Understanding the Security Implications of Internal Headers

Today, we’re examining CVE-2025-29927, a critical security vulnerability affecting Next.js middleware that could allow attackers to bypass authorization checks under certain conditions. This exploit highlights important security considerations when using Next.js middleware for authentication and access control.

What is Next.js?

Next.js is a popular React-based framework for building modern web applications. It provides features like:

• Server-side rendering (SSR)
• Static site generation (SSG)
• API routes
• Middleware for edge-based logic
• Automatic code splitting
• Built-in optimizations (image, font, and script optimization)

Next.js middleware allows developers to run code before a request is processed, enabling functionalities like:

• Authentication & authorization
• Redirects & rewrites
• A/B testing
• Bot protection
• Logging & monitoring

About the http header that allowed the vulnerability

The X-Middleware-Subrequest: src/middleware:nowaf header is an internal marker used by Next.js to indicate that a request is being processed by middleware.

Key Details:

Purpose

• Identifies requests containing the X-Middleware-Subrequest: src/middleware:nowaf header as internal subrequests
• Helps distinguish between external user requests and internal middleware-triggered requests

Header Structure

• src/middleware → Directly references the middleware file inside the “src” folder handling the request
• nowaf → Meant to indicate NO WAF Explicitly indicates Web Application Firewall (WAF) checks should be skipped for this request

When This Header Appears

• Automatically added when Next.js processes the X-Middleware-Subrequest: src/middleware:nowaf header during:
  • Internal rewrites/redirects
  • Edge function executions
  • Middleware-triggered fetches

Example Use Case
If middleware rewrites a request from /old-path to /new-path, Next.js might attach this header to ensure proper internal handling.

The X-Middleware-Subrequest header is part of Next.js’s internal routing system, helping middleware execute efficiently. While developers don’t usually interact with it directly, understanding it can help debug middleware-related issues.

X-Middleware-Subrequest: src/middleware:nowaf

This internal header is a critical component of Next.js’s routing system. Let’s break it down:

Key Characteristics:

• Purpose: Identifies internal subrequests triggered by middleware
• Appearance: Automatically added when:
• Middleware performs rewrites or redirects
• Next.js processes edge functions
• Internal fetches are made
• Security Note: Should never be manually modified or trusted without validation

While developers don’t normally interact with this header, understanding it is crucial for debugging and security purposes.

Understanding the Vulnerability

What is the Issue?

The vulnerability stems from improper authorization handling in Next.js middleware, where an attacker could manipulate request headers (particularly X-Middleware-Subrequest: src/middleware:nowaf) to trick the middleware into skipping security checks.

Root Cause Analysis

• Next.js middleware relies on internal headers like X-Middleware-Subrequest: src/middleware:nowaf to distinguish between user requests and internal subrequests
• If an attacker forges this header, they might bypass middleware-based authentication
• This becomes possible when:
• The middleware fails to validate request sources properly
• The application trusts internal headers without strict verification

Proof of Concept Using PinewoodStore Code

Examining the Vulnerable Implementation

// app/middleware.js from PinewoodStore
import { NextResponse } from "next/server";
import { parseCookies } from "./utils/cookies";
import { jwtVerify } from "jose";

export async function middleware(req) {
  const { pathname } = req.nextUrl;

  if (pathname.startsWith("/admin")) {
    const cookies = parseCookies(req);
    let token = cookies["auth-token"];

    if (!token) {
      return NextResponse.redirect(new URL("/login", req.url));
    }

    try {
      const secret = new TextEncoder().encode(process.env.JWT_SECRET);
      const { payload } = await jwtVerify(token, secret);

      if (payload.role !== "admin") {
         return NextResponse.redirect(new URL("/unauthorized-page", req.url)); // Redirect if not an admin
      }

      return NextResponse.next();
    } catch (error) {
      return NextResponse.redirect(new URL("/login", req.url));
    }
  }
  return NextResponse.next();
}

Exploitation Scenario

Normal Flow:

1. Regular user accesses /admin → JWT validation occurs
2. Unauthenticated user → Redirected to /login
3. Non-admin user → Redirected to /

Attack Flow:

1. Attacker crafts request with:

   GET /admin HTTP/1.1
   Host: vulnerable-app.com
   X-Middleware-Subrequest: src/middleware:nowaf

2. Middleware:

• Detects X-Middleware-Subrequest: src/middleware:nowaf
• May treat as legitimate internal request
• Potentially skips JWT verification

2. Attacker gains unauthorized admin access

Mitigation Strategies

Immediate Fix for PinewoodStore

// Add at start of middleware
if (req.headers.get('x-middleware-subrequest')) {
  return new NextResponse('Invalid request', { status: 403 });
}

Comprehensive Protection Measures

1. Header Validation: Explicitly block external requests with internal headers
2. Zero-Trust Architecture: Verify all requests regardless of source
3. WAF Integration: Detect and block header manipulation attempts
4. Enhanced Monitoring: Log all requests with internal headers for audit

Conclusion

The X-Middleware-Subrequest: src/middleware:nowaf header serves an important internal purpose in Next.js, but its misuse can lead to serious security vulnerabilities. The PinewoodStore example demonstrates how proper JWT validation can be undermined by missing header verification.

Key Takeaways:

• Never trust internal headers for authentication decisions
• Always implement multiple layers of security validation
• Regularly audit middleware for proper request verification

#WebSecurity #NextJS #CVEAnalysis #MiddlewareSecurity

Below is link to my YouTube channel to Demo of the Practical attack on the web application PinewoodStore which is vulnerable to CVE-2025-29927 and allowed an attacker to bypass Authorization check to get access to the Admin Page.

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