NIST SP 800-53 Rev. 5 AC-4(21) — Physical Or Logical Separation Of Information Flows

What this control requires

Organizations must separate information flows logically or physically using defined mechanisms or techniques to accomplish required separations by types of information. This ensures that information is not commingled during transit and enables flow control based on transmission paths that would otherwise be unachievable. This control supports the Governance capability within the Access Control family.

In plain English

Organizations must separate information flows logically or physically using defined mechanisms or techniques to accomplish required separations by types of information. 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-4(21)) and look for objective evidence on the latest two assessment cycles.

How to implement

Physical Security

Inheritance. For cloud-only operations, inherit physical controls from the provider's SOC 2 / ISO 27001 report and document the inheritance in the SoA.

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.
• Access. Document the trigger, the owner and the cadence.
• Information. Document the trigger, the owner and the cadence.
• Flow Control. Document the trigger, the owner and the cadence.
• Security. Document the trigger, the owner and the cadence.
• Scope: management, access control, information, capability, techniques, transmission, organizations, security policy.

Evidence to collect

• Policy Document. A signed and version-controlled policy / standard / procedure document.

Signed policy or standard documents the safeguard. Focus: Physical Or Logical Separation Of Information Flows.

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 3.1.5 — Least Privilege
• ISO/IEC 27001:2022 A.6.7 — Remote working
• HIPAA (45 CFR Part 164) 164.308(a)(4)(ii)(A) — Information Access Management - Isolating Healthcare Clearinghouse Functions
• BSI C5:2020 SP-01 — Documentation, communication and provision of policies and instructions

Related controls in NIST SP 800-53 Rev. 5

• Control AC-4(27) — Redundant/Independent Filtering Mechanisms
• Control AC-4(20) — Approved Solutions
• Control AC-4(30) — Filter Mechanisms Using Multiple Processes
• Control AC-3(3) — Mandatory Access Control
• Control AC-4(5) — Embedded Data Types
• Control AC-4(4) — Flow Control Of Encrypted Information

References

• NIST Computer Security Resource Center — NIST SP 800-53 Rev. 5
• Catalogue identifier: Control AC-4(21)

Topics: governance riskphysical securitypolicy management