CVE-2025-29927: Nextjs Middleware Authorization Bypass

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Many systems using Next.js software have vulnerabilities. This exploit is very simple and easy to exploit and needs to be fixed urgently in vulnerable systems.

Affected Versions:

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.

Official Patches:

Vercel, the company behind Next.js, has released patches for the vulnerability:
– For Next.js 15.x, the issue is fixed in version 15.2.3
– For Next.js 14.x, the issue is fixed in version 14.2.25
– For Next.js versions 11.1.4 through 13.5.6, users are advised to implement the workaround described below.

CVSS 3.x Severity and Vector Strings:

  • Base Score: 9.1 CRITICAL
  • Vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`

Method of Exploitation:

Example Exploit Request:

GET /admin HTTP/1.1 
Host: vulnerable-site.com 
x-middleware-subrequest:middleware:middleware:middleware:middleware:middleware

References:

You Should Know:

  1. How to Check if Your Next.js Version is Vulnerable

Run the following command in your project directory:

npm list next

If the version falls within the affected range, upgrade immediately.

2. Manual Workaround (For Unpatched Versions)

Modify your middleware to explicitly reject unexpected `x-middleware-subrequest` headers: 

// middleware.js 
export function middleware(request) { 
if (request.headers.get('x-middleware-subrequest')) { 
return new Response('Unauthorized', { status: 403 }); 
} 
// Rest of your auth logic 
}

3. Upgrading Next.js

To upgrade to the latest secure version:

npm install next@latest

Or for a specific fixed version:

npm install [email protected]

4. Testing for the Vulnerability

Use cURL to test if your middleware is vulnerable: 

curl -H "x-middleware-subrequest: bypass" http://yoursite.com/admin

If you get a 403, the fix is working. If you get a 200, you’re still vulnerable.

5. Monitoring & Logging Suspicious Requests

Add logging in `middleware.js` to detect exploit attempts:

export function middleware(request) { 
if (request.headers.get('x-middleware-subrequest')) { 
console.warn('Possible exploit attempt:', request.url); 
return new Response('Blocked', { status: 403 }); 
} 
}

6. Automated Scanning with Nmap

Check exposed endpoints for misconfigurations:

nmap -p 80,443 --script http-security-headers yoursite.com
  1. Linux Firewall Rule to Block Exploit Patterns
    Add an iptables rule to block repeated `x-middleware-subrequest` headers: 

    iptables -A INPUT -p tcp --dport 80 -m string --string "x-middleware-subrequest" --algo bm -j DROP

What Undercode Say:

This vulnerability highlights the risks of improper middleware authorization checks in modern web frameworks. Immediate patching is critical, but defensive coding practices—such as strict header validation—should be standard.

Expected Output:

  • A 403 Forbidden response when testing with x-middleware-subrequest.
  • No unauthorized access to protected routes after applying patches.
  • Logs showing blocked exploit attempts.

Prediction:

  • Attackers will quickly weaponize this CVE due to its simplicity.
  • Unpatched systems will see increased scanning and exploitation attempts within weeks.
  • Future Next.js updates will enforce stricter middleware security by default.

References:

Reported By: La Pyae – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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