Exploiting CVE-2025-29927: A Red-Teamer’s Guide to Next.js Middleware Bypass

Affected Systems and Versions

This critical vulnerability impacts Next.js applications running:

• Versions prior to 14.2.25
• Versions prior to 15.2.3

Technical Deep Dive: The Middleware Vulnerability

The core security failure stems from improper validation of the x-middleware-subrequest header. Next.js middleware typically intercepts HTTP requests to enforce security policies, but attackers can exploit this vulnerability by injecting the malicious header to bypass authorization checks.

Exploitation Walkthrough

1. Craft Malicious Requests: Use tools like Burp Suite or Postman to create requests containing:

   GET /protected-route HTTP/1.1
   Host: vulnerable-app.com
   x-middleware-subrequest: true

2. Test Authorization Endpoints: Target routes relying on middleware for access control
3. Escalate Access: Demonstrate impact through privilege escalation or data exfiltration

Why This Matters for Red-Team Operations

Detection and Mitigation Strategies

Identifying Exploitation Attempts

Monitor HTTP logs for suspicious patterns:

GET /protected-route HTTP/1.1 200 - x-middleware-subrequest: true

Patching Recommendations

Next.js has released security patches in versions 14.2.25 and 15.2.3. For immediate mitigation:

export function middleware(request: NextRequest) {
  if (request.headers.has('x-middleware-subrequest')) {
    return new Response('Unauthorized', { status: 401 });
  }
  return NextResponse.next();
}

Vendor Security Posture

Vercel (Next.js maintainers) maintains a strong security track record, with rapid response to vulnerabilities like CVE-2024-51479. Their transparent advisory system enables quick organizational response.