NIST 800-171 Controls · Access Control

AC.2.003 — Control CUI Flow per Authorizations

CMMC Level 2SPRS if unmet: -5CRITICAL priority~16h to implement

Control the flow of CUI in accordance with approved authorizations.

What AC.2.003 means in plain English

Know exactly where your government contract information (called CUI — Controlled Unclassified Information) lives and travels. Don't let it drift to personal email, personal cloud storage, or unauthorized thumb drives. If you receive a drawing marked "CUI" from a prime contractor, it should only live in your approved, secure folder — not in a Gmail inbox or on a personal USB stick.

The assessment question

“Do you have controls in place to prevent CUI from flowing to unauthorized locations — such as blocking CUI from being emailed to personal accounts, uploaded to unapproved cloud services like personal Google Drive, or copied to unauthorized USB drives?”

How to implement AC.2.003

  1. Designate a specific, protected folder or SharePoint site as the only approved location for CUI. Label it clearly (e.g., "CUI - Controlled Documents") and restrict access as described in AC.1.002.
  2. In Microsoft 365 GCC, enable Microsoft Purview Information Protection (formerly AIP) sensitivity labels to tag CUI documents and prevent forwarding or downloading to unmanaged devices.
  3. Configure Exchange Online mail flow rules in Microsoft 365 GCC admin center to block outbound emails with CUI labels going to personal email domains (gmail.com, yahoo.com, etc.).
  4. Use Windows Group Policy to disable AutoRun on USB drives and consider blocking unknown USB storage devices via Device Control settings in Microsoft Defender for Business.
  5. Train employees to recognize CUI markings and document the training so you have a record. A 30-minute lunch-and-learn with a sign-in sheet satisfies this evidence requirement.

Evidence your assessor will ask for

  • Data flow diagram or written description showing where CUI enters, is stored, processed, and exits your environment
  • Screenshot of SharePoint/OneDrive CUI folder with restricted permissions
  • Mail flow rule configurations in Microsoft 365 admin center
  • Sensitivity label policy configuration screenshots
  • USB device control policy or written procedure
  • Employee training records on CUI handling

Does AI prompt monitoring help with this control? Yes — directly.

AC.2.003 is one of the requirements a local AI prompt firewall concretely evidences. When employees send prompts to ChatGPT, Copilot, or Claude, that traffic crosses your external boundary — HoundShield inspects it on your own infrastructure (self-hosted Docker, Mode B), blocks CUI patterns before transmission, and writes every allowed/blocked event to a SHA-256 hash-chained log attributable to the user. Architecture diagram, active pattern set, and a log sample are the evidence an assessor tests this against for the AI data path.

Full mapping: which 800-171 controls AI prompt monitoring evidences →

More Access Control controls

AC.1.001

Limit System Access to Authorized Users

AC.1.002

Limit System Access to Authorized Transaction Types

AC.2.004

Separate Duties to Reduce Risk

AC.2.005

Employ Least Privilege

AC.2.006

Use Non-Privileged Accounts for Non-Security Functions

AC.2.007

Prevent Non-Privileged Users from Executing Privileged Functions

Score yourself against all 110 controls

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