CVE‑2025‑29927 POC: Next.js Middleware Authorization Bypass
This repository (MuhammadWaseem29/CVE‑2025‑29927‑POC) provides a Proof of Concept (PoC) for CVE‑2025‑29927, demonstrating how a specially crafted HTTP request can bypass middleware-based authorization in Next.js applications. Designed for educational and security‑research use only—testing without explicit consent is unethical and illegal.
📋 Table of Contents
- Prerequisites
- Vulnerability Overview
- PoC Walkthrough
- Built‑in Exploitation Logic
- Detection & Exploitation Template
- Affected Versions & Impact
- Mitigation Strategies
- Why It Matters
✅ Prerequisites
- A Next.js application using middleware for protecting routes
- An HTTP client (e.g.
curl, Burp Suite, custom script)
- Understanding of HTTP headers, especially
x-middleware-subrequest
🔍 Vulnerability Overview
- CVE‑2025‑29927 is a critical authorization bypass vulnerability in Next.js
- Attackers can bypass middleware security controls by adding a malicious
x-middleware-subrequest header to HTTP requests
- Affected Next.js versions include:
- 11.1.4 ≤ v < 12.3.5
- 12.x < 12.3.5; 13.x < 13.5.9; 14.x < 14.2.25; 15.x < 15.2.3
🧪 PoC Walkthrough in the Repository
The PoC repository walks through two key steps:
Step 1: Initial Request (Unauthorized)
- Request to protected endpoint (e.g.
/dashboard) without custom header is redirected or denied.
Step 2: Modified Request (Authorization Bypassed)
- Same endpoint accessed with
X-Middleware-Subrequest header containing middleware path (e.g. middleware or src/middleware).
- Access granted, demonstrating bypass of middleware logic
📝 Detection & Exploitation Template
A Nuclei template provided by ProjectDiscovery helps scan for CVE‑2025‑29927 automatically:
| Phase | Action |
|--------------------------------|--------|
| Detect Next.js | Request base URL; check for "_next/static" in response body. |
| Enumerate endpoints | Extract URLs via regex href=['"](\/[^.\"']+). |
| Identify protected routes | Check for x-middleware-rewrite, x-middleware-next, or x-middleware-redirect headers in responses (non-200). |
| Attempt bypass | Resend with header x-middleware-subrequest: middleware:middleware:… |
| Validate bypass | If response status is 200, vulnerability is confirmed. |
Template applies to typical middleware naming conventions: middleware, src/middleware, or pages/_middleware for older versions.
⚠️ Impact & Affected Versions
Affected Next.js versions:
- Fixed in:
- Next.js 12.3.5, 13.5.9, 14.2.25, 15.2.3 and later
Potential impacts:
- Unauthorized access to protected routes
- Bypass of authentication, CSP enforcement, redirects
- Cache poisoning / DoS if middleware also handles localization or rewriting and attacker forces error responses to cache
🔧 Mitigation Strategies
- Upgrade Next.js to patched versions:
- 12.3.5, 13.5.9, 14.2.25, or 15.2.3 and above
- Block or strip
x-middleware-subrequest header at:
- Load balancer (e.g. AWS ELB, Cloudflare)
- Web server (e.g. Nginx:
proxy_set_header x-middleware-subrequest "";)
- Custom front‑end middleware to remove the header before Next.js processes it
- Implement redundant authorization checks downstream of middleware logic.
🎯 Why It Matters
- Next.js powers many modern web applications (e-commerce, dashboards, content platforms)
- Use of middleware for routing, redirects, auth, and headers is common
- A single HTTP header can override security logic—simple but extremely dangerous
📚 Further Reading & Reference
- NVD entry for CVE‑2025‑29927
- Datadog Security Research blog on the exploit mechanism
- JFrog Advanced Security analysis with technical insights and impact assessment
- ProjectDiscovery Nuclei template and scan guidance
- Rapid7 analysis highlighting context and variable impact
📝 Conclusion
This Proof of Concept clearly demonstrates that CVE‑2025‑29927 enables attackers to bypass middleware-based authorization in vulnerable Next.js applications by injecting a crafted x-middleware-subrequest header. With no complex exploitation required, it's urgent that developers patch vulnerable versions and implement header filtering or redundant auth checks to protect against this critical vulnerability.
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