How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927, a critical Next.js Middleware Authorization Bypass vulnerability (CVSS 9.1), has been identified. This flaw allows unauthorized access to protected routes in Next.js applications. Immediate action is required to secure your deployments.
What is Next.js Middleware Authorization Bypass?
This critical vulnerability stems from improper handling of the x-middleware-subrequest header within Next.js middleware. Attackers can craft requests that manipulate this header, effectively bypassing intended authorization logic. This grants unauthorized access to routes that should be protected by your application's middleware.
Impact and Risks for your Infrastructure
The primary impact is a complete authentication and authorization bypass in Next.js applications. Attackers can gain unauthorized access to sensitive data, protected user accounts, and critical API endpoints. This could lead to data breaches, unauthorized actions, and significant reputational damage.
Step-by-Step Mitigation Guide
To mitigate, update your Next.js application to version 15.2.3+, 14.2.25+, 13.5.9+, or 12.3.5+. Verify the fix by ensuring all x-middleware-subrequest header manipulations are correctly blocked by your updated middleware, preventing unauthorized access to protected routes.
- 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.