NIST SP 800-53 Rev. 5 AC-21(1) — Automated Decision Support

What this control requires

Uses automated mechanisms to enforce information-sharing decisions based on partner access authorizations and information restrictions, ensuring secure data sharing. This control supports the Automated Policy Enforcement Tool capability within the Access Control family.

In plain English

Uses automated mechanisms to enforce information-sharing decisions based on partner access authorizations and information restrictions, ensuring secure data sharing. In practice, this means you can prove you policy management-level controls on a defined cadence.

Why it matters

SP 800-53 is the catalogue every U.S. federal authorisation (FedRAMP, FISMA, CMMC) and most regulated industries point at. A missing control surfaces as a Plan of Action & Milestones (POA&M) item that an Authorizing Official must approve before the Authority To Operate is granted or renewed.

This control is rated high. Auditors expect a named owner, a remediation SLA, and a tooling artefact that evidences continuous operation.

What an auditor looks for

A 3PAO (FedRAMP) or Independent Assessor (NIST RMF) tests against NIST SP 800-53A assessment procedures. For each control they expect a documented implementation that addresses every assessment objective (Examine, Interview, Test) and an implementation summary in the System Security Plan.

Within Access Control, the assessor will trace your implementation back to the catalogue reference (Control AC-21(1)) and look for objective evidence on the latest two assessment cycles.

How to implement

1. Write a short policy stating intent, scope and named owner.
2. Encode it as a platform guardrail — SCP, Azure Policy initiative, Organisation Policy, or repository policy.
3. Wire a monitoring signal that catches drift (CSPM rule, SIEM rule, scheduled query, Workspace alert) and ticket it.
4. Review on a documented cadence and capture the review in the risk register or compliance tracker.

Operational checklist

• Policy. Document the trigger, the owner and the cadence.
• Restrictions. Document the trigger, the owner and the cadence.
• Access. Document the trigger, the owner and the cadence.
• Information. Document the trigger, the owner and the cadence.
• Data. Document the trigger, the owner and the cadence.
• Scope: management, enforcement, access control, information, access authorizations, decision support, decisions, identity.

Evidence to collect

• Identity Scan. IAM / IdP exports — Entra ID, Okta, IAM Identity Center, Workspace directory — showing accounts, roles, MFA state and last sign-in.

Identity analytics (Okta, SailPoint, Access Analyzer) demonstrates least-privilege. Focus: Automated Decision Support.

How it maps to other frameworks

Computed from shared tags, control component and key-operation subjects — handy when you are reconciling controls across an audit.

• NIST SP 800-171 Rev. 3 3.1.4 — Separation of Duties
• CIS Controls v8 6.7 — Centralize Access Control
• ISO/IEC 27001:2022 A.5.18 — Access rights
• BSI C5:2020 PS-04 — Physical site access control

Related controls in NIST SP 800-53 Rev. 5

• Control AC-24(1) — Transmit Access Authorization Information
• Control AC-21(2) — Information Search And Retrieval
• Control AC-5 — Separation Of Duties
• Control AC-25 — Reference Monitor
• Control AC-17(6) — Protection Of Mechanism Information
• Control SA-8(23) — Secure Defaults

References

• NIST Computer Security Resource Center — NIST SP 800-53 Rev. 5
• Catalogue identifier: Control AC-21(1)

Topics: identity accessvendor riskidentity managementaccess reviewprivileged access