Next.js Middleware Bypass Vulnerability

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The Next.js Middleware Bypass vulnerability threatens web application security. This critical flaw allows attackers to circumvent authentication mechanisms. The vulnerability was disclosed in March 2025 with identifier CVE-2025-29927.

Next.js middleware typically protects sensitive routes and resources. However, this vulnerability creates a dangerous gap in security. Attackers can bypass middleware logic entirely including authentication and authorization mechanisms.

Technical Overview of CVE-2025-29927

How the Vulnerability Works

The Next.js Middleware Bypass Vulnerability stems from improper header validation. Next.js uses middleware to enforce security policies before routing requests. To avoid infinite loops during internal redirects or server-side rendering (SSR), it includes a special header x-middleware-subrequest.

The vulnerability affects multiple Next.js versions significantly. Critical vulnerability identified as CVE-2025-29927 was disclosed in Next.js. The flaw affects versions prior to 12.3.5, 13.5.9, 14.2.25, and 15.2.3.

Attack Vector Analysis

The flaw lies in the fact that this header is blindly trusted by the framework without verifying its origin. An attacker can spoof this header in a request, tricking the server into skipping the middleware layer entirely. This simple technique makes exploitation extremely dangerous.

// Vulnerable middleware example
export function middleware(request) {
  const token = request.cookies.get('auth-token');
  
  if (!token) {
    return NextResponse.redirect('/login');
  }
  
  return NextResponse.next();
}

Impact on Web Application Security

Authentication Bypass Risks

The Next.js authorization bypass vulnerability poses severe security threats. This effectively bypasses all access control logic enforced by middleware, granting unauthorized access to protected routes. Security teams must understand this threat to protect their systems.

Companies conducting web application pen test activities should prioritise this vulnerability. CVE-2025-29927 has a CVSS Score of 9.1 indicating critical severity. EPSS Score: 92.56 suggests high likelihood of exploitation.

Data Exposure Concerns

Protected resources become accessible without proper authentication checks. Successful exploitation can allow attackers to access protected routes without authentication. Financial information, personal details, and proprietary data remain at risk.

Identifying Vulnerable Applications

Version Check Methods

Security teams should immediately check their Next.js versions. You are vulnerable if you’re running a version below: 12.3.5, 13.5.9, 14.2.25, 15.2.3. The following command reveals your current Next.js version:

npm list next
# Or with yarn
yarn list next

Testing for Vulnerability

Teams can test for the vulnerability using simple curl commands. The attacker can abuse this functionality by supplying the header with the maximum number of middleware calls. Monitor whether middleware authentication executes properly.

# Example test request
curl -H "x-middleware-subrequest: middleware:middleware:middleware:middleware:middleware" \
https://your-app.com/protected-route

Exploitation Scenarios and Examples

Step-by-Step Exploitation Process

Identify protected routes using middleware authentication
Craft malicious request with x-middleware-subrequest header
Send request to bypass middleware checks
Access protected content without authentication
Extract sensitive data from unprotected endpoints

An attacker sends a request with a spoofed x-middleware-subrequest header to impersonate an internal request. The simplicity makes widespread exploitation highly probable.

Real-World Attack Example

Consider an e-commerce platform using Next.js middleware for authentication. The middleware protects customer order history and payment details. This tricks the server into thinking it’s a trusted internal request, bypassing middleware logic such as session or role validation.

GET /admin HTTP/1.1
Host: vulnerable-app.com
x-middleware-subrequest: middleware

Mitigation Strategies and Fixes

Immediate Actions Required

The Next.js patch addresses this critical vulnerability completely. Upgrade Next.js to: 12.3.5+, 13.5.9+, 14.2.25+, 15.2.3+. This update prevents header manipulation and restores middleware security.

Development teams should implement additional security layers. Don’t rely solely on middleware for authentication/authorization. Implement fallback access controls at the route/controller level.

Implementation of Security Headers

// Enhanced middleware with additional checks
export function middleware(request) {
  // Block x-middleware-subrequest header
  if (request.headers.get('x-middleware-subrequest')) {
    return new Response('Forbidden', { status: 403 });
  }
  
  // Continue with authentication logic
  const token = request.cookies.get('auth-token');
  
  if (!token || !validateToken(token)) {
    return NextResponse.redirect('/login');
  }
  
  return NextResponse.next();
}

Long-Term Security Improvements

Defence in Depth Approach

Implement multiple security layers throughout your application architecture. Middleware allows you to redirect, rewrite, or modify the incoming request before producing a response. Regular network penetration testing identifies potential vulnerabilities early.

Monitor application logs for suspicious header patterns. Check access logs for unusual or unauthorized access attempts using x-middleware-subrequest. Proactive monitoring prevents successful exploitation attempts.

Security Best Practices

Update Next.js and dependencies regularly to receive security patches. We published a blog post on the Next.js site explaining the CVE. Implement automated dependency scanning in your CI/CD pipeline.

Patch Implementation Guide

Upgrade Process Steps

Backup your application before making any changes
Update package.json with the latest Next.js version
Run npm update or yarn upgrade commands
Test thoroughly in development environment first
Deploy to staging for comprehensive testing
Monitor production deployment carefully

Verification Methods

After applying the Next.js vulnerability fix, verify the patch effectiveness. Monitor for requests that explicitly include the x-middleware-subrequest header from external IPs. Ensure middleware executes correctly for all protected routes.

// Test script for verification
const testVulnerability = async () => {
  const response = await fetch('/api/protected', {
    headers: {
      'x-middleware-subrequest': 'middleware'
    }
  });
  
  if (response.status === 403) {
    console.log('Vulnerability patched successfully');
  } else {
    console.error('System still vulnerable!');
  }
};

Workaround Solutions

Reverse Proxy Configuration

Block or strip the x-middleware-subrequest header at the reverse proxy or edge layer. This provides immediate protection while planning upgrades. Configure your web server to reject malicious requests.

For Nginx configuration:

location / {
    proxy_set_header x-middleware-subrequest "";
    proxy_pass http://localhost:3000;
}

For Apache configuration:

RequestHeader unset x-middleware-subrequest

Cloud Provider Protections

We did verify Netlify and Cloudflare Workers were not impacted. Vercel announced that this vulnerability does not impact applications hosted on the Vercel hosting platform. However, self-hosted deployments remain vulnerable without patching.

Frequently Asked Questions

What is the Next.js Middleware Bypass vulnerability?

The Next.js Middleware Bypass vulnerability allows attackers to skip authentication checks. By spoofing this header, attackers can bypass middleware logic entirely. The flaw affects middleware execution in vulnerable Next.js versions.

How serious is CVE-2025-29927?

CVE-2025-29927 represents a critical security vulnerability. CVSS Score: 9.1 indicates maximum severity rating. Immediate patching remains essential for all production systems.

Which Next.js versions are affected?

The flaw affects versions prior to 12.3.5, 13.5.9, 14.2.25, and 15.2.3. Applications using middleware for authentication face immediate risk. Check your version and upgrade immediately if affected.

Can attackers exploit this vulnerability remotely?

Yes, attackers can exploit this vulnerability remotely. They only need to send HTTP requests with specific headers. No special tools or advanced knowledge is required.

What data could attackers access?

Successful exploitation can allow attackers to perform privilege escalation and reach internal-only functionality. This includes user profiles, payment information, and API endpoints. The exact impact depends on your application’s architecture.

How quickly should I patch this vulnerability?

Patch this vulnerability immediately upon discovery. An attacker could leverage the publicly available PoC to compromise vulnerable systems. Every hour of delay expands your attack surface.

Prevention and Monitoring

Continuous Security Assessment

Regular security assessments identify vulnerabilities before attackers do. Organizations are advised to patch their vulnerable Next.js applications without delay. Schedule periodic manual reviews of critical security components.

Professional penetration testing services provide comprehensive vulnerability assessments. Expert testers simulate real-world attacks against your applications. Their findings help strengthen your security posture significantly.

Logging and Detection

Configure detailed logging for all middleware executions. Header Inspection: Monitor for requests that explicitly include the x-middleware-subrequest header. Alert on multiple failed authentication attempts from single sources.

// Enhanced logging middleware
export function middleware(request) {
  const suspiciousHeader = request.headers.get('x-middleware-subrequest');
  
  if (suspiciousHeader) {
    console.error('Potential bypass attempt detected:', {
      ip: request.ip,
      url: request.url,
      timestamp: new Date().toISOString()
    });
    
    return new Response('Forbidden', { status: 403 });
  }
  
  // Continue with normal flow
}

Industry Response and Community Actions

Next.js Team Response

On 27 Feb 2025 06:03:00 GMT, the vulnerability was disclosed to the Next.js team through GitHub private reporting. They released patches within days of disclosure. Their transparent communication helped developers understand the risks.

Thank you to security researchers Rachid Allam (zhero) and Yassir Alam (inzo_) for responsibly disclosing this issue. Future middleware implementations will undergo stricter validation. These changes prevent similar vulnerabilities from recurring.

Community Contributions

Security researchers worldwide contributed to identifying affected applications. The vulnerability was originally discovered and analyzed by Rachid Allam (zhero). The community’s collaborative response minimised exploitation attempts.

Future Security Considerations

Emerging Threat Landscape

Web application vulnerabilities continue evolving rapidly. Given the popularity of Next.js, with millions of downloads weekly, the vulnerability may have a severe impact. Development teams must stay informed about emerging threats.

Regular training keeps developers aware of security best practices. Code reviews should include security-focused assessments. Automated tools complement but don’t replace human expertise.

Framework Security Evolution

Modern frameworks must balance features with security. CVE-2025-29927 occurs due to improper handling of the x-middleware-subrequest header internally by Next.js. Future versions will include enhanced protection mechanisms by default.

Technical Glossary

Middleware: Server-side code that executes before route handlers process requests

CVE: Common Vulnerabilities and Exposures – a standardised vulnerability identification system

HTTP Headers: Metadata sent with HTTP requests and responses

Authentication Bypass: Circumventing security checks to gain unauthorised access

Subrequest: An internal request generated during main request processing

Patch: Software update that fixes security vulnerabilities or bugs

API Endpoint: Specific URL where applications can access server resources

CI/CD Pipeline: Continuous Integration and Deployment automated workflow

Secure Your Applications with Professional Testing

The Next.js Middleware Bypass Vulnerability highlights the importance of regular security assessments. Due to the widespread reliance on middleware in Next.js apps, the impact is significant. Aardwolf Security specialises in identifying and resolving security weaknesses before attackers find them.

Our expert team provides comprehensive penetration testing services for modern web applications. We understand Next.js architecture and its unique security challenges. Our assessments cover authentication bypasses, API vulnerabilities, and infrastructure weaknesses.

Take proactive steps to protect your applications and users. Contact Aardwolf Security today to schedule your security assessment. Our team will help identify vulnerabilities and provide actionable remediation guidance.