and obtaining management approvals.
It is possible to overmanage a system:
to spend more time planning, designing, and analyzing risk than is necessary.
Planning, by itself, does not further the mission or business of an organization.
Therefore, while security life cycle management can yield significant benefits,
the effort should be commensurate with the system's size, complexity, and sensitivity
and the risks associated with the system. In general, the higher the value of
the system, the newer the system's architecture, technologies, and practices,
and the worse the impact if the system security fails, the more effort should
be spent on life cycle management.
Communications Security Establishment. A Framework for Security Risk Management in Information Technology Systems. Canada.
Dykman, Charlene A. ed., and Charles K. Davis, asc. ed. Control Objectives -- Controls in an Information Systems Environment: Objectives, Guidelines, and Audit Procedures. (Fourth edition). Carol Stream, IL: The EDP Auditors Foundation, Inc., April 1992.
Guttman, Barbara. Computer Security Considerations in Federal Procurements: A Guide for Procurement Initiators, Contracting Officers, and Computer Security Officials. Special Publication 800-4. Gaithersburg, MD: National Institute of Standards and Technology, March 1992.
Institute of Internal Auditors Research Foundation. System Auditability and Control Report. Altamonte Springs, FL: The Institute of Internal Auditors, 1991.
Murphy, Michael, and Xenia Ley Parker. Handbook of EDP Auditing, especially Chapter 2 "The Auditing Profession," and Chapter 3, "The EDP Auditing Profession." Boston, MA: Warren, Gorham & Lamont, 1989.
National Bureau of Standards. Guideline for Computer Security Certification and Accreditation. Federal Information Processing Standard Publication 102. September 1983.
National Institute of Standards and Technology. "Disposition of Sensitive Automated Information." Computer Systems Laboratory Bulletin. October 1992.
National Institute of Standards and Technology. "Sensitivity of Information." Computer Systems Laboratory Bulletin. November 1992.
Office of Management and Budget. "Guidance for Preparation of Security Plans for Federal Computer Systems That Contain Sensitive Information." OMB Bulletin 90-08. 1990.
Ruthberg, Zella G, Bonnie T. Fisher and John W. Lainhart IV. System Development Auditor. Oxford, England: Elsevier Advanced Technology, 1991.
Ruthberg, Z., et al. Guide to Auditing for Controls and Security: A System Development Life Cycle Approach. Special Publication 500-153. Gaithersburg, MD: National Institute of Standards. April 1988.
Vickers Benzel, T. C. Developing Trusted Systems Using DOD-STD-2167A. Oakland, CA: IEEE Computer Society Press, 1990.
Wood, C. "Building Security
Into Your System Reduces the Risk of a Breach." LAN Times, 10(3),
1993. p 47.
65. A computer system refers to a collection of processes, hardware, and software that perform a function. This includes applications, networks, or support systems.
66. Although this chapter addresses a life cycle process that starts with system initiation, the process can be initiated at any point in the life cycle.
67. An organization will typically have many computer security plans. However, it is not necessary that a separate and distinct plan exist for every physical system (e.g., PCs). Plans may address, for example, the computing resources within an operational element, a major application, or a group of similar systems (either technologically or functionally).
68. For brevity and because of the uniqueness of each system, none of these discussions can include the details of all possible security activities at any particular life cycle phase.
69. This is an example of a risk-based decision.
Some federal agencies use a broader definition of the term certification
to refer to security reviews or evaluations, formal or information,
that take place prior to and are used to support accreditation.
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