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AC-4(21) AC · Access Control

Physical or Logical Separation of Information Flows

LOW MODERATE HIGH
1 Overlay 1 Related Control
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Requirements NIST SOURCE

Separate information flows logically or physically using [organization-defined mechanisms and/or techniques] to accomplish [required separations].

Discussion (NIST Supplemental Guidance)

Enforcing the separation of information flows associated with defined types of data can enhance protection by ensuring that information is not commingled while in transit and by enabling flow control by transmission paths that are not otherwise achievable. Types of separable information include inbound and outbound communications traffic, service requests and responses, and information of differing security impact or classification levels.

Implementation Guidance

Engineering Interpretation

Original engineering commentary written for this explorer — not NIST source text and not authoritative guidance.

No engineering interpretation has been authored for AC-4(21) yet. This section is architected to receive it — see the Requirements and Assessment sections above for the authoritative NIST source content in the meantime.

Assessment

NIST SP 800-53A REV 5.2.0

Assessment Objectives

  1. mechanisms and/or techniques used to logically separate information flows are defined (if selected);
  2. mechanisms and/or techniques used to physically separate information flows are defined (if selected);
  3. required separations by types of information are defined;
  4. information flows are separated logically using <AC-04(21)_ODP[01] mechanisms and/or techniques> to accomplish <AC-04(21)_ODP[03] required separations>;
  5. information flows are separated physically using <AC-04(21)_ODP[02] mechanisms and/or techniques> to accomplish <AC-04(21)_ODP[03] required separations>.

Examine

[SELECT FROM: Information flow enforcement policy; information flow control policies; procedures addressing information flow enforcement; system design documentation; system configuration settings and associated documentation; list of required separation of information flows by information types; list of mechanisms and/or techniques used to logically or physically separate information flows; system audit records; system security plan; other relevant documents or records].

Interview

[SELECT FROM: Organizational personnel with information flow enforcement responsibilities; system/network administrators; organizational personnel with information security responsibilities; system developers].

Test

[SELECT FROM: Mechanisms implementing information flow enforcement functions].

Overlays

Showing the OT/ICS overlay for the parent control AC-4 — see the AC-4(21) entries within each baseline below.

OT/ICS Overlay SP 800-82r3

NIST SP 800-82r3 Appendix F, Table 22. Blank baseline means the control/control enhancement is not selected in that initial OT baseline.

LOW

Not applicable at this tier.

MODERATE

  • Base control: Included (matches standard baseline)

HIGH

  • Base control: Included (matches standard baseline)
  • Included: (4)

STIGs & CCIs

No STIG checks or CCI mappings are currently loaded for AC-4(21). This section is architected to display, per product: STIG ID, Finding ID, Severity, Title, Description, Check, Fix, CCI, and NIST control mapping — but nothing is populated here until a real DISA STIG/CCI dataset is ingested.

Learn more about STIG/CCI integration →

Evidence

Potential Evidence — Derived

Categorized from the SP 800-53A "Examine"/"Test" artifact list above by keyword — not an authoritative NIST evidence list.

Policy

  • Information flow enforcement policy
  • information flow control policies
  • system security plan

Configuration

  • system design documentation
  • system configuration settings and associated documentation

Testing

  • Mechanisms implementing information flow enforcement functions

Other Records

  • procedures addressing information flow enforcement
  • list of required separation of information flows by information types
  • list of mechanisms and/or techniques used to logically or physically separate information flows
  • system audit records
  • other relevant documents or records