Control statement
- a.Enable authorized users to determine whether access authorizations assigned to a sharing partner match the information’s access and use restrictions for [Organization-defined: information-sharing circumstances] ; and
- b.Employ [Organization-defined: automated mechanisms] to assist users in making information sharing and collaboration decisions.
Discussion
Information sharing applies to information that may be restricted in some manner based on some formal or administrative determination. Examples of such information include, contract-sensitive information, classified information related to special access programs or compartments, privileged information, proprietary information, and personally identifiable information. Security and privacy risk assessments as well as applicable laws, regulations, and policies can provide useful inputs to these determinations. Depending on the circumstances, sharing partners may be defined at the individual, group, or organizational level. Information may be defined by content, type, security category, or special access program or compartment. Access restrictions may include non-disclosure agreements (NDA). Information flow techniques and security attributes may be used to provide automated assistance to users making sharing and collaboration decisions.
Organization-defined parameters
These values must be resolved through the organization’s tailoring and governance process. Bracketed parameter references in the control text identify where a decision is required.
From control text to operational evidence
Use Information Sharing as a testable risk decision. Translate the official statement into accountable people, repeatable processes, configured technology, and evidence that demonstrates the outcome over time. In this family, pay particular attention to identity, authorization, least privilege, session boundaries, and access lifecycle governance.
Implementation workflow
- Define the control boundary, responsible owner, inherited portions, and systems or processes in scope.
- Resolve each organization-defined parameter before declaring the control implemented.
- Document how the implementation satisfies every clause of the official control statement.
- Collect evidence as a normal byproduct of operation rather than only before an assessment.
- Review exceptions, changes, and monitoring results on a risk-based cadence.
Evidence examples
- access approvals and entitlement records
- role and group configuration exports
- periodic access review results
- authentication and authorization logs
Common failure patterns
- standing privileges that outlive business need
- shared or orphaned accounts
- access rules implemented differently across systems
- approvals that cannot be traced to actual permissions
Questions practitioners should ask
- What risk decision is this control intended to support in this system?
- Which parts are implemented locally, inherited, shared, or not applicable—and what evidence supports that decision?
- Do the documented narrative, deployed configuration, operating process, and collected evidence agree?
- What event or threshold requires the implementation to be reviewed or changed?
Assessment objectives and methods
Show the assessment objective
- AC-21a.authorized users are enabled to determine whether access authorizations assigned to a sharing partner match the information’s access and use restrictions for [Organization-defined: information-sharing circumstances];
- AC-21b.[Organization-defined: automated mechanisms] are employed to assist users in making information-sharing and collaboration decisions.
Examine
- Access control policy
- procedures addressing user-based collaboration and information sharing (including restrictions)
- system design documentation
- system configuration settings and associated documentation
- list of users authorized to make information-sharing/collaboration decisions
- list of information-sharing circumstances requiring user discretion
- non-disclosure agreements
- acquisitions/contractual agreements
- system security plan
- privacy plan
- privacy impact assessment
- security and privacy risk assessments
- other relevant documents or records
Interview
- Organizational personnel responsible for information-sharing/collaboration decisions
- organizational personnel with responsibility for acquisitions/contractual agreements
- system/network administrators
- organizational personnel with information security and privacy responsibilities
Test
- Automated mechanisms or manual process implementing access authorizations supporting information-sharing/user collaboration decisions
Related controls
These relationships come from the official OSCAL catalog. They indicate useful dependencies or context, not automatic inheritance or equivalence.
Control enhancements
Enhancements add specificity, strength, or scope to the base control. Baseline badges show explicit selections in the official SP 800-53B OSCAL profiles.
AC-21(1) — Automated Decision Support
Employ [Organization-defined: automated mechanisms] to enforce information-sharing decisions by authorized users based on access authorizations of sharing partners and access restrictions on information to be shared.
Official discussion
Automated mechanisms are used to enforce information sharing decisions.
Organization-defined parameters (1)
Assessment objectives and methods
[Organization-defined: automated mechanisms] are employed to enforce information-sharing decisions by authorized users based on access authorizations of sharing partners and access restrictions on information to be shared.
Examine
- Access control policy
- procedures addressing user-based collaboration and information sharing (including restrictions)
- system design documentation
- system configuration settings and associated documentation
- system-generated list of users authorized to make information-sharing/collaboration decisions
- system-generated list of sharing partners and access authorizations
- system-generated list of access restrictions regarding information to be shared
- system security plan
- other relevant documents or records
Interview
- System/network administrators
- organizational personnel with information security responsibilities
- system developers
Test
- Automated mechanisms implementing access authorizations supporting information-sharing/user collaboration decisions
AC-21(2) — Information Search and Retrieval
Implement information search and retrieval services that enforce [Organization-defined: information-sharing restrictions].
Official discussion
Information search and retrieval services identify information system resources relevant to an information need.
Organization-defined parameters (1)
Assessment objectives and methods
information search and retrieval services that enforce [Organization-defined: information-sharing restrictions] are implemented.
Examine
- Access control policy
- procedures addressing user-based collaboration and information sharing (including restrictions)
- system design documentation
- system configuration settings and associated documentation
- system-generated list of access restrictions regarding information to be shared
- information search and retrieval records
- system audit records
- system security plan
- other relevant documents or records
Interview
- Organizational personnel with access enforcement responsibilities for system search and retrieval services
- system/network administrators
- organizational personnel with information security responsibilities
- system developers
Test
- System search and retrieval services enforcing information-sharing restrictions
Authoritative sources
Bare Metal Cyber is an independent educational publisher and is not affiliated with or endorsed by NIST. Official control requirements and interpretations remain with NIST and the responsible authorizing organization.