# AC-4(21)_physical-or-logical-separation-of-information-flows

> Separate information flows logically or physically using [organization-defined] to accomplish [organization-defined].

- Skill: `cyberstrikeus/ac-4-21-physical-or-logical-separation-of-information-flows` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/ac-4-21-physical-or-logical-separation-of-information-flows`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/ac-4-21-physical-or-logical-separation-of-information-flows/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: cyberstrikeus (https://skillmd.com/u/cyberstrikeus)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cyberstrikeus/ac-4-21-physical-or-logical-separation-of-information-flows

---


# AC-4(21) Physical or Logical Separation of Information Flows

> **Enhancement of:** AC-4

## High-Level Description

**Family:** Access Control (AC)
**Framework:** NIST SP 800-53 Rev 5

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.

## What to Check

- [ ] Verify AC-4(21) Physical or Logical Separation of Information Flows is documented in SSP
- [ ] Confirm control is operating effectively
- [ ] Review evidence of continuous monitoring for AC-4(21)
- [ ] Verify enhancement builds upon base control AC-4

## How to Test

### Step 1: Review Documentation

Examine the System Security Plan (SSP) and related artifacts for AC-4(21) implementation details. Verify the organization has documented how this control is satisfied.

### Step 2: Validate Implementation

```
# For cloud environments, use cloud-audit-mcp tools
# For on-premises, review system configurations directly

# Example: Check if account management policies exist
grep -r "account.management\|access.control" /etc/security/ 2>/dev/null
```

### Step 3: Test Operating Effectiveness

Verify the control is actively functioning, not just documented. Check logs, configurations, and operational evidence.

## Tools

| Tool            | Purpose                                | Usage                      |
| --------------- | -------------------------------------- | -------------------------- |
| cloud-audit-mcp | Check IAM policies and access controls | `cloud_audit_iam_policies` |
| hackbrowser | Test web application access controls   | `hackbrowser`        |

## Remediation Guide

### Control Statement

Separate information flows logically or physically using [organization-defined] to accomplish [organization-defined].

### Implementation 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.

## Risk Assessment

| Finding                                                                      | Severity | Impact                    |
| ---------------------------------------------------------------------------- | -------- | ------------------------- |
| AC-4(21) Physical or Logical Separation of Information Flows not implemented | High     | Access Control            |
| AC-4(21) partially implemented                                               | Medium   | Incomplete Access Control |

## CWE Categories

| CWE ID  | Title                   |
| ------- | ----------------------- |
| CWE-284 | Improper Access Control |

## References

- [NIST SP 800-53 Rev 5 - AC-4(21)](https://csrc.nist.gov/projects/cprt/catalog#/cprt/framework/version/SP_800_53_5_1_1/home?element=ac-4.21)
- [NIST SP 800-53A Rev 5 (Assessment Procedures)](https://csrc.nist.gov/pubs/sp/800/53/a/r5/final)
- [NIST SP 800-53 Rev 5 Full Catalog](https://csrc.nist.gov/pubs/sp/800/53/r5/upd1/final)

## Checklist

- [ ] Control documented in SSP
- [ ] Implementation evidence collected
- [ ] Operating effectiveness validated
- [ ] Continuous monitoring in place
- [ ] Related controls (SC-32) reviewed