CVE-2025-29927: Understanding the Vulnerability and How to Protect Your Next.js Application

Overview

CVE-2025-29927 is a critical security vulnerability found in certain versions of the Next.js framework. The issue arises when authentication and authorization checks are implemented solely in middleware. Under specific conditions, an attacker may bypass these checks by sending a crafted request with the x-middleware-subrequest header. This bypass can allow unauthorized access to protected pages or resources.

We will explain how this vulnerability works, demonstrate a proof-of-concept (PoC) scenario, and provide workarounds and patches to mitigate the risk.

How the Vulnerability Works

The Root Cause

In affected Next.js versions (e.g., versions up to 14.2.14 or similar vulnerable releases), internal subrequests are managed with a header (x-middleware-subrequest). When this header is added to a request, Next.js may treat it as an internal call and skip some of the normal middleware checks. If your middleware is solely relying on checking for an authentication token, then an attacker who adds this header might bypass the authentication mechanism.

Exploitation Scenario

1. Protected Endpoint:

   A Next.js application has a protected page (/protected) that requires a valid authentication cookie.

2. Middleware Check:

   A middleware is implemented to check for the presence of authToken. If the cookie is missing, it redirects the user to /login.

3. Bypass with Header:

   In vulnerable environments, if an attacker sends an HTTP request to /protected with the header:

x-middleware-subrequest: middleware
or
x-middleware-subrequest: middleware:middleware:middleware:middleware:middleware

the framework might incorrectly treat this as an internal subrequest, bypassing the middleware check and granting access to the protected content without proper authentication.

How to Protect Your Application

1. Update Your Next.js Version

   The primary and most effective way to mitigate this vulnerability is to update your Next.js version to a release where the issue is patched. If you’re using a vulnerable version (e.g., ≤14.2.14), upgrade to at least version 14.2.25 or the latest stable release. Updating ensures that internal subrequest handling has been hardened against such bypass attempts.

npm install next@latest

1. Implement a Header-Based Workaround

   If updating immediately is not an option, you can implement an additional check in your middleware to explicitly block requests that include the x-middleware-subrequest header. Here’s an example of how to modify your middleware:

// middleware.js
import { NextResponse } from 'next/server';

export function middleware(req) {
  // Reject requests that include the x-middleware-subrequest header
  if (req.headers.get('x-middleware-subrequest')) {
    return NextResponse.json({ error: 'Unauthorized' }, { status: 403 });
  }

  // Normal authentication check: verify the presence of authToken cookie
  const token = req.cookies.get('authToken');
  if (!token) {
    return NextResponse.redirect(new URL('/login', req.url));
  }
  
  return NextResponse.next();
}

export const config = {
  matcher: ['/protected'],
};

This workaround adds an explicit check: if the header is present, the request is immediately rejected with a 403 error. This prevents potential abuse of the vulnerability even if you are using an older Next.js version.

Conclusion

CVE-2025-29927 serves as a powerful reminder that even robust frameworks can harbor hidden vulnerabilities when security assumptions are made about internal request handling. By understanding the mechanics behind this vulnerability, developers can proactively protect their applications through a combination of updates and strategic workarounds. Upgrading to a patched version of Next.js should always be your primary line of defense. In parallel, implementing explicit checks—such as rejecting requests that include the x-middleware-subrequest header—adds an essential layer of security.

Ultimately, security is an evolving process that demands constant vigilance. Regular updates, thorough testing in controlled environments, and a multi-layered authentication strategy will help ensure that your application remains resilient against emerging threats. Stay informed, be proactive, and continuously refine your security practices to protect both your users and your infrastructure.