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Controls / System and Communications Protection / 3.13.6
3.13.6
SC.L2-3.13.6 · Level 2

Deny network communications traffic by default and allow network communications traffic by…

NIST SP 800-171 Rev 2 · §3.13.6

Deny network communications traffic by default and allow network communications traffic by exception (i.e., deny all, permit by exception).

Assessment objectives

From NIST SP 800-171A · 2 objectives must be met to satisfy this control
3.13.6[a] network communications traffic is denied by default.
Examine
[SELECT FROM: System and communications protection policy; procedures addressing boundary protection; security plan; system design documentation; system configuration settings and associated documentation; system audit logs and records; other relevant documents or records].
Interview
[SELECT FROM: System or network administrators; personnel with information security responsibilities; system developer; personnel with boundary protection responsibilities].
Test
[SELECT FROM: Mechanisms implementing traffic management at managed interfaces].
3.13.6[b] network communications traffic is allowed by exception.
Examine
[SELECT FROM: System and communications protection policy; procedures addressing boundary protection; security plan; system design documentation; system configuration settings and associated documentation; system audit logs and records; other relevant documents or records].
Interview
[SELECT FROM: System or network administrators; personnel with information security responsibilities; system developer; personnel with boundary protection responsibilities].
Test
[SELECT FROM: Mechanisms implementing traffic management at managed interfaces].

What we look for in practice

The following are placeholder notes that should be refined based on practice experience.

What assessors look for in practice. Documented evidence that this control is implemented across all CUI-handling systems within scope. Specific artifacts vary, but expect requests for written procedures, system configurations, and operational records demonstrating the control is active.

Common failure patterns. Typical issues include incomplete documentation, inconsistent implementation across systems, and missing periodic review records. Refine this section as your practice accumulates direct assessment experience.

Strong evidence looks like. Formal policy referencing this control, technical configurations demonstrating enforcement, periodic review logs with sign-off, and procedures for handling exceptions.

Scoring and POA&M context. 5-point control under the DoD Assessment Methodology. Highest scoring impact — treat as a foundational requirement. Not POA&M-eligible — must be implemented at assessment time.