How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927 identifies a critical authorization bypass vulnerability in Next.js. This flaw allows malicious actors to circumvent security checks implemented through Next.js middleware. Addressing this is paramount for maintaining the security of your Next.js applications.
What is Next.js Middleware Authorization Bypass?
The vulnerability, named Next.js Middleware Authorization Bypass, 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 resources that are intended to be protected.
Impact and Risks for your Infrastructure
This critical flaw enables unauthorized access to protected pages and API routes within Next.js applications. Attackers can bypass authentication and authorization controls, potentially leading to sensitive data exposure or unauthorized actions. This directly impacts the confidentiality and integrity of your application's data.
Step-by-Step Mitigation Guide
To fix this, update your Next.js installation to versions 15.2.3+, 14.2.25+, 13.5.9+, or 12.3.5+. Verify the update by checking your `package.json` and running `npm install` or `yarn install`. After deployment, confirm that all middleware-protected routes correctly enforce authorization as expected.
- 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.