How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927, "Next.js Middleware Authorization Bypass," is a critical vulnerability (CVSS 9.1) affecting Next.js applications. This flaw allows attackers to bypass security controls, granting unauthorized access to protected resources. Immediate action is crucial to safeguard your deployments.
What is Next.js Middleware Authorization Bypass?
This vulnerability arises from improper handling of the `x-middleware-subrequest` header in Next.js middleware. Attackers can manipulate this header to trick the middleware into bypassing intended authorization checks. This allows them to access routes and resources that should be restricted to authenticated or authorized users.
Impact and Risks for your Infrastructure
The primary impact is a severe authentication and authorization bypass. Attackers can gain unauthorized access to protected pages and API routes, potentially leading to data breaches, unauthorized actions, or full application compromise. This directly undermines the security posture of affected Next.js applications.
Step-by-Step Mitigation Guide
To fix this, update your Next.js application to versions 15.2.3+, 14.2.25+, 13.5.9+, or 12.3.5+. Verify the update by checking your `package.json` and ensuring the new version is deployed. Confirm the fix by testing access to previously protected routes to ensure authorization is enforced.
- 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.