Critical Authorization Bypass Vulnerability in Nextjs (CVE-2025-29927)

The Next.js team has disclosed a critical authorization bypass vulnerability in Next.js versions before 14.2.25 and 15.2.3. This vulnerability, identified as CVE-2025-29927, allows attackers to bypass authorization by simply passing a specific header to the web server. If authorization is supposed to occur in middleware, it will be bypassed entirely. This issue is trivial to exploit and has been rated as a CVSS 9.1, indicating its high severity.

To help identify and mitigate this vulnerability, a Nuclei template has been created to scan web applications for CVE-2025-29927. The repository for this template can be found here: Nuclei Template for CVE-2025-29927.

For more details on the original research, visit: Next.js and the Corrupt Middleware.

You Should Know:

1. How to Identify the Vulnerability

To check if your Next.js application is vulnerable, you can use the Nuclei template provided. Below are the steps to run the scan:

1. Install Nuclei (if not already installed):

go install -v github.com/projectdiscovery/nuclei/v2/cmd/nuclei@latest

2. Run the Nuclei Scan:

nuclei -t https://github.com/6mile/nextjs-CVE-2025-29927 -u <your-target-url>

3. Interpret the Results:

• If the scan returns a positive result, your application is vulnerable.
• Immediately update your Next.js version to 14.2.25 or 15.2.3.

2. Mitigation Steps

To mitigate this vulnerability, follow these steps:

1. Update Next.js:

Update your Next.js application to the latest patched version:

npm install next@latest

2. Verify Middleware Authorization:

Ensure that your middleware is correctly validating authorization headers. Example middleware code:

import { NextResponse } from 'next/server';

export function middleware(request) {
const authHeader = request.headers.get('authorization');
if (!authHeader || authHeader !== 'valid-token') {
return NextResponse.redirect(new URL('/unauthorized', request.url));
}
return NextResponse.next();
}

3. Test Your Application:

After updating, re-run the Nuclei scan to confirm the vulnerability is resolved.

3. Additional Security Measures

• Enable HTTPS: Ensure your application is served over HTTPS to prevent header tampering.

  </li>
  </ul>
  
  <h1>Generate a self-signed certificate (for testing)</h1>
  
  openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 -nodes
  

  • Use Web Application Firewalls (WAF): Deploy a WAF to filter malicious traffic.

    </li>
    </ul>
    
    <h1>Example: Install and configure ModSecurity on Apache</h1>
    
    sudo apt-get install libapache2-mod-security2
    sudo a2enmod security2
    

    • Monitor Logs: Regularly monitor server logs for suspicious activity.

      </li>
      </ul>
      
      <h1>Tail Apache logs in real-time</h1>
      
      tail -f /var/log/apache2/access.log
      

      What Undercode Say:

      This vulnerability highlights the importance of keeping dependencies up to date and thoroughly testing middleware logic. The simplicity of the exploit underscores the need for robust authorization mechanisms. Always validate headers and implement layered security measures to protect your applications.

      Expected Output:

      • Vulnerability Scan Results: Positive or negative identification of CVE-2025-29927.
      • Updated Next.js Application: Confirmation of successful update to a patched version.
      • Secure Middleware: Verified middleware logic to prevent authorization bypass.
      • Enhanced Security: Implementation of HTTPS, WAF, and log monitoring.

      For further reading, visit:

      • Nuclei Template for CVE-2025-29927
      • Next.js and the Corrupt Middleware

      References:

      Reported By: Mccartypaul Softwaresupplychain – Hackers Feeds
      Extra Hub: Undercode MoN
      Basic Verification: Pass ✅

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