03.01.03 – Information Flow Enforcement (NIST 800-171 Rev. 3)

2025-05-27

Welcome to our breakdown of 03.01.03 – Information Flow Enforcement, the third control in the Access Control family under NIST SP 800-171 Rev. 3.

We’ve already discussed who gets access (03.01.01) and how to enforce it (03.01.02).
Now we’re asking: Where can that information go once access is granted?

03.01.03 – Information Flow Enforcement

This control ensures that once a user has access to CUI, that data can only flow where it's explicitly allowed — and not where it shouldn’t.

That includes:

• Preventing sensitive data from being sent to unauthorized destinations
• Ensuring internal traffic isn’t spoofed or misrouted
• Blocking CUI from being transmitted in the clear over public networks

It’s all about defining which pathways are allowed — and making sure your systems technically enforce those boundaries.

03.01.03 – Key Requirement

You must enforce approved authorizations for controlling the flow of CUI,
within your system and across connected systems.

That means:

• Having documented policies that define valid information flow
• And technical controls in place to prevent unauthorized movement

It’s not enough to say “CUI shouldn’t leave the internal network” —
Your system must actually prevent it.

03.01.03 – Implementation Tips

Here are practical steps to enforce secure information flow:

• Define approved data flow paths in your network and system architecture
• Use firewalls, proxies, and access control lists (ACLs) to restrict flow
• Implement data labeling or content filtering for CUI
• Apply network segmentation to prevent internal spoofing
• Restrict external transfers of CUI to authorized endpoints only
• Document systems and services permitted to exchange CUI
• Enforce encryption and boundary protections for cross-environment flows

03.01.03 – Evidence

Auditors may ask to see:

• Information flow policy and enforcement procedures
• System or network configuration files showing flow restrictions
• A diagram mapping allowed data flows between systems
• Audit logs of blocked or permitted flow events
• Role-based or data-type-based flow restrictions

These artifacts also support your System Security Plan (SSP) by showing how flow control maps to specific components and users.

Include in your SSP

You’ll want to document:

• How your organization defines and enforces information flow paths
• Which technologies (e.g., firewall, DLP, proxies) you use to enforce flow control
• How flow enforcement is monitored and tested over time

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Why it matters..

Control 03.01.03 ensures that CUI doesn’t just stay in the system — it stays where it’s supposed to.

Without clear boundaries and technical enforcement, sensitive information can leak, bypass controls, or get exfiltrated unnoticed.
Information Flow Enforcement is the firewall between access and exposure.

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• Enforceable information flow policies tied to user roles
• Uploadable diagrams and system configurations
• Audit log visibility and flow anomaly detection
• SSP guidance linked directly to control objectives

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Next up in this NIST 800-171 Rev. 3 series:
03.01.04 – Separation of Duties

We’ll break down how role separation keeps your system honest, and why it’s critical for preventing internal abuse.

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