Security Alert: How Attackers Can Bypass Next.js Middleware With a Single HTTP Header

The Critical Vulnerability Every Next.js Developer Should Know About

Did you know that a simple HTTP header manipulation could completely bypass your authentication and authorization systems in Next.js applications? Recently, a critical vulnerability (CVE-2025-29927) was disclosed in the popular Next.js framework, allowing attackers to circumvent middleware execution—including security checks—by leveraging an internal header.

Understanding Next.js Architecture and Why It Matters

According to Next.js documentation:

Middleware [in Next.js] allows you to run code before a request is completed. Then, based on the incoming request, you can modify the response by rewriting, redirecting, modifying the request or response headers, or responding directly.

In practical terms, middleware serves as the gatekeeper for your application, handling critical functions like:

• Authentication & Authorization
• Rate limiting
• Request logging
• Response manipulation
• Routing

When middleware execution is bypassed, all of these security controls fail simultaneously.

How the Attack Works

As detailed in research by security researchers Allam Rachid (zhero) and Allam Yasser (inzo_), the attack exploits the x-middleware-subrequest header—an internal implementation detail that was never meant to be exposed to users.

The Exploit Method

By supplying a specific value to this header—easily inferred from the target application's structure—attackers can completely bypass Next.js middleware execution. For example, accessing a protected admin section at dashboard/panel/admin would simply require trying requests with headers like:

For modern versions (14.x and 15.x):

x-middleware-subrequest: middleware:middleware:middleware:middleware:middleware

For older versions (11.1.4 through 12.1.x):

x-middleware-subrequest: pages/_middleware
x-middleware-subrequest: pages/dashboard/_middleware
x-middleware-subrequest: pages/dashboard/panel/_middleware

You can find more details about the vulnerability here.

Real-World Impact

This vulnerability has severe implications:

• Unauthorized access
• Bypassing of authentication and authorization
• Exposure of sensitive data
• Circumvention of rate limiting and other protective measures
• Potential access to internal APIs not meant for public consumption
• Denial of Service through cache poisoning

The Response from Next.js

The Next.js team addressed this issue with a combination of patches and workarounds that vary by version:

• Next.js 15.x: Fixed in version 15.2.3
• Next.js 14.x: Fixed in version 14.2.25
• Next.js 11.1.4 through 13.5.6: No direct patches available; the official recommendation is to implement a workaround, though 13.x is a maintained version.

Importantly, the official workaround recommendation from Next.js is to prevent the x-middleware-subrequest header from reaching your application - which can be implemented through various means, including WAFs, API gateways, reverse proxies, or load balancers.

This version-specific approach creates notable challenges for organizations:

• Teams face trade-offs between performing emergency upgrades or rapidly deploying header-filtering solutions, both of which can strain resources.
• Organizations managing multiple Next.js apps across different versions must tailor their mitigation approach for each one. Organizations with multiple Next.js applications across different versions need different strategies
• Older versions lack direct patches, requiring teams to rely on network-level filtering—and if that’s not feasible, they may need to refactor sensitive parts of their codebase to mitigate the risk.
• For legacy versions, the responsibility shifts from developers to operations and network teams, as code-based fixes aren't an option.
• Non-managed deployments require self-managed defenses, increasing the overhead for teams hosting Next.js independently.

For many organizations, leveraging an API Gateway and a WAF—or similar network-level protection—is essential for ensuring consistent security across their entire application portfolio. Teams that already have these in place are in a stronger position to mitigate such attacks, reinforcing the importance of adopting the GW pattern as a proactive investment in security.

The Operational Advantage: API Gateway Layer Protection vs. Application Upgrades

One of the most compelling yet often overlooked benefits of protection through an API Gateway is the dramatic operational advantage it provides compared to codebase and tooling upgrades. Consider the contrasting approaches:

Upgrades approach:

• Upgrades may require product downtime
• Requires end to end testing of the product
• May be blocked by release freezes or change management policies
• Demands coordination across multiple teams (development, QA, operations)
• Requires upgrading to Next.js 14.2.25+ or 15.2.3+ depending on your current version
• May introduce compatibility issues with other dependencies and a cascade of other dependency updates

API Gateway protection approach:

• Allows for rapid response to the vulnerability
• Can be deployed independently of the application
• Can be tested in DetectOnly mode before enforcing
• Is typically reversible with minimal consequences
• Targets only the specific vulnerability without touching the application code
• Can be managed by operations teams without developer involvement
• And most importantly, will bring security governance to your platform

The difference is stark for organizations managing dozens or hundreds of applications. Implementing API Gateway protection at the network edge can shield all vulnerable applications simultaneously—while application upgrades might require weeks of coordinated work across multiple teams and systems.

Patching remains essential for comprehensive security. However, the ability to deploy an immediate API Gateway-based solution creates a critical time buffer that allows for proper planning, testing, and deployment of application upgrades without leaving systems exposed in the interim. A good security strategy is the one that relies on multiple layers of protection.

Immediate Mitigation with Traefik and Coraza: Blocking the Vulnerable Header

If you're unable to upgrade to the fixed versions mentioned above, the official Next.js guidance is to block the x-middleware-subrequest header from reaching your application. Traefik offers a streamlined and less invasive way to implement this protection—regardless of whether you are pursuing a temporary workaround or a long-term mitigation strategy. 

By leveraging Traefik with its Coraza WAF extension, teams can enforce this critical safeguard at the network edge, avoiding disruptive changes to application code or infrastructure. This approach allows for rapid, centralized protection that scales easily across multiple services and environments.

http:
  middlewares:
    waf:
      plugin:
        coraza:
          directives:
            - Include @coraza.conf-recommended
            - SecRuleEngine On
            - SecRule &REQUEST_HEADERS:x-middleware-subrequest "@gt 0" "id:1000001,phase:1,msg:'Request contains x-middleware-subrequest header',deny,status:403"

The above configuration blocks any request containing the vulnerable header, providing immediate protection while you plan your patching strategy.

Even without Coraza WAF, Traefik's native header manipulation capabilities can implement this protection using the Headers middleware:

http:
  middlewares:
    patch-nextjs-apps:
      headers:
        customRequestHeaders:
          x-middleware-subrequest: "" # Removes the header

The key requirement is blocking this header before it reaches your Next.js application, and Traefik provides multiple flexible approaches to accomplish this critical security task.

Broader Protection: Mitigating High-Profile Attacks with Traefik Coraza WAF

The power of Traefik's WAF extends far beyond mitigating specific vulnerabilities like the Next.js middleware bypass. It provides protection against many of the most common and devastating web application attacks, including:

SQL Injection (SQLi)

SQL injection attacks remain one of the OWASP Top 10 web application vulnerabilities year after year. These attacks can lead to:

• Unauthorized access to sensitive database information
• Data theft or manipulation
• Complete system compromise

Traefik's Coraza WAF can detect and block SQLi attempts with rules like:

http:
  middlewares:
    xss-protection:
      plugin:
        coraza:
          directives:
            - Include @coraza.conf-recommended
            - SecRuleEngine On
            - Include @owasp_crs/REQUEST-942-APPLICATION-ATTACK-SQLI.conf

Cross-Site Scripting (XSS)

XSS attacks allow attackers to inject malicious client-side scripts into web pages viewed by other users. These attacks can:

• Steal session cookies and user credentials
• Deface websites
• Redirect users to malicious sites
• Perform actions on behalf of authenticated users

Traefik WAF can protect against XSS with rules like:

http:
  middlewares:
    xss-protection:
      plugin:
        coraza:
          directives:
            - Include @coraza.conf-recommended
            - SecRuleEngine On
            - Include @owasp_crs/REQUEST-941-APPLICATION-ATTACK-XSS.conf
            - Include @owasp_crs/REQUEST-949-BLOCKING-EVALUATION.conf

Defense in Depth: Beyond Patching

This vulnerability reminds us of the importance of defense in depth. Even with the Next.js patch, consider implementing these additional protections:

• Use layered protection mechanisms rather than relying solely on Next.js middleware
• Follow the principle of least privilege for all application components
• Implement an API gateway architecture for additional security layers
• Integrate WAF capabilities with the API gateway architecture
• Regularly audit your dependencies for known vulnerabilities

Traefik's flexibility allows it to act not just as a simple reverse proxy but as a comprehensive security gateway that can adapt to evolving threats without requiring application changes.

Real-World Impact

Several recent reports such as the 2024 Verizon Data Breach Investigations Report, Kaspersky Incident Response Report 2024, and 2024 Unit 42 Incident Response Report, have found that web application attacks continue to be the most common attack vector in confirmed breaches. Implementing a WAF like Traefik Coraza provides a crucial defense layer against these prevalent threats.

A single deployment of Traefik with properly configured WAF rules can protect entire application ecosystems from a wide range of attacks, often without requiring changes to the underlying applications themselves.

Conclusion

HTTP header-based attacks represent a sophisticated yet increasingly common attack vector. The Next.js vulnerability demonstrates how a single header manipulation can bypass critical security controls, but it's just one of many threats targeting modern web applications.

The integration of Traefik with its Coraza WAF extension provides a powerful first line of defense against not only specialized attacks like the Next.js middleware bypass, but also persistent, high-impact threats like SQL injection and XSS attacks that continue to plague applications across industries.

This operational approach—implementing Traefik protection rather than rushing application changes—represents a fundamental shift in vulnerability response strategy. It allows organizations to implement immediate protection while planning thoughtful, well-tested application updates on normal release cycles. This balance of security and operational stability is increasingly critical as application complexity and the pace of discovered vulnerabilities both continue to accelerate.

For development teams, this serves as a reminder to:

• Keep frameworks and libraries updated
• Monitor security advisories from your dependencies
• Have a clear incident response plan for vulnerabilities
• Prioritize operational resilience through non-invasive security controls
• Consider gateway-level protections that can shield multiple applications simultaneously
• Implement multiple layers of security, with an API Gateway as a critical component

By understanding these risks and implementing proper mitigations with Traefik, you can significantly reduce your attack surface and better protect your applications from the ever-evolving threat landscape while maintaining operational stability.

Related Articles and Resources

• Glossary Article: Understanding API Security and the Top 4 Methods to Implement It
• 7 Critical API Protection Strategies to Fortify Your API Security
• Stop SQLi and XSS Attacks Easily with Traefik's WAF Integration
• Traefik Coraza WAF Documentation

References

• Official Next.js security advisory (GHSA-f82v-jwr5-mffw)
• Next.js middleware documentation
• CVE-2025-29927 details
• OWASP Top 10 Web Application Security Risks
• Verizon 2024 Data Breach Investigations Report
• Kaspersky Incident Response Report 2024
• 2024 Unit 42 Incident Response Report
• Traefik Coraza WAF Documentation
• Next.js and the corrupt middleware: the authorizing artifact - zhero_web_security
• Credits: Vulnerability discovered by Allam Rachid (zhero) and Allam Yasser (inzo_).