Next.js Middleware Bypass (CVE-2025-29927) Proof of Concept

This repository contains a proof of concept (POC) for CVE-2025-29927, a critical vulnerability in Next.js that allows attackers to bypass authorization checks implemented in middleware.

📋 Table of Contents

1. Running the POC Application
2. Using the Exploit Script
3. How to Fix the Vulnerability
4. Vulnerability Details

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Running the POC Application

This Next.js application demonstrates the vulnerability with a protected dashboard route.

Setup Instructions

1. Clone the repository
2. Install and run:

   npm install
   npm run build
   npm run start

3. Access http://localhost:3000 in your browser
4. Try accessing the protected dashboard at /dashboard

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Using the Exploit Script

The exploit script can test any Next.js application for this vulnerability.

Usage

npm run exploit <target-url>

Example

npm run exploit http://localhost:3000/dashboard

How it Works

The exploit tool:

1. Takes a target URL as input
2. Makes a normal request to check if the route is protected
3. Attempts to bypass middleware using the x-middleware-subrequest header
4. Compares responses to determine vulnerability

Sample Output

=== Next.js CVE-2025-29927 Middleware Bypass Tester ===
Target: http://localhost:3000/dashboard

Testing vulnerability...
Normal request status: 307
Bypass request status: 200

⚠️  VULNERABLE
The route is protected but accessible with the bypass header

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How to Fix the Vulnerability

A fix implementation is available in the how-to-fix branch.

Quick Fixes

1. Update Next.js Version (Recommended)

   • Update to one of these patched versions:
     • Next.js 15.x → 15.2.3
     • Next.js 14.x → 14.2.25
     • Next.js 13.x → 13.5.9
     • Next.js 12.x → 12.3.5

2. Implement Secondary Validation

   • Add authentication checks in your API routes/pages
   • Don't rely solely on middleware for security

Sample Output

=== Next.js CVE-2025-29927 Middleware Bypass Tester ===

Target: http://localhost:3000/dashboard

Testing vulnerability...
Making request without bypass header...
Making request with bypass header...
Normal request status: 307
Bypass request status: 307

✓ NOT VULNERABLE - Protected Route
The route is protected and the bypass attempt was unsuccessful
Normal request redirected to: /
Bypass request redirected to: /

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Vulnerability Details

CVE Information

• CVE ID: CVE-2025-29927
• Affected Versions:
  • 13.0.0 - 13.5.8
  • 14.0.0 - 14.2.24
  • 15.0.0 - 15.2.2
  • 11.1.4 - 12.3.4

Description

Next.js uses an internal header x-middleware-subrequest to prevent recursive requests. This vulnerability allows attackers to bypass middleware security checks by manipulating this header.

Impact

• Unauthorized access to protected routes
• Bypass of authentication checks
• Circumvention of security middleware

Affected Environments

• Self-hosted Next.js applications using Middleware

References

• Next.js Security Advisory
• GitHub Security Advisory

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Practice This Exploit in a Real CTF Environment

Want to go further? This CVE is implemented as a flag in OSS OopsSec Store, a deliberately vulnerable lab designed for offensive security training.

👉 Try the challenge here → kOaDT/oss-oopssec-store

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Disclaimer

This tool is provided for educational and testing purposes only. Only use this tool on systems you own or have explicit permission to test.