How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927, a critical Next.js Middleware Authorization Bypass (CVSS 9.1), has been identified. This vulnerability 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 vulnerability stems from improper handling of the x-middleware-subrequest header within Next.js middleware. Attackers can craft requests that manipulate this header, tricking the middleware into bypassing intended authorization logic. This effectively circumvents access controls designed to protect specific routes and resources.
Impact and Risks for your Infrastructure
The primary impact is a complete bypass of authentication and authorization mechanisms in Next.js applications. Attackers can gain unauthorized access to sensitive data, administrative functions, or confidential API endpoints. This could lead to data breaches, unauthorized actions, and significant reputational damage.
Step-by-Step Mitigation Guide
To mitigate, update Next.js to versions 15.2.3+, 14.2.25+, 13.5.9+, or 12.3.5+. Verify the fix by deploying the updated application and confirming that protected routes correctly enforce authorization checks, even with manipulated x-middleware-subrequest headers. Regularly monitor logs for unusual access patterns.
- 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.