A critical security vulnerability, identified as CVE-2025-29927, has been discovered in the Next.js framework, a widely used React-based tool for building web applications. This flaw enables attackers to bypass authorization mechanisms, potentially granting unauthorized access to sensitive areas of applications, such as administrative dashboards and user data.
Understanding the Vulnerability
The core issue lies in the handling of the `x-middleware-subrequest` HTTP header within Next.js middleware. Originally designed to prevent infinite loops by identifying internal subrequests, this header can be manipulated by external actors to bypass middleware functions entirely. By crafting requests that include this header with specific values, attackers can trick the application into skipping critical authorization checks.
Affected Versions
The vulnerability impacts self-hosted Next.js applications running versions 11.1.4 through 15.2.2, particularly those deployed using the `next start` command with the `output: ‘standalone’` configuration. Applications hosted on platforms like Vercel or Netlify, or deployed as static exports, are not affected due to differences in how middleware is executed in these environments.
Potential Impact
Exploitation of this vulnerability can lead to several severe consequences:
1. Authorization Bypass: Attackers can access restricted areas without proper credentials, compromising sensitive information and administrative functions.
2. Content Security Policy (CSP) Bypass: If CSP headers are set via middleware, bypassing these can expose the application to cross-site scripting (XSS) attacks.
3. Cache Poisoning and Denial-of-Service (DoS): Manipulating cache behavior by bypassing middleware that sets cache headers can lead to DoS scenarios.
Mitigation Steps
To address this vulnerability, the following actions are recommended:
1. Update Next.js: Upgrade to the latest patched versions:
– For Next.js 15.x, update to version 15.2.3.
– For Next.js 14.x, update to version 14.2.25.
– For Next.js 13.x, update to version 13.5.9.
– For Next.js 12.x, update to version 12.3.5.
2. Implement Workarounds: If immediate patching isn’t feasible:
– Filter Incoming Requests: Configure load balancers or reverse proxies to remove the `x-middleware-subrequest` header from incoming requests.
– Web Server Configuration: For Nginx, use the `proxy_set_header` directive to set `x-middleware-subrequest` to an empty value. In Apache, utilize the `RequestHeader` unset directive to remove the header.
3. Enhance Authorization Mechanisms: Implement additional server-side authorization checks beyond middleware to validate user permissions.
Conclusion
The discovery of CVE-2025-29927 underscores the importance of robust security practices in web application development. Developers and administrators must promptly update their Next.js installations and review their security configurations to mitigate potential risks associated with this vulnerability.
