How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927 (Next.js Middleware Authorization Bypass) is a <strong>CRITICAL severity</strong> vulnerability with a CVSS score of 9.1/10. A vulnerability in Next.js middleware allows attackers to bypass authorization checks by manipulating the x-middleware-subrequest header, granting unauthorized access to protected routes. Follow the step-by-step guide below to remediate this vulnerability in your infrastructure.
What is Next.js Middleware Authorization Bypass?
Next.js Middleware Authorization Bypass (CVE-2025-29927) affects <strong>Next.js</strong>. A vulnerability in Next.js middleware allows attackers to bypass authorization checks by manipulating the x-middleware-subrequest header, granting unauthorized access to protected routes. It was published on 2025-03-21 and affects <strong>Next.js < 15.2.3, < 14.2.25, < 13.5.9, < 12.3.5</strong>. The fixed version is <strong>Next.js 15.2.3+, 14.2.25+, 13.5.9+, 12.3.5+</strong>.
Impact and Risks for your Infrastructure
Authentication and authorization bypass in Next.js applications relying on middleware for access control. Attackers can access protected pages and API routes without valid credentials.
Step-by-Step Mitigation Guide
To remediate CVE-2025-29927, follow the prioritized mitigation steps: Upgrade Next.js immediately to 15.2.3+, 14.2.25+, 13.5.9+, or 12.3.5+. → Block x-middleware-subrequest header at CDN/reverse proxy level. → Move critical authorization checks from middleware into route handlers/server components. → Audit all middleware.ts files for security-critical authorization logic. → Deploy Cloudflare WAF rule or equivalent to block the header manipulation. → Rotate session tokens and audit access logs for potential exploitation.. Verify the fix using the verification commands below and confirm the patched version is deployed across all affected systems.
- 1Upgrade Next.js immediately to 15.2.3+, 14.2.25+, 13.5.9+, or 12.3.5+.
- 2Block x-middleware-subrequest header at CDN/reverse proxy level.
- 3Move critical authorization checks from middleware into route handlers/server components.
- 4Audit all middleware.ts files for security-critical authorization logic.
- 5Deploy Cloudflare WAF rule or equivalent to block the header manipulation.
- 6Rotate session tokens and audit access logs for potential exploitation.