CVE-2025-29927: Vercel Next.js Auth Bypass Vulnerability

CVE-2025-29927 Overview

Next.js is a React framework for building full-stack web applications. Starting in version 1.11.4 and prior to versions 12.3.5, 13.5.9, 14.2.25, and 15.2.3, it is possible to bypass authorization checks within a Next.js application, if the authorization check occurs in middleware. This vulnerability is fixed in 12.3.5, 13.5.9, 14.2.25, and 15.2.3.

Critical Impact
Unauthorized access to sensitive resources due to bypassed authorization checks.

Affected Products

Vercel Next.js

Discovery Timeline

Not Available - Vulnerability discovered by Not Available
Not Available - Responsible disclosure to Vercel
Not Available - CVE CVE-2025-29927 assigned
Not Available - Vercel releases security patch
2025-03-21 - CVE CVE-2025-29927 published to NVD
2025-09-10 - Last updated in NVD database

Technical Details for CVE-2025-29927

Vulnerability Analysis

The vulnerability allows an attacker to bypass authorization checks by manipulating the x-middleware-subrequest header. This oversight enables unauthorized access to sensitive data within Next.js applications.

Root Cause

The core issue resides in the improper handling of authorization checks within middleware, which can be bypassed by exploiting the x-middleware-subrequest header.

Attack Vector

The attack is conducted over the Network, utilizing specially crafted HTTP requests to bypass authorization via middleware.

javascript

// Example exploitation code (sanitized)
fetch('/api/private', {
  method: 'GET',
  headers: {
    'x-middleware-subrequest': 'yes',
    'Authorization': 'Bearer invalidtoken'
  }
});

Detection Methods for CVE-2025-29927

Indicators of Compromise

Unexpected entries in server logs with x-middleware-subrequest headers.
Unauthorized access records in application logs.
Anomalous HTTP request patterns.

Detection Strategies

Utilize log monitoring to detect instances of HTTP requests containing the x-middleware-subrequest header. Monitor for any unauthorized access attempts and analyze user access patterns.

Monitoring Recommendations

Implement comprehensive log analysis tools to continuously monitor HTTP headers in incoming requests. Setup alerts for detection of unusual patterns in requests containing specific headers like x-middleware-subrequest.

How to Mitigate CVE-2025-29927

Immediate Actions Required

Update Next.js to versions 12.3.5, 13.5.9, 14.2.25, or 15.2.3.
Block external requests containing the x-middleware-subrequest header.
Review and enhance middleware authorization logic.

Patch Information

Patches are available, and it's crucial to update to versions 12.3.5, 13.5.9, 14.2.25, or 15.2.3. More details can be found in Vercel's advisory.

Workarounds

If updating is infeasible, configure the application server to filter out requests containing the x-middleware-subrequest header before they reach the application.

bash

# Example Nginx configuration to block specific headers
server {
  location / {
    if ($http_x_middleware_subrequest) {
      return 403;
    }
    proxy_pass http://localhost:3000;
  }
}