Next.js Authorization Bypass Vulnerability CVE-2025-29927: What You Need to Know

On March 21, 2025, a critical security flaw, identified as CVE-2025-29927, was disclosed in the Next.js framework, a popular React-based platform for building modern web applications. This vulnerability permits attackers to bypass middleware-based security controls simply by manipulating HTTP headers.

Understanding CVE-2025-29927: Key Details

• Nature of vulnerability: A remote authorization bypass affecting Next.js middleware.
• Affected versions: Includes Next.js 11.1.4 through 13.5.6 (unpatched), 14.x before 14.2.25, and 15.x prior to 15.2.3.
• Impact: Enables attackers to circumvent all middleware logic, including critical authentication, authorization, security headers enforcement, and path rewriting.
• Mitigation: Upgrading to patched versions is the only guaranteed fix; temporary proxy-level header blocking is possible but not recommended long term.

How This Next.js Middleware Bypass Works

Next.js middleware utilizes an internal HTTP header, x-middleware-subrequest, to prevent recursive middleware executions that could lead to infinite loops. The vulnerability arises because attackers can forge this header with specific crafted values that instruct Next.js to completely skip middleware processing.

• Older versions (pre-12.2): The header x-middleware-subrequest: pages/_middleware triggers the bypass.
• Newer versions: Use repeated segments such as x-middleware-subrequest: middleware:middleware:middleware:middleware:middleware or the src/middleware variant.

This allows malicious requests to directly access protected resources without any security checks, effectively nullifying middleware security measures.

Assessing Your Risk to Next.js Middleware Bypass

Your application is likely vulnerable if it meets all the following criteria:

1. Relies on Next.js middleware for security controls like authentication and authorization.
2. Runs a self-hosted Next.js setup using next start with output: 'standalone' configuration.

Additional factors increasing risk include:

• Middleware governs enforcement of security headers such as Content Security Policy (CSP).
• Middleware is used to restrict access through path rewriting.

Applications deployed on managed platforms like Vercel or Netlify are generally not affected due to mitigations implemented at the platform edge. Static export deployments (without middleware execution) are also unaffected.

Real-World Implications and Case Studies

The widespread adoption of Next.js means this vulnerability can expose numerous web applications. For example, companies hosting sensitive customer data or financial information via self-hosted Next.js apps are at heightened risk. A case study from a recent penetration test on a fintech platform revealed that unpatched Next.js middleware allowed attackers to access restricted pages without authentication, posing a severe compliance risk.

According to a 2024 web application security report by OWASP, unauthorized access vulnerabilities remain among the top three application security risks, emphasizing the critical need to address authorization bypass flaws like CVE-2025-29927 promptly.

Effective Detection Methods for CVE-2025-29927

Active Detection Using Dynamic Application Security Testing (DAST)

Security testing tools scan for CVE-2025-29927 by verifying the presence of Next.js middleware characteristics and then sending crafted requests that include the bypass headers. Detection methodology includes:

1. Identifying Next.js middleware: Look for unique 307 redirects where the response body matches the ‘Location’ header.
2. Confirming Next.js usage: Check for x-powered-by: Next.js headers in HTTP responses.
3. Testing bypass payloads: Use multiple header values depending on Next.js version.
4. Validating responses: Compare requests with bypass headers against control requests to reduce false positives.
5. Confirm vulnerability: Confirm consistent bypass behavior across multiple tests.

Passive Detection via Security Context Analysis (SCA)

Some advanced tools monitor normal traffic for indications of vulnerable Next.js versions without actively attacking the application. This involves analyzing HTTP headers and JavaScript environment data loaded in browsers, such as the next.version function, cross-referencing them against CVE databases.

Mitigating the Next.js Authorization Bypass Vulnerability

1. Update Next.js Immediately:
   • Next.js 15.x: Upgrade to version 15.2.3 or later
   • Next.js 14.x: Upgrade to 14.2.25 or later
   • Next.js 13.x: Upgrade to 13.5.9 or later
   • Next.js 12.x: Upgrade to 12.3.5 or later
2. Interim Mitigation: If immediate upgrades are not feasible, block the x-middleware-subrequest header at your proxy or edge layer (do not rely on middleware itself for this). For example, Cloudflare offers a Managed WAF rule to block this attack vector, although it is opt-in and may affect some legitimate third-party authentication frameworks.

Conclusion

The CVE-2025-29927 vulnerability represents a significant risk for self-hosted Next.js applications relying on middleware for critical security functions. Given the simplicity of the exploit—manipulating one HTTP header—prompt action is imperative. Upgrading to the latest patched Next.js versions remains the most reliable safeguard, complemented by robust security testing and monitoring practices.

Continuing research and monitoring from the security community and vendors ensure new detections and mitigation strategies will evolve as more insights emerge.

Further Reading and Resources

• Original research on Next.js middleware by Rachid Allam and Yasser Allam
• OWASP Top Ten Web Application Security Risks
• Official Next.js Documentation

Keywords: Next.js authorization bypass, CVE-2025-29927, middleware bypass vulnerability, Next.js security risk