How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927 is a critical Next.js vulnerability allowing authorization bypass. Named 'Next.js Middleware Authorization Bypass', it impacts applications relying on Next.js middleware for access control. Published on March 21, 2025, this flaw demands immediate attention.
What is Next.js Middleware Authorization Bypass?
This vulnerability, CVE-2025-29927, stems from improper handling of the x-middleware-subrequest header within Next.js middleware. Attackers can craft requests to manipulate this header, tricking the middleware into bypassing intended authorization logic. This allows unauthorized access to routes that should be protected by your application's access control mechanisms.
Impact and Risks for your Infrastructure
The critical impact of CVE-2025-29927 is unauthorized access to protected Next.js pages and API routes. Attackers can bypass authentication and authorization checks, potentially leading to data breaches, privilege escalation, or unauthorized data manipulation in affected applications.
Step-by-Step Mitigation Guide
To mitigate CVE-2025-29927, upgrade your Next.js application immediately to a fixed version: 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 routes, even with manipulated x-middleware-subrequest headers. Re-deploy and monitor logs for unauthorized access attempts.
- 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.