CVE-2025-29927 - A Deep Dive Into the Next.js Authorization Bypass Vulnerability

Next.js is a popular React framework that developers utilize to build full-stack web applications. A recent vulnerability, with the CVE identifier CVE-2025-29927, was discovered affecting Next.js versions prior to 14.2.25 and 15.2.3. This post will dive into the details of this vulnerability, provide code snippets, discuss the exploit, and offer some recommended mitigation steps.

Vulnerability Details

The vulnerability exists within the Next.js framework, specifically when authorization checks occur in middleware. By exploiting this vulnerability, attackers can bypass these authorization checks and infiltrate the application, potentially causing severe consequences.

The crux of this authorization bypass lies in how the middleware handles requests containing the "x-middleware-subrequest" header. By intercepting requests and injecting this header, an attacker can bypass the verification process and gain unauthorized access to sensitive data or restricted functionalities.

Exploit Details

An attacker may exploit this vulnerability by crafting a malicious request that includes the "x-middleware-subrequest" header. The following example demonstrates this:

curl -H "x-middleware-subrequest: true" http://example.com/secure-endpoint

By appending the "x-middleware-subrequest" header, an attacker can access the "secure-endpoint," even if they do not have appropriate authorization.

Original References

The official Next.js GitHub Repository provides thorough documentation on this vulnerability, including an explanation of the issue and links to official patches (fixed in 14.2.25 and 15.2.3):

- Next.js GitHub Advisory: https://github.com/vercel/next.js/security/advisories/GHSA-c99v-c7c7-gccw
- Next.js 14.2.25 Release Notes: https://github.com/vercel/next.js/releases/tag/v14.2.25
- Next.js 15.2.3 Release Notes: https://github.com/vercel/next.js/releases/tag/v15.2.3

Mitigation

The safest and most effective way to mitigate this vulnerability is to update your Next.js application to the fixed versions – 14.2.25 or 15.2.3 – as soon as possible. However, if patching right away is not feasible, you can implement a temporary workaround by preventing external user requests containing the "x-middleware-subrequest" header from reaching your application. An example of this using Nginx is provided below:

location / {
  if ($http_x_middleware_subrequest) {
    return 403;
  }
  proxy_pass http://nextjs_app;
}

This configuration would prevent any requests containing the "x-middleware-subrequest" header from accessing your Next.js application, effectively blocking the exploit.

Conclusion

In summary, CVE-2025-29927 is a critical vulnerability found in the Next.js framework, allowing malicious actors to bypass authorization checks by exploiting middleware handling. To minimize the risk of succumbing to this vulnerability, it is crucial to update to Next.js 14.2.25 or 15.2.3, or at the very least apply a temporary workaround that prevents requests containing the "x-middleware-subrequest" header from accessing your application.

Timeline

Published on: 03/21/2025 15:15:42 UTC