NIST SP 800-53 Rev. 5 Common Control Library

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

Develop a system-level continuous monitoring strategy and implement continuous monitoring in accordance with the organization-level continuous monitoring strategy that includes: a. Establishing the following system-level metrics to be monitored: [Assignment: system-level metrics]; b. Establishing [Assignment: frequencies] for monitoring and [Assignment: frequencies] for assessment of control effectiveness; c. Ongoing control assessments in accordance with the continuous monitoring strategy; d. Ongoing monitoring of system and organization-defined metrics in accordance with the continuous monitoring strategy; e. Correlation and analysis of information generated by control assessments and monitoring; f. Response actions to address results of the analysis of control assessment and monitoring information; and g. Reporting the security and privacy status of the system to [Assignment: organization-defined personnel or roles] [Assignment: organization-defined frequency].

Discussion

Continuous monitoring at the system level facilitates ongoing awareness of the system security and privacy posture to support organizational risk management decisions. The terms "continuous" and "ongoing" imply that organizations assess and monitor their controls and risks at a frequency sufficient to support risk-based decisions. Different types of controls may require different monitoring frequencies. The results of continuous monitoring generate risk response actions by organizations. When monitoring the effectiveness of multiple controls that have been grouped into capabilities, a root-cause analysis may be needed to determine the specific control that has failed. Continuous monitoring programs allow organizations to maintain the authorizations of systems and common controls in highly dynamic environments of operation with changing mission and business needs, threats, vulnerabilities, and technologies. Having access to security and privacy information on a continuing basis through reports and dashboards gives organizational officials the ability to make effective and timely risk management decisions, including ongoing authorization decisions. Automation supports more frequent updates to hardware, software, and firmware inventories, authorization packages, and other system information. Effectiveness is further enhanced when continuous monitoring outputs are formatted to provide information that is specific, measurable, actionable, relevant, and timely. Continuous monitoring activities are scaled in accordance with the security categories of systems. Monitoring requirements, including the need for specific monitoring, may be referenced in other controls and control enhancements, such as [AC-2g](#ac-2_smt.g), [AC-2(7)](#ac-2.7), [AC-2(12)(a)](#ac-2.12_smt.a), [AC-2(7)(b)](#ac-2.7_smt.b), [AC-2(7)(c)](#ac-2.7_smt.c), [AC-17(1)](#ac-17.1), [AT-4a](#at-4_smt.a), [AU-13](#au-13), [AU-13(1)](#au-13.1), [AU-13(2)](#au-13.2), [CM-3f](#cm-3_smt.f), [CM-6d](#cm-6_smt.d), [CM-11c](#cm-11_smt.c), [IR-5](#ir-5), [MA-2b](#ma-2_smt.b), [MA-3a](#ma-3_smt.a), [MA-4a](#ma-4_smt.a), [PE-3d](#pe-3_smt.d), [PE-6](#pe-6), [PE-14b](#pe-14_smt.b), [PE-16](#pe-16), [PE-20](#pe-20), [PM-6](#pm-6), [PM-23](#pm-23), [PM-31](#pm-31), [PS-7e](#ps-7_smt.e), [SA-9c](#sa-9_smt.c), [SR-4](#sr-4), [SC-5(3)(b)](#sc-5.3_smt.b), [SC-7a](#sc-7_smt.a), [SC-7(24)(b)](#sc-7.24_smt.b), [SC-18b](#sc-18_smt.b), [SC-43b](#sc-43_smt.b) , and [SI-4](#si-4).

NIST control statement

(a) Identify [Assignment: software programs]; (b) Employ a deny-all, permit-by-exception policy to allow the execution of authorized software programs on the system; and (c) Review and update the list of authorized software programs [Assignment: frequency].

Discussion

Authorized software programs can be limited to specific versions or from a specific source. To facilitate a comprehensive authorized software process and increase the strength of protection for attacks that bypass application level authorized software, software programs may be decomposed into and monitored at different levels of detail. These levels include applications, application programming interfaces, application modules, scripts, system processes, system services, kernel functions, registries, drivers, and dynamic link libraries. The concept of permitting the execution of authorized software may also be applied to user actions, system ports and protocols, IP addresses/ranges, websites, and MAC addresses. Organizations consider verifying the integrity of authorized software programs using digital signatures, cryptographic checksums, or hash functions. Verification of authorized software can occur either prior to execution or at system startup. The identification of authorized URLs for websites is addressed in [CA-3(5)](#ca-3.5) and [SC-7](#sc-7).

NIST control statement

Develop an organization-wide continuous monitoring strategy and implement continuous monitoring programs that include: a. Establishing the following organization-wide metrics to be monitored: [Assignment: metrics]; b. Establishing [Assignment: monitoring frequencies] and [Assignment: assessment frequencies] for control effectiveness; c. Ongoing monitoring of organizationally-defined metrics in accordance with the continuous monitoring strategy; d. Correlation and analysis of information generated by control assessments and monitoring; e. Response actions to address results of the analysis of control assessment and monitoring information; and f. Reporting the security and privacy status of organizational systems to [Assignment: organization-defined personnel or roles] [Assignment: organization-defined frequency].

Discussion

Continuous monitoring at the organization level facilitates ongoing awareness of the security and privacy posture across the organization to support organizational risk management decisions. The terms "continuous" and "ongoing" imply that organizations assess and monitor their controls and risks at a frequency sufficient to support risk-based decisions. Different types of controls may require different monitoring frequencies. The results of continuous monitoring guide and inform risk response actions by organizations. Continuous monitoring programs allow organizations to maintain the authorizations of systems and common controls in highly dynamic environments of operation with changing mission and business needs, threats, vulnerabilities, and technologies. Having access to security- and privacy-related information on a continuing basis through reports and dashboards gives organizational officials the capability to make effective, timely, and informed risk management decisions, including ongoing authorization decisions. To further facilitate security and privacy risk management, organizations consider aligning organization-defined monitoring metrics with organizational risk tolerance as defined in the risk management strategy. Monitoring requirements, including the need for monitoring, may be referenced in other controls and control enhancements such as, [AC-2g](#ac-2_smt.g), [AC-2(7)](#ac-2.7), [AC-2(12)(a)](#ac-2.12_smt.a), [AC-2(7)(b)](#ac-2.7_smt.b), [AC-2(7)(c)](#ac-2.7_smt.c), [AC-17(1)](#ac-17.1), [AT-4a](#at-4_smt.a), [AU-13](#au-13), [AU-13(1)](#au-13.1), [AU-13(2)](#au-13.2), [CA-7](#ca-7), [CM-3f](#cm-3_smt.f), [CM-6d](#cm-6_smt.d), [CM-11c](#cm-11_smt.c), [IR-5](#ir-5), [MA-2b](#ma-2_smt.b), [MA-3a](#ma-3_smt.a), [MA-4a](#ma-4_smt.a), [PE-3d](#pe-3_smt.d), [PE-6](#pe-6), [PE-14b](#pe-14_smt.b), [PE-16](#pe-16), [PE-20](#pe-20), [PM-6](#pm-6), [PM-23](#pm-23), [PS-7e](#ps-7_smt.e), [SA-9c](#sa-9_smt.c), [SC-5(3)(b)](#sc-5.3_smt.b), [SC-7a](#sc-7_smt.a), [SC-7(24)(b)](#sc-7.24_smt.b), [SC-18b](#sc-18_smt.b), [SC-43b](#sc-43_smt.b), [SI-4](#si-4).

NIST control statement

a. Monitor and control communications at the external managed interfaces to the system and at key internal managed interfaces within the system; b. Implement subnetworks for publicly accessible system components that are [Selection (one): physically; logically] separated from internal organizational networks; and c. Connect to external networks or systems only through managed interfaces consisting of boundary protection devices arranged in accordance with an organizational security and privacy architecture.

Discussion

Managed interfaces include gateways, routers, firewalls, guards, network-based malicious code analysis, virtualization systems, or encrypted tunnels implemented within a security architecture. Subnetworks that are physically or logically separated from internal networks are referred to as demilitarized zones or DMZs. Restricting or prohibiting interfaces within organizational systems includes restricting external web traffic to designated web servers within managed interfaces, prohibiting external traffic that appears to be spoofing internal addresses, and prohibiting internal traffic that appears to be spoofing external addresses. [SP 800-189](#f5edfe51-d1f2-422e-9b27-5d0e90b49c72) provides additional information on source address validation techniques to prevent ingress and egress of traffic with spoofed addresses. Commercial telecommunications services are provided by network components and consolidated management systems shared by customers. These services may also include third party-provided access lines and other service elements. Such services may represent sources of increased risk despite contract security provisions. Boundary protection may be implemented as a common control for all or part of an organizational network such that the boundary to be protected is greater than a system-specific boundary (i.e., an authorization boundary).

NIST control statement

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

NIST control statement

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

NIST control statement

Limit the number of external network connections to the system.

Discussion

Limiting the number of external network connections facilitates monitoring of inbound and outbound communications traffic. The Trusted Internet Connection [DHS TIC](#4f42ee6e-86cc-403b-a51f-76c2b4f81b54) initiative is an example of a federal guideline that requires limits on the number of external network connections. Limiting the number of external network connections to the system is important during transition periods from older to newer technologies (e.g., transitioning from IPv4 to IPv6 network protocols). Such transitions may require implementing the older and newer technologies simultaneously during the transition period and thus increase the number of access points to the system.

NIST control statement

(a) Implement a managed interface for each external telecommunication service; (b) Establish a traffic flow policy for each managed interface; (c) Protect the confidentiality and integrity of the information being transmitted across each interface; (d) Document each exception to the traffic flow policy with a supporting mission or business need and duration of that need; (e) Review exceptions to the traffic flow policy [Assignment: frequency] and remove exceptions that are no longer supported by an explicit mission or business need; (f) Prevent unauthorized exchange of control plane traffic with external networks; (g) Publish information to enable remote networks to detect unauthorized control plane traffic from internal networks; and (h) Filter unauthorized control plane traffic from external networks.

Discussion

External telecommunications services can provide data and/or voice communications services. Examples of control plane traffic include Border Gateway Protocol (BGP) routing, Domain Name System (DNS), and management protocols. See [SP 800-189](#f5edfe51-d1f2-422e-9b27-5d0e90b49c72) for additional information on the use of the resource public key infrastructure (RPKI) to protect BGP routes and detect unauthorized BGP announcements.

NIST control statement

Deny network communications traffic by default and allow network communications traffic by exception [Selection (one-or-more): at managed interfaces; for [Assignment: systems] ].

Discussion

Denying by default and allowing by exception applies to inbound and outbound network communications traffic. A deny-all, permit-by-exception network communications traffic policy ensures that only those system connections that are essential and approved are allowed. Deny by default, allow by exception also applies to a system that is connected to an external system.

NIST control statement

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

NIST control statement

Prevent split tunneling for remote devices connecting to organizational systems unless the split tunnel is securely provisioned using [Assignment: safeguards].

Discussion

Split tunneling is the process of allowing a remote user or device to establish a non-remote connection with a system and simultaneously communicate via some other connection to a resource in an external network. This method of network access enables a user to access remote devices and simultaneously, access uncontrolled networks. Split tunneling might be desirable by remote users to communicate with local system resources, such as printers or file servers. However, split tunneling can facilitate unauthorized external connections, making the system vulnerable to attack and to exfiltration of organizational information. Split tunneling can be prevented by disabling configuration settings that allow such capability in remote devices and by preventing those configuration settings from being configurable by users. Prevention can also be achieved by the detection of split tunneling (or of configuration settings that allow split tunneling) in the remote device, and by prohibiting the connection if the remote device is using split tunneling. A virtual private network (VPN) can be used to securely provision a split tunnel. A securely provisioned VPN includes locking connectivity to exclusive, managed, and named environments, or to a specific set of pre-approved addresses, without user control.

NIST control statement

Route [Assignment: internal communications traffic] to [Assignment: external networks] through authenticated proxy servers at managed interfaces.

Discussion

External networks are networks outside of organizational control. A proxy server is a server (i.e., system or application) that acts as an intermediary for clients requesting system resources from non-organizational or other organizational servers. System resources that may be requested include files, connections, web pages, or services. Client requests established through a connection to a proxy server are assessed to manage complexity and provide additional protection by limiting direct connectivity. Web content filtering devices are one of the most common proxy servers that provide access to the Internet. Proxy servers can support the logging of Transmission Control Protocol sessions and the blocking of specific Uniform Resource Locators, Internet Protocol addresses, and domain names. Web proxies can be configured with organization-defined lists of authorized and unauthorized websites. Note that proxy servers may inhibit the use of virtual private networks (VPNs) and create the potential for "man-in-the-middle" attacks (depending on the implementation).

NIST control statement

(a) Detect and deny outgoing communications traffic posing a threat to external systems; and (b) Audit the identity of internal users associated with denied communications.

Discussion

Detecting outgoing communications traffic from internal actions that may pose threats to external systems is known as extrusion detection. Extrusion detection is carried out within the system at managed interfaces. Extrusion detection includes the analysis of incoming and outgoing communications traffic while searching for indications of internal threats to the security of external systems. Internal threats to external systems include traffic indicative of denial-of-service attacks, traffic with spoofed source addresses, and traffic that contains malicious code. Organizations have criteria to determine, update, and manage identified threats related to extrusion detection.

NIST control statement

(a) Prevent the exfiltration of information; and (b) Conduct exfiltration tests [Assignment: frequency].

Discussion

Prevention of exfiltration applies to both the intentional and unintentional exfiltration of information. Techniques used to prevent the exfiltration of information from systems may be implemented at internal endpoints, external boundaries, and across managed interfaces and include adherence to protocol formats, monitoring for beaconing activity from systems, disconnecting external network interfaces except when explicitly needed, employing traffic profile analysis to detect deviations from the volume and types of traffic expected, call backs to command and control centers, conducting penetration testing, monitoring for steganography, disassembling and reassembling packet headers, and using data loss and data leakage prevention tools. Devices that enforce strict adherence to protocol formats include deep packet inspection firewalls and Extensible Markup Language (XML) gateways. The devices verify adherence to protocol formats and specifications at the application layer and identify vulnerabilities that cannot be detected by devices that operate at the network or transport layers. The prevention of exfiltration is similar to data loss prevention or data leakage prevention and is closely associated with cross-domain solutions and system guards that enforce information flow requirements.

NIST control statement

Only allow incoming communications from [Assignment: authorized sources] to be routed to [Assignment: authorized destinations].

Discussion

General source address validation techniques are applied to restrict the use of illegal and unallocated source addresses as well as source addresses that should only be used within the system. The restriction of incoming communications traffic provides determinations that source and destination address pairs represent authorized or allowed communications. Determinations can be based on several factors, including the presence of such address pairs in the lists of authorized or allowed communications, the absence of such address pairs in lists of unauthorized or disallowed pairs, or meeting more general rules for authorized or allowed source and destination pairs. Strong authentication of network addresses is not possible without the use of explicit security protocols, and thus, addresses can often be spoofed. Further, identity-based incoming traffic restriction methods can be employed, including router access control lists and firewall rules.

NIST control statement

Implement [Assignment: host-based boundary protection mechanisms] at [Assignment: system components].

Discussion

Host-based boundary protection mechanisms include host-based firewalls. System components that employ host-based boundary protection mechanisms include servers, workstations, notebook computers, and mobile devices.

NIST control statement

Isolate [Assignment: information security tools, mechanisms, and support components] from other internal system components by implementing physically separate subnetworks with managed interfaces to other components of the system.

Discussion

Physically separate subnetworks with managed interfaces are useful in isolating computer network defenses from critical operational processing networks to prevent adversaries from discovering the analysis and forensics techniques employed by organizations.

NIST control statement

Protect against unauthorized physical connections at [Assignment: managed interfaces].

Discussion

Systems that operate at different security categories or classification levels may share common physical and environmental controls, since the systems may share space within the same facilities. In practice, it is possible that these separate systems may share common equipment rooms, wiring closets, and cable distribution paths. Protection against unauthorized physical connections can be achieved by using clearly identified and physically separated cable trays, connection frames, and patch panels for each side of managed interfaces with physical access controls that enforce limited authorized access to these items.

NIST control statement

Route networked, privileged accesses through a dedicated, managed interface for purposes of access control and auditing.

Discussion

Privileged access provides greater accessibility to system functions, including security functions. Adversaries attempt to gain privileged access to systems through remote access to cause adverse mission or business impacts, such as by exfiltrating information or bringing down a critical system capability. Routing networked, privileged access requests through a dedicated, managed interface further restricts privileged access for increased access control and auditing.

NIST control statement

Prevent the discovery of specific system components that represent a managed interface.

Discussion

Preventing the discovery of system components representing a managed interface helps protect network addresses of those components from discovery through common tools and techniques used to identify devices on networks. Network addresses are not available for discovery and require prior knowledge for access. Preventing the discovery of components and devices can be accomplished by not publishing network addresses, using network address translation, or not entering the addresses in domain name systems. Another prevention technique is to periodically change network addresses.

NIST control statement

Enforce adherence to protocol formats.

Discussion

System components that enforce protocol formats include deep packet inspection firewalls and XML gateways. The components verify adherence to protocol formats and specifications at the application layer and identify vulnerabilities that cannot be detected by devices operating at the network or transport layers.

NIST control statement

Prevent systems from entering unsecure states in the event of an operational failure of a boundary protection device.

Discussion

Fail secure is a condition achieved by employing mechanisms to ensure that in the event of operational failures of boundary protection devices at managed interfaces, systems do not enter into unsecure states where intended security properties no longer hold. Managed interfaces include routers, firewalls, and application gateways that reside on protected subnetworks (commonly referred to as demilitarized zones). Failures of boundary protection devices cannot lead to or cause information external to the devices to enter the devices nor can failures permit unauthorized information releases.

NIST control statement

Block inbound and outbound communications traffic between [Assignment: communication clients] that are independently configured by end users and external service providers.

Discussion

Communication clients independently configured by end users and external service providers include instant messaging clients and video conferencing software and applications. Traffic blocking does not apply to communication clients that are configured by organizations to perform authorized functions.

NIST control statement

Provide the capability to dynamically isolate [Assignment: system components] from other system components.

Discussion

The capability to dynamically isolate certain internal system components is useful when it is necessary to partition or separate system components of questionable origin from components that possess greater trustworthiness. Component isolation reduces the attack surface of organizational systems. Isolating selected system components can also limit the damage from successful attacks when such attacks occur.