# AC-4(27)_redundantindependent-filtering-mechanisms

> AC-4(27)_redundantindependent-filtering-mechanisms is an Agent Skill for devops & security published in the CyberStrikeus/CyberStrike repository on GitHub: a SKILL.md instruction file an AI agent can load to do this task.

When transferring information between different security domains, implement content filtering solutions that provide redundant and independent filteri

- Skill name: `AC-4(27)_redundantindependent-filtering-mechanisms`
- Category: [DevOps & Security](https://www.openparable.com/skill/devops-security/1)
- Source: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/NIST/SP800-53_rev5/AC_access-control/AC-4(27)_redundantindependent-filtering-mechanisms/SKILL.md
- Repository: CyberStrikeus/CyberStrike (1,211 GitHub stars when collected)

## SKILL.md

# AC-4(27) Redundant/Independent Filtering Mechanisms

> **Enhancement of:** AC-4

## High-Level Description

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

Content filtering is the process of inspecting information as it traverses a cross-domain solution and determines if the information meets a predefined policy. Redundant and independent content filtering eliminates a single point of failure filtering system. Independence is defined as the implementation of a content filter that uses a different code base and supporting libraries (e.g., two JPEG filters using different vendors’ JPEG libraries) and multiple, independent system processes.

## What to Check

- [ ] Verify AC-4(27) Redundant/Independent Filtering Mechanisms is documented in SSP
- [ ] Confirm control is operating effectively
- [ ] Review evidence of continuous monitoring for AC-4(27)
- [ ] 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(27) 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-mcp | Test web application access controls   | `browser_auth_test`        |

## Remediation Guide

### Control Statement

When transferring information between different security domains, implement content filtering solutions that provide redundant and independent filtering mechanisms for each data type.

### Implementation Guidance

Content filtering is the process of inspecting information as it traverses a cross-domain solution and determines if the information meets a predefined policy. Redundant and independent content filtering eliminates a single point of failure filtering system. Independence is defined as the implementation of a content filter that uses a different code base and supporting libraries (e.g., two JPEG filters using different vendors’ JPEG libraries) and multiple, independent system processes.

## Risk Assessment

| Finding                                                             | Severity | Impact                    |
| ------------------------------------------------------------------- | -------- | ------------------------- |
| AC-4(27) Redundant/Independent Filtering Mechanisms not implemented | High     | Access Control            |
| AC-4(27) 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(27)](https://csrc.nist.gov/projects/cprt/catalog#/cprt/framework/version/SP_800_53_5_1_1/home?element=ac-4.27)
- [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 (none) reviewed

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