How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927, known as 'Next.js Middleware Authorization Bypass', is a critical vulnerability (CVSS 9.1) affecting Next.js applications. It allows unauthorized access to protected routes by bypassing authorization checks enforced by middleware.
What is Next.js Middleware Authorization Bypass?
This critical Next.js vulnerability arises from improper handling of the `x-middleware-subrequest` header. Attackers can manipulate this header to bypass authorization logic within Next.js middleware. This grants unauthorized access to routes and API endpoints intended to be protected by access control mechanisms.
Impact and Risks for your Infrastructure
The primary impact is a severe authentication and authorization bypass. Attackers can gain unauthorized access to sensitive data, administrative functions, or restricted API routes. This could lead to data breaches, privilege escalation, or full system compromise.
Step-by-Step Mitigation Guide
To fix CVE-2025-29927, immediately upgrade your Next.js application to versions 15.2.3+, 14.2.25+, 13.5.9+, or 12.3.5+. Verify the fix by ensuring your middleware correctly enforces access control for all protected routes after the update. No further configuration changes are typically required.
- 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.