NIST SP 800-53 Rev. 5 Common Control Library

NIST control statement

Enforce approved authorizations for logical access to information and system resources in accordance with applicable access control policies.

Discussion

Access control policies control access between active entities or subjects (i.e., users or processes acting on behalf of users) and passive entities or objects (i.e., devices, files, records, domains) in organizational systems. In addition to enforcing authorized access at the system level and recognizing that systems can host many applications and services in support of mission and business functions, access enforcement mechanisms can also be employed at the application and service level to provide increased information security and privacy. In contrast to logical access controls that are implemented within the system, physical access controls are addressed by the controls in the Physical and Environmental Protection ( [PE](#pe) ) family.

NIST control statement

Withdrawn in this source release; review the official source for disposition.

NIST control statement

Enforce dual authorization for [Assignment: privileged commands and/or other actions].

Discussion

Dual authorization, also known as two-person control, reduces risk related to insider threats. Dual authorization mechanisms require the approval of two authorized individuals to execute. To reduce the risk of collusion, organizations consider rotating dual authorization duties. Organizations consider the risk associated with implementing dual authorization mechanisms when immediate responses are necessary to ensure public and environmental safety.

NIST control statement

Enforce [Assignment: organization-defined mandatory access control policy] over the set of covered subjects and objects specified in the policy, and where the policy: (a) Is uniformly enforced across the covered subjects and objects within the system; (b) Specifies that a subject that has been granted access to information is constrained from doing any of the following; (1) Passing the information to unauthorized subjects or objects; (2) Granting its privileges to other subjects; (3) Changing one or more security attributes (specified by the policy) on subjects, objects, the system, or system components; (4) Choosing the security attributes and attribute values (specified by the policy) to be associated with newly created or modified objects; and (5) Changing the rules governing access control; and (c) Specifies that [Assignment: subjects] may explicitly be granted [Assignment: privileges] such that they are not limited by any defined subset (or all) of the above constraints.

Discussion

Mandatory access control is a type of nondiscretionary access control. Mandatory access control policies constrain what actions subjects can take with information obtained from objects for which they have already been granted access. This prevents the subjects from passing the information to unauthorized subjects and objects. Mandatory access control policies constrain actions that subjects can take with respect to the propagation of access control privileges; that is, a subject with a privilege cannot pass that privilege to other subjects. The policy is uniformly enforced over all subjects and objects to which the system has control. Otherwise, the access control policy can be circumvented. This enforcement is provided by an implementation that meets the reference monitor concept as described in [AC-25](#ac-25) . The policy is bounded by the system (i.e., once the information is passed outside of the control of the system, additional means may be required to ensure that the constraints on the information remain in effect). The trusted subjects described above are granted privileges consistent with the concept of least privilege (see [AC-6](#ac-6) ). Trusted subjects are only given the minimum privileges necessary for satisfying organizational mission/business needs relative to the above policy. The control is most applicable when there is a mandate that establishes a policy regarding access to controlled unclassified information or classified information and some users of the system are not authorized access to all such information resident in the system. Mandatory access control can operate in conjunction with discretionary access control as described in [AC-3(4)](#ac-3.4) . A subject constrained in its operation by mandatory access control policies can still operate under the less rigorous constraints of AC-3(4), but mandatory access control policies take precedence over the less rigorous constraints of AC-3(4). For example, while a mandatory access control policy imposes a constraint that prevents a subject from passing information to another subject operating at a different impact or classification level, AC-3(4) permits the subject to pass the information to any other subject with the same impact or classification level as the subject. Examples of mandatory access control policies include the Bell-LaPadula policy to protect confidentiality of information and the Biba policy to protect the integrity of information.

NIST control statement

Enforce [Assignment: organization-defined discretionary access control policy] over the set of covered subjects and objects specified in the policy, and where the policy specifies that a subject that has been granted access to information can do one or more of the following: (a) Pass the information to any other subjects or objects; (b) Grant its privileges to other subjects; (c) Change security attributes on subjects, objects, the system, or the system’s components; (d) Choose the security attributes to be associated with newly created or revised objects; or (e) Change the rules governing access control.

Discussion

When discretionary access control policies are implemented, subjects are not constrained with regard to what actions they can take with information for which they have already been granted access. Thus, subjects that have been granted access to information are not prevented from passing the information to other subjects or objects (i.e., subjects have the discretion to pass). Discretionary access control can operate in conjunction with mandatory access control as described in [AC-3(3)](#ac-3.3) and [AC-3(15)](#ac-3.15) . A subject that is constrained in its operation by mandatory access control policies can still operate under the less rigorous constraints of discretionary access control. Therefore, while [AC-3(3)](#ac-3.3) imposes constraints that prevent a subject from passing information to another subject operating at a different impact or classification level, [AC-3(4)](#ac-3.4) permits the subject to pass the information to any subject at the same impact or classification level. The policy is bounded by the system. Once the information is passed outside of system control, additional means may be required to ensure that the constraints remain in effect. While traditional definitions of discretionary access control require identity-based access control, that limitation is not required for this particular use of discretionary access control.

NIST control statement

Prevent access to [Assignment: security-relevant information] except during secure, non-operable system states.

Discussion

Security-relevant information is information within systems that can potentially impact the operation of security functions or the provision of security services in a manner that could result in failure to enforce system security and privacy policies or maintain the separation of code and data. Security-relevant information includes access control lists, filtering rules for routers or firewalls, configuration parameters for security services, and cryptographic key management information. Secure, non-operable system states include the times in which systems are not performing mission or business-related processing, such as when the system is offline for maintenance, boot-up, troubleshooting, or shut down.

NIST control statement

Withdrawn in this source release; review the official source for disposition.

NIST control statement

Enforce a role-based access control policy over defined subjects and objects and control access based upon [Assignment: organization-defined roles and users authorized to assume such roles].

Discussion

Role-based access control (RBAC) is an access control policy that enforces access to objects and system functions based on the defined role (i.e., job function) of the subject. Organizations can create specific roles based on job functions and the authorizations (i.e., privileges) to perform needed operations on the systems associated with the organization-defined roles. When users are assigned to specific roles, they inherit the authorizations or privileges defined for those roles. RBAC simplifies privilege administration for organizations because privileges are not assigned directly to every user (which can be a large number of individuals) but are instead acquired through role assignments. RBAC can also increase privacy and security risk if individuals assigned to a role are given access to information beyond what they need to support organizational missions or business functions. RBAC can be implemented as a mandatory or discretionary form of access control. For organizations implementing RBAC with mandatory access controls, the requirements in [AC-3(3)](#ac-3.3) define the scope of the subjects and objects covered by the policy.

NIST control statement

Enforce the revocation of access authorizations resulting from changes to the security attributes of subjects and objects based on [Assignment: rules].

Discussion

Revocation of access rules may differ based on the types of access revoked. For example, if a subject (i.e., user or process acting on behalf of a user) is removed from a group, access may not be revoked until the next time the object is opened or the next time the subject attempts to access the object. Revocation based on changes to security labels may take effect immediately. Organizations provide alternative approaches on how to make revocations immediate if systems cannot provide such capability and immediate revocation is necessary.

NIST control statement

Release information outside of the system only if: (a) The receiving [Assignment: system or system component] provides [Assignment: controls] ; and (b) [Assignment: controls] are used to validate the appropriateness of the information designated for release.

Discussion

Organizations can only directly protect information when it resides within the system. Additional controls may be needed to ensure that organizational information is adequately protected once it is transmitted outside of the system. In situations where the system is unable to determine the adequacy of the protections provided by external entities, as a mitigation measure, organizations procedurally determine whether the external systems are providing adequate controls. The means used to determine the adequacy of controls provided by external systems include conducting periodic assessments (inspections/tests), establishing agreements between the organization and its counterpart organizations, or some other process. The means used by external entities to protect the information received need not be the same as those used by the organization, but the means employed are sufficient to provide consistent adjudication of the security and privacy policy to protect the information and individuals’ privacy. Controlled release of information requires systems to implement technical or procedural means to validate the information prior to releasing it to external systems. For example, if the system passes information to a system controlled by another organization, technical means are employed to validate that the security and privacy attributes associated with the exported information are appropriate for the receiving system. Alternatively, if the system passes information to a printer in organization-controlled space, procedural means can be employed to ensure that only authorized individuals gain access to the printer.

NIST control statement

Employ an audited override of automated access control mechanisms under [Assignment: conditions] by [Assignment: roles].

Discussion

In certain situations, such as when there is a threat to human life or an event that threatens the organization’s ability to carry out critical missions or business functions, an override capability for access control mechanisms may be needed. Override conditions are defined by organizations and used only in those limited circumstances. Audit events are defined in [AU-2](#au-2) . Audit records are generated in [AU-12](#au-12).

NIST control statement

Restrict access to data repositories containing [Assignment: information types].

Discussion

Restricting access to specific information is intended to provide flexibility regarding access control of specific information types within a system. For example, role-based access could be employed to allow access to only a specific type of personally identifiable information within a database rather than allowing access to the database in its entirety. Other examples include restricting access to cryptographic keys, authentication information, and selected system information.

NIST control statement

(a) Require applications to assert, as part of the installation process, the access needed to the following system applications and functions: [Assignment: system applications and functions]; (b) Provide an enforcement mechanism to prevent unauthorized access; and (c) Approve access changes after initial installation of the application.

Discussion

Asserting and enforcing application access is intended to address applications that need to access existing system applications and functions, including user contacts, global positioning systems, cameras, keyboards, microphones, networks, phones, or other files.

NIST control statement

Enforce attribute-based access control policy over defined subjects and objects and control access based upon [Assignment: attributes].

Discussion

Attribute-based access control is an access control policy that restricts system access to authorized users based on specified organizational attributes (e.g., job function, identity), action attributes (e.g., read, write, delete), environmental attributes (e.g., time of day, location), and resource attributes (e.g., classification of a document). Organizations can create rules based on attributes and the authorizations (i.e., privileges) to perform needed operations on the systems associated with organization-defined attributes and rules. When users are assigned to attributes defined in attribute-based access control policies or rules, they can be provisioned to a system with the appropriate privileges or dynamically granted access to a protected resource. Attribute-based access control can be implemented as either a mandatory or discretionary form of access control. When implemented with mandatory access controls, the requirements in [AC-3(3)](#ac-3.3) define the scope of the subjects and objects covered by the policy.

NIST control statement

Provide [Assignment: mechanisms] to enable individuals to have access to the following elements of their personally identifiable information: [Assignment: elements].

Discussion

Individual access affords individuals the ability to review personally identifiable information about them held within organizational records, regardless of format. Access helps individuals to develop an understanding about how their personally identifiable information is being processed. It can also help individuals ensure that their data is accurate. Access mechanisms can include request forms and application interfaces. For federal agencies, [PRIVACT](#18e71fec-c6fd-475a-925a-5d8495cf8455) processes can be located in systems of record notices and on agency websites. Access to certain types of records may not be appropriate (e.g., for federal agencies, law enforcement records within a system of records may be exempt from disclosure under the [PRIVACT](#18e71fec-c6fd-475a-925a-5d8495cf8455) ) or may require certain levels of authentication assurance. Organizational personnel consult with the senior agency official for privacy and legal counsel to determine appropriate mechanisms and access rights or limitations.

NIST control statement

(a) Enforce [Assignment: organization-defined mandatory access control policy] over the set of covered subjects and objects specified in the policy; and (b) Enforce [Assignment: organization-defined discretionary access control policy] over the set of covered subjects and objects specified in the policy.

Discussion

Simultaneously implementing a mandatory access control policy and a discretionary access control policy can provide additional protection against the unauthorized execution of code by users or processes acting on behalf of users. This helps prevent a single compromised user or process from compromising the entire system.

NIST control statement

a. Identify and document [Assignment: duties of individuals] ; and b. Define system access authorizations to support separation of duties.

Discussion

Separation of duties addresses the potential for abuse of authorized privileges and helps to reduce the risk of malevolent activity without collusion. Separation of duties includes dividing mission or business functions and support functions among different individuals or roles, conducting system support functions with different individuals, and ensuring that security personnel who administer access control functions do not also administer audit functions. Because separation of duty violations can span systems and application domains, organizations consider the entirety of systems and system components when developing policy on separation of duties. Separation of duties is enforced through the account management activities in [AC-2](#ac-2) , access control mechanisms in [AC-3](#ac-3) , and identity management activities in [IA-2](#ia-2), [IA-4](#ia-4) , and [IA-12](#ia-12).

NIST control statement

Prevent non-privileged users from executing privileged functions.

Discussion

Privileged functions include disabling, circumventing, or altering implemented security or privacy controls, establishing system accounts, performing system integrity checks, and administering cryptographic key management activities. Non-privileged users are individuals who do not possess appropriate authorizations. Privileged functions that require protection from non-privileged users include circumventing intrusion detection and prevention mechanisms or malicious code protection mechanisms. Preventing non-privileged users from executing privileged functions is enforced by [AC-3](#ac-3).

NIST control statement

a. Establish and document usage restrictions, configuration/connection requirements, and implementation guidance for each type of remote access allowed; and b. Authorize each type of remote access to the system prior to allowing such connections.

Discussion

Remote access is access to organizational systems (or processes acting on behalf of users) that communicate through external networks such as the Internet. Types of remote access include dial-up, broadband, and wireless. Organizations use encrypted virtual private networks (VPNs) to enhance confidentiality and integrity for remote connections. The use of encrypted VPNs provides sufficient assurance to the organization that it can effectively treat such connections as internal networks if the cryptographic mechanisms used are implemented in accordance with applicable laws, executive orders, directives, regulations, policies, standards, and guidelines. Still, VPN connections traverse external networks, and the encrypted VPN does not enhance the availability of remote connections. VPNs with encrypted tunnels can also affect the ability to adequately monitor network communications traffic for malicious code. Remote access controls apply to systems other than public web servers or systems designed for public access. Authorization of each remote access type addresses authorization prior to allowing remote access without specifying the specific formats for such authorization. While organizations may use information exchange and system connection security agreements to manage remote access connections to other systems, such agreements are addressed as part of [CA-3](#ca-3) . Enforcing access restrictions for remote access is addressed via [AC-3](#ac-3).

NIST control statement

a. Identify the types of events that the system is capable of logging in support of the audit function: [Assignment: event types]; b. Coordinate the event logging function with other organizational entities requiring audit-related information to guide and inform the selection criteria for events to be logged; c. Specify the following event types for logging within the system: [Assignment: organization-defined event types (subset of the event types defined in [AU-2a.](#au-2_smt.a)) along with the frequency of (or situation requiring) logging for each identified event type]; d. Provide a rationale for why the event types selected for logging are deemed to be adequate to support after-the-fact investigations of incidents; and e. Review and update the event types selected for logging [Assignment: frequency].

Discussion

An event is an observable occurrence in a system. The types of events that require logging are those events that are significant and relevant to the security of systems and the privacy of individuals. Event logging also supports specific monitoring and auditing needs. Event types include password changes, failed logons or failed accesses related to systems, security or privacy attribute changes, administrative privilege usage, PIV credential usage, data action changes, query parameters, or external credential usage. In determining the set of event types that require logging, organizations consider the monitoring and auditing appropriate for each of the controls to be implemented. For completeness, event logging includes all protocols that are operational and supported by the system. To balance monitoring and auditing requirements with other system needs, event logging requires identifying the subset of event types that are logged at a given point in time. For example, organizations may determine that systems need the capability to log every file access successful and unsuccessful, but not activate that capability except for specific circumstances due to the potential burden on system performance. The types of events that organizations desire to be logged may change. Reviewing and updating the set of logged events is necessary to help ensure that the events remain relevant and continue to support the needs of the organization. Organizations consider how the types of logging events can reveal information about individuals that may give rise to privacy risk and how best to mitigate such risks. For example, there is the potential to reveal personally identifiable information in the audit trail, especially if the logging event is based on patterns or time of usage. Event logging requirements, including the need to log specific event types, may be referenced in other controls and control enhancements. These include [AC-2(4)](#ac-2.4), [AC-3(10)](#ac-3.10), [AC-6(9)](#ac-6.9), [AC-17(1)](#ac-17.1), [CM-3f](#cm-3_smt.f), [CM-5(1)](#cm-5.1), [IA-3(3)(b)](#ia-3.3_smt.b), [MA-4(1)](#ma-4.1), [MP-4(2)](#mp-4.2), [PE-3](#pe-3), [PM-21](#pm-21), [PT-7](#pt-7), [RA-8](#ra-8), [SC-7(9)](#sc-7.9), [SC-7(15)](#sc-7.15), [SI-3(8)](#si-3.8), [SI-4(22)](#si-4.22), [SI-7(8)](#si-7.8) , and [SI-10(1)](#si-10.1) . Organizations include event types that are required by applicable laws, executive orders, directives, policies, regulations, standards, and guidelines. Audit records can be generated at various levels, including at the packet level as information traverses the network. Selecting the appropriate level of event logging is an important part of a monitoring and auditing capability and can identify the root causes of problems. When defining event types, organizations consider the logging necessary to cover related event types, such as the steps in distributed, transaction-based processes and the actions that occur in service-oriented architectures.

NIST control statement

Define, document, approve, and enforce physical and logical access restrictions associated with changes to the system.

Discussion

Changes to the hardware, software, or firmware components of systems or the operational procedures related to the system can potentially have significant effects on the security of the systems or individuals’ privacy. Therefore, organizations permit only qualified and authorized individuals to access systems for purposes of initiating changes. Access restrictions include physical and logical access controls (see [AC-3](#ac-3) and [PE-3](#pe-3) ), software libraries, workflow automation, media libraries, abstract layers (i.e., changes implemented into external interfaces rather than directly into systems), and change windows (i.e., changes occur only during specified times).

NIST control statement

Manage system authenticators by: a. Verifying, as part of the initial authenticator distribution, the identity of the individual, group, role, service, or device receiving the authenticator; b. Establishing initial authenticator content for any authenticators issued by the organization; c. Ensuring that authenticators have sufficient strength of mechanism for their intended use; d. Establishing and implementing administrative procedures for initial authenticator distribution, for lost or compromised or damaged authenticators, and for revoking authenticators; e. Changing default authenticators prior to first use; f. Changing or refreshing authenticators [Assignment: time period by authenticator type] or when [Assignment: events] occur; g. Protecting authenticator content from unauthorized disclosure and modification; h. Requiring individuals to take, and having devices implement, specific controls to protect authenticators; and i. Changing authenticators for group or role accounts when membership to those accounts changes.

Discussion

Authenticators include passwords, cryptographic devices, biometrics, certificates, one-time password devices, and ID badges. Device authenticators include certificates and passwords. Initial authenticator content is the actual content of the authenticator (e.g., the initial password). In contrast, the requirements for authenticator content contain specific criteria or characteristics (e.g., minimum password length). Developers may deliver system components with factory default authentication credentials (i.e., passwords) to allow for initial installation and configuration. Default authentication credentials are often well known, easily discoverable, and present a significant risk. The requirement to protect individual authenticators may be implemented via control [PL-4](#pl-4) or [PS-6](#ps-6) for authenticators in the possession of individuals and by controls [AC-3](#ac-3), [AC-6](#ac-6) , and [SC-28](#sc-28) for authenticators stored in organizational systems, including passwords stored in hashed or encrypted formats or files containing encrypted or hashed passwords accessible with administrator privileges. Systems support authenticator management by organization-defined settings and restrictions for various authenticator characteristics (e.g., minimum password length, validation time window for time synchronous one-time tokens, and number of allowed rejections during the verification stage of biometric authentication). Actions can be taken to safeguard individual authenticators, including maintaining possession of authenticators, not sharing authenticators with others, and immediately reporting lost, stolen, or compromised authenticators. Authenticator management includes issuing and revoking authenticators for temporary access when no longer needed.

NIST control statement

Mark [Assignment: system hardware components] indicating the impact level or classification level of the information permitted to be processed, stored, or transmitted by the hardware component.

Discussion

Hardware components that may require marking include input and output devices. Input devices include desktop and notebook computers, keyboards, tablets, and smart phones. Output devices include printers, monitors/video displays, facsimile machines, scanners, copiers, and audio devices. Permissions controlling output to the output devices are addressed in [AC-3](#ac-3) or [AC-4](#ac-4) . Components are marked to indicate the impact level or classification level of the system to which the devices are connected, or the impact level or classification level of the information permitted to be output. Security marking refers to the use of human-readable security attributes. Security labeling refers to the use of security attributes for internal system data structures. Security marking is generally not required for hardware components that process, store, or transmit information determined by organizations to be in the public domain or to be publicly releasable. However, organizations may require markings for hardware components that process, store, or transmit public information in order to indicate that such information is publicly releasable. Marking of system hardware components reflects applicable laws, executive orders, directives, policies, regulations, and standards.

NIST control statement

Verify that individuals accessing a system processing, storing, or transmitting classified information are cleared and indoctrinated to the highest classification level of the information to which they have access on the system.

Discussion

Classified information is the most sensitive information that the Federal Government processes, stores, or transmits. It is imperative that individuals have the requisite security clearances and system access authorizations prior to gaining access to such information. Access authorizations are enforced by system access controls (see [AC-3](#ac-3) ) and flow controls (see [AC-4](#ac-4)).