3.1.3 — Where CUI Can Flow

The One-Liner

If you can’t draw a map showing exactly where CUI enters, moves through, and leaves your environment — and prove the map is enforced — you fail this requirement.

What It Says

Control the flow of CUI in accordance with approved authorizations.

What It Actually Means

You need to know three things and enforce all three:

1. Where CUI comes from — which systems, people, and networks send CUI into your environment
2. Where CUI goes — which internal systems handle it and which external destinations are approved
3. What stops it going anywhere else — firewalls, DLP, proxies, network segmentation

The word that matters here is enforce. A policy saying “don’t email CUI to personal accounts” isn’t enough. You need DLP rules that block it. A policy saying “CUI stays on the internal network” isn’t enough. You need firewall rules that enforce it.

This is one of the most commonly failed requirements because it demands both knowledge (where does CUI flow?) and technical controls (how do you stop unauthorized flows?).

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Pass or Fail

Your assessor needs a “yes” to every row:

#	Question	What “yes” looks like
1	Are flow control policies defined?	Documented rules for where CUI can and can’t go
2	Are CUI sources and destinations identified?	You know which systems create, receive, and store CUI
3	Are enforcement mechanisms in place?	Firewalls, DLP, proxies — not just policy documents
4	Are flow authorizations actually enforced?	Unauthorized transfers get blocked, not just logged

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What to Have Ready on Assessment Day

Documents they’ll review: Information flow control policy, system security plan, network diagrams showing CUI flow paths, firewall rules, DLP policy configs, proxy settings, list of approved flow authorizations, audit logs

People they’ll talk to: Network admins, information security staff, system developers

Live demos they’ll ask for: “Try to email a CUI document to a personal email address — show me it’s blocked.” “Try to copy CUI to a USB drive — show me what happens.”

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The Assessor’s Playbook

These are the actual questions. Have answers ready.

• “Show me where CUI originates in your environment and where it’s allowed to flow.”
• “What technical mechanisms enforce your flow control policies?”
• “Can you demonstrate that an unauthorized CUI transfer is actually blocked?”
• “How do you handle CUI flow to cloud services and email?”

Real-World Example

A defense sub uses Microsoft Purview DLP to enforce CUI flow. Sensitivity labels mark CUI documents. DLP rules block labeled documents from being emailed outside the organization, uploaded to personal OneDrive, or copied to USB drives. A proxy server inspects all outbound traffic. The IT team shows the assessor a blocked email attempt in the DLP incident report.

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Where Companies Trip Up

No DLP. Relying on policy alone without technical enforcement. The assessor will ask for a live demo.

Email is a free-for-all. CUI attached to emails going to personal accounts, partners, or unencrypted destinations.

Cloud storage blind spots. CUI uploaded to personal Dropbox, Google Drive, or unapproved cloud services.

Flat network. CUI flows freely across the entire network with no segmentation or zone boundaries.

Shadow IT. Users moving CUI through WhatsApp, personal file sharing, or tools the security team doesn’t know about.

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How to Talk About This

To a CEO or Board

We can draw you a map of exactly where CUI enters our environment, where it moves, and where it exits. Every path is authorized and technically enforced — not just documented.

To an Engineering Team

Purview DLP with sensitivity labels on CUI. Outbound proxy inspection. Firewall rules segment CUI zones. USB blocked via Intune device restrictions. Every unauthorized transfer attempt is logged and blocked.

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Connected Requirements

Requirement	Why it matters here
3.13.1 — Guard the Boundaries	Monitors communications at network boundaries
3.13.6 — Deny Everything by Default	Default-deny firewall rules that support flow control
3.13.8 — Encrypt in Transit	Encrypting CUI when it moves between systems

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Implementation

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Step-by-step guides for Microsoft 365, AWS, Azure, and GCP are available to Ancitus clients.

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CMMC Practice ID: AC.L2-3.1.3 | SPRS Weight: 1 point | POA&M Eligible: Yes