03.13.06 — Network Communications – Deny by Default – Allow by Exception

Official NIST requirement content

Security requirement

Deny network communications traffic by default, and allow network communications traffic by exception.

Official NIST discussion

Discussion

This requirement applies to inbound and outbound network communications traffic at the system boundary and at identified points within the system. A deny-all, allow-by-exception network communications traffic policy ensures that only essential and approved connections are allowed.

Bare Metal Cyber interpretation

Implementation perspective

Treat Network Communications – Deny by Default – Allow by Exception as a CUI protection outcome that must be reflected in the system boundary, documented implementation, operational behavior, and assessment evidence. Pay particular attention to secure architecture, boundary protection, communications security, cryptography, and isolation of CUI processing.

Confirm the requirement is in scope for the CUI system components, services, users, and external connections being assessed.
Resolve every organization-defined parameter through an approved governance and tailoring process.
Map each clause of the requirement to an accountable owner, implementation mechanism, and evidence source.
Verify that inherited and shared implementations are supported by current provider evidence and responsibility boundaries.
Collect evidence during normal operation and review changes, exceptions, and deficiencies on a risk-based cadence.

Questions to ask

Which CUI assets, data flows, users, and services are protected by this requirement?
Which portions are implemented locally, inherited, shared, or not applicable, and what evidence supports that determination?
Do the system security plan, deployed configuration, operating process, and assessment evidence tell the same story?
What change, incident, or threshold should trigger reassessment?

Evidence and validation

network and trust-boundary diagrams
firewall and gateway configurations
cryptographic configuration and key records
segmentation and isolation test results

Common failure patterns

diagrams omitting cloud and third-party paths
encryption enabled without key governance
flat trust zones permitting unnecessary lateral movement
boundary rules accumulating without owner review

Official NIST SP 800-171A content

Assessment objectives and methods

Assessment objectives (2)

SR-03.13.6.
network communications traffic is denied by default.
SR-03.13.6.
network communications traffic is allowed by exception.

Examine

system and communications protection policy and procedures
procedures for boundary protection
system design documentation
system configuration settings
system audit records
system security plan
other relevant documents or records

Interview

personnel with boundary protection responsibilities
personnel with information security responsibilities
system developers
system administrators

Test

mechanisms for implementing traffic management at managed interfaces

Official source-control relationships

Source NIST SP 800-53 controls

These controls are referenced by the official SP 800-171 Rev. 3 OSCAL record. Open the corresponding control pages for complete control text, enhancements, D3FEND mappings, and related learning.

Source controlSC-7(5)Open SP 800-53 control →

Source record

Authoritative sources

Bare Metal Cyber is an independent educational publisher and is not affiliated with or endorsed by NIST. The official publications, the responsible federal agency, and the governing contract or agreement determine applicability, tailoring, assessment depth, and required implementation.