How to fix CVE-2025-29927 – Step-by-Step Guide
CVE-2025-29927, a critical Next.js Middleware Authorization Bypass (CVSS 9.1), was published on March 21, 2025. This vulnerability allows attackers to circumvent access controls in Next.js applications. Immediate action is required to protect your systems from unauthorized access.
What is Next.js Middleware Authorization Bypass?
This Next.js vulnerability stems from improper handling of the `x-middleware-subrequest` header within middleware functions. Attackers can craft requests that exploit this flaw, causing the middleware to incorrectly process authorization logic. This bypasses intended access controls, allowing unauthorized access to restricted routes and resources.
Impact and Risks for your Infrastructure
The primary impact is unauthorized access to sensitive data and functionality in Next.js applications. Attackers can bypass authentication and authorization checks, accessing protected pages, API routes, and potentially critical business logic. This could lead to data breaches, unauthorized operations, and reputational damage.
Step-by-Step Mitigation Guide
To mitigate CVE-2025-29927, update your Next.js installation to version 15.2.3+, 14.2.25+, 13.5.9+, or 12.3.5+. Verify the fix by ensuring your `package.json` reflects the updated versions and running `npm install` or `yarn install`. Thoroughly test your application's authorization flows to confirm proper access control enforcement.
- 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.