How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927, known as 'Next.js Middleware Authorization Bypass', is a critical vulnerability (CVSS 9.1) affecting Next.js applications. It allows attackers to circumvent authorization controls, granting unauthorized access to protected resources.
What is Next.js Middleware Authorization Bypass?
This vulnerability arises from a flaw in Next.js middleware processing. Attackers can manipulate the `x-middleware-subrequest` header, tricking the middleware into bypassing intended authorization checks. This allows requests to access routes that should be restricted based on access control policies.
Impact and Risks for your Infrastructure
Successful exploitation leads to unauthorized access to protected pages and API routes. This can result in sensitive data exposure, unauthorized actions, and compromise the integrity of your application. Business operations and user trust can be severely impacted.
Step-by-Step Mitigation Guide
To fix this, immediately upgrade 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 running `npm list next`. Re-test critical authorization flows to ensure proper access control 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.