Simon Willison’s Weblog

23rd March 2025 - Link Blog

Next.js and the corrupt middleware: the authorizing artifact. Good, detailed write-up of the Next.js vulnerability CVE-2025-29927 by Allam Rachid, one of the researchers who identified the issue.

The vulnerability is best illustrated by this code snippet:

const subreq = params.request.headers['x-middleware-subrequest'];
const subrequests = typeof subreq === 'string' ? subreq.split(':') : [];
// ...
for (const middleware of this.middleware || []) {
  // ...
  if (subrequests.includes(middlewareInfo.name)) {
    result = {
      response: NextResponse.next(),
      waitUntil: Promise.resolve(),
    };
    continue;
  }
}

This was part of Next.js internals used to help avoid applying middleware recursively to requests that are re-dispatched through the framework.

Unfortunately it also meant that attackers could send a x-middleware-subrequest HTTP header with a colon-separated list of middleware names to skip. If a site used middleware to apply an authentication gate (as suggested in the Next.js documentation) an attacker could bypass that authentication using this trick.

The vulnerability has been fixed in Next.js 15.2.3 - here's the official release announcement talking about the problem.