03.01.03OFFICIAL TITLEPENDING NIST SME REVIEW

03.01.03 — Information Flow Enforcement

03.01 Access Control · NIST SP 800-171 Rev. 3

Independent summary of the official requirement

Requires enforcing approved authorizations for controlling the flow of CUI within the system and between connected systems (aligned to SP 800-53 AC-4). Flow control governs where CUI may travel — across internal segments, to connected systems, and out of the environment — as distinct from who may access it.

Rev. 3 requirement text is multi-part and parameterized with organization-defined values, so this site summarizes rather than reproduces it. The summary is independent — read the official publication for the binding wording.

NIST SP 800-171 Rev. 3 — Protecting CUI in Nonfederal Systems ↗NIST SP 800-171A Rev. 3 — Assessing Security Requirements for CUI ↗

Independent interpretation

What this requirement is after

Access control decides who opens the file; flow control decides where the file may travel. In practice this is your mail-flow rules, DLP policies, firewall zones, and tenant boundaries — the mechanisms that stop CUI drifting to personal email, an unvetted SaaS tool, or a network segment that was never assessed for it. Small contractors usually start by naming the few places CUI is allowed to live and blocking the obvious exits.

Across revisions

Carried forward from 3.1.3 with the same substance, restated in AC-4's enforcement language: approved authorizations for CUI flow, enforced within the system and between connected systems.

Mapped practices

Brilliant at the Basics practices that support this requirement

IT-05 — Logical segmentationIT

Partial implementation supportModerate confidence

Why: Network segmentation is a flow-enforcement mechanism: zone boundaries with deny-by-default policy are one of the concrete ways approved CUI flows get enforced and unapproved ones get blocked.

What this does not claim: Segmentation enforces flows at the network layer only, and only where zones were drawn with CUI in mind. Application-layer flows — email, cloud sharing, SaaS integrations — cross zones legitimately and need flow policy of their own, and the approved-authorization side of the requirement (documenting which CUI flows are approved at all) is analysis the practice does not perform.

Practice-side activities

Place CUI repositories in zones whose ingress and egress rules reflect the approved flows
Review inter-zone rules against the CUI flow documentation on a cadence

Evidence this produces

Zone diagrams and firewall policy exports for CUI segments
Rule-review records tying rules to approved flows

Review status: Pending NIST SME review · Reviewed by Brilliant at the Basics editorial — practitioner-authored; NIST SME review pending · updated 2026-08-06

Doing the work

Implementation considerations and evidence

Implementation considerationsIndependent guidance — tailor to your environment

Document the approved CUI flows first — between which repositories, to which partners, over which channels; enforcement without a flow map is guesswork.
Use the boundary mechanisms you already own: mail transport rules, DLP policy, tenant restrictions, and firewall zone policy are all flow enforcement.
Watch the unmanaged exits — personal webmail, consumer cloud storage, unsanctioned AI tools — because that is where CUI actually leaves small environments.

What operating evidence looks likeRecords worth retaining, not a submission checklist

A documented CUI flow map or table naming the approved paths
DLP, mail-flow, or firewall policy exports implementing those flows
Blocked-flow or DLP incident records showing the enforcement operates

Suggested owners, derived from the mapped practices and artifacts: Network administrator. Ownership is a named person in your organization, not a role on a website.

Artifacts

Templates and worksheets with a mapped relationship

No artifact in the library names this requirement yet. The library index groups everything by category and practice.

The other revision

Where this came from in Rev. 2

3.1.3 — CUI flow control

Provenance

Sources and review status

Primary sources	NIST SP 800-171 Rev. 3 — Protecting CUI in Nonfederal Systems · NIST SP 800-171A Rev. 3 — Assessing Security Requirements for CUI
Review status	Pending NIST SME review
Content version	1.0
Updated	2026-08-06