# NIST SP 800-53 Rev. 5 AC-21 — Information Sharing - Source: https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-21-information-sharing - Section: [Compliance](https://support.icompaas.com/compliance) › [NIST SP 800-53 Rev. 5 Controls](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5) › AC — Access Control - Last updated: 2026-06-05 - Publisher: iCompaas, Inc. (https://www.icompaas.com) **NIST SP 800-53 Rev. 5**·Control `AC-21`·medium·Component: **Access Control** ## What this control requires > Enables authorized users to verify sharing partner access matches information restrictions; employs automated or manual processes to aid sharing decisions for sensitive data like PII. This control supports the Access Control System capability within the Access Control family. > > — NIST SP 800-53 Rev. 5, Control AC-21 ### In plain English Enables authorized users to verify sharing partner access matches information restrictions; employs automated or manual processes to aid sharing decisions for sensitive data like PII. 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. ## 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) and look for objective evidence on the latest two assessment cycles. ## How to implement #### Policy Management **Docs.** Keep each policy in version-controlled Markdown alongside the code that implements it; ship a signed PDF snapshot at each annual review and link the SHA into the SoA. ### 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, access control, information, privacy, system, decisions, capability, authorized. ## Evidence to collect - **Policy Document.** A signed and version-controlled policy / standard / procedure document. Signed policy or standard documents the safeguard. Focus: Information Sharing. ## Common pitfalls - Policies that no engineer has read; the version in Confluence drifts from the version that's actually being enforced. ## 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 3.1.9** — System Use Notification](https://support.icompaas.com/compliance/nist-sp-800-171-rev-3/3-1-9-system-use-notification) - [**HIPAA (45 CFR Part 164) 164.308(a)(1)(ii)(D)** — Security Management Process -Information System Activity Review](https://support.icompaas.com/compliance/hipaa/164-308-a-1-ii-d-security-management-process-information-system-activity-review) - [**ISO/IEC 27001:2022 A.8.3** — Information access restriction](https://support.icompaas.com/compliance/iso-27001-2022/a-8-3-information-access-restriction) ## Related controls in NIST SP 800-53 Rev. 5 - [**Control AC-16(2)** — Attribute Value Changes By Authorized Individuals](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-16-2-attribute-value-changes-by-authorized-individuals) - [**Control AC-4(9)** — Human Reviews](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-4-9-human-reviews) - [**Control AC-16(4)** — Association Of Attributes By Authorized Individuals](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-16-4-association-of-attributes-by-authorized-individuals) - [**Control AC-16(5)** — Attribute Displays On Objects To Be Output](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-16-5-attribute-displays-on-objects-to-be-output) - [**Control AC-16(8)** — Association Techniques And Technologies](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-16-8-association-techniques-and-technologies) - [**Control AC-16(3)** — Maintenance Of Attribute Associations By System](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-16-3-maintenance-of-attribute-associations-by-system) ## References - [NIST Computer Security Resource Center — NIST SP 800-53 Rev. 5](https://csrc.nist.gov/Projects/risk-management/sp800-53-controls) - Catalogue identifier: `Control AC-21` Topics: governance riskvendor riskpolicy managementprivacy management ## Related compliance guidance - [NIST SP 800-53 Rev. 5: a practical implementation guide for cloud teams](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5): Start with the full NIST SP 800-53 implementation guide — scope, roadmap, evidence and how the assessment runs. - [NIST SP 800-171 and CMMC: a practical guide to protecting CUI in the cloud](https://support.icompaas.com/compliance/nist-sp-800-171-rev-3): If your obligation is protecting CUI in your own systems for a federal or defence contract, this derived subset — not the full catalogue — is the requirement. - [SOC 2 compliance: a practical implementation guide for cloud-native SaaS teams](https://support.icompaas.com/compliance/soc-2-trust-services-criteria): Most commercial SaaS already hold a SOC 2 Type 2, and FedRAMP 20x Class A is designed for providers that have one.