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CVE-2026-53609: CVE-2026-53609: Server-Side Prototype Pollution in ApostropheCMS

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CVE-2026-53609: Server-Side Prototype Pollution in ApostropheCMS

Vulnerability ID: CVE-2026-53609
CVSS Score: 9.1
Published: 2026-07-31

A critical server-side prototype pollution vulnerability in ApostropheCMS versions up to and including 4.30.0 allows authenticated editors to write arbitrary properties to the global Object.prototype via patch operators. Exploiting a confirmed gadget in publicApiCheck() bypasses authorization on all piece-type REST API endpoints framework-wide, persisting for the lifetime of the Node.js process.

TL;DR

Server-side prototype pollution in ApostropheCMS <= 4.30.0 allows authenticated editors to inject properties into Object.prototype, enabling full process-wide REST API authorization bypass.

⚠️ Exploit Status: POC

Technical Details

  • CWE ID: CWE-1321
  • Attack Vector: Network (AV:N)
  • CVSS v3.1 Score: 9.1 (Critical)
  • EPSS Score: 0.00237 (Percentile: 14.86%)
  • Impact: Authentication and Authorization Bypass
  • Exploit Status: PoC Available
  • KEV Status: Not listed

Affected Systems

  • ApostropheCMS
  • apostrophe: <= 4.30.0 (Fixed in: > 4.30.0)

Code Analysis

Commit: 5a88e96

Fix prototype pollution vulnerability in apos.util.set and apos.util.get

Exploit Details

  • GitHub Security Advisory: Exploit methodology and regression test verification showcasing prototype pollution of publicApiProjection

Mitigation Strategies

  • Upgrade ApostropheCMS to a version higher than 4.30.0
  • Deploy Web Application Firewall (WAF) rule to block keys containing prototype pollution keywords
  • Enforce input sanitization on all incoming REST API PATCH bodies

Remediation Steps:

  1. Identify all active instances of ApostropheCMS running versions <= 4.30.0
  2. Update package dependencies in package.json to the latest fixed release
  3. Run npm install or yarn install to apply the patch
  4. Restart the Node.js application process to clear any polluted prototype memory
  5. Validate REST API PATCH endpoints to ensure proper rejection of unsafe paths

References

Read the full report for CVE-2026-53609 on our website for more details including interactive diagrams and full exploit analysis.

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