Next.js Middleware CVE-2025-29927 Vulnerability

Executive Summary

A critical vulnerability identified as CVE-2025-29927 was discovered in Next.js Middleware. Attackers can bypass authorization checks handled by Middleware. The vulnerability affects Next.js versions 11 through 15. Patched versions and a workaround have been made available.

Background

Next.js is a React framework used to build web applications [2]. React is a JavaScript library designed to construct user interfaces and manage user interactions. Middleware is a feature in Next.js to modify code to run before a request reaches its destination [3]. It’s commonly used for enforcing authentication and authorization checks, handling redirects, adding security headers, cookie validation, etc. [1]. The flaw of CVE-2025-29927 comes from the misuse of the x-middleware-subrequest header due to incorrect handling. It occurs within the application layer, where Middleware performs critical security checks.

Exploitation

The exploitation of CVE-2025-29927 involves an attacker inserting the x-middleware-subrequest header into an HTTP request [4]. This deceptive insertion tricks the Next.js application using Middleware into bypassing security checks. The problem with this vulnerability is that it can be exploited by anyone using the correct header value to the request.

Significance and Impact

This vulnerability is dangerous because it requires minimal technical knowledge and does not need sophisticated tools. A crafted header is enough to bypass Middleware security. This gives attackers unauthorized access to sensitive data and protected routes. Also, if Content Security Policy headers are added, threat actors can execute cross-site scripting attacks. Further attacks such as denial of service, privilege escalation, and data breaches can happen. 

Mitigation

To address this vulnerability, users are strongly advised to upgrade to the latest patched versions of Next.js. For older versions without available fixes, a recommended workaround is to block external requests that contain the x-middleware-subrequest header before they reach the application [5]. This can be done by configuring the web server, such as Nginx or Apache, to reject that header. Implementing a defense-in-depth approach can limit risk if one security layer is compromised.

Conclusion

CVE-2025-29927 is a reminder of how minor flaws in web frameworks can lead to significant security risks. Without proper safeguards, something as simple as a malicious crafted header can bypass security measures. Therefore, users must act quickly to apply the latest patches or the necessary workarounds to protect sensitive data and maintain application integrity.

References

[1] Chaddha, P. (2025 March 23). “CVE-2025-29927: Next.js Middleware Authorization Bypass – Technical Analysis.” ProjectDiscovery. https://projectdiscovery.io/blog/nextjs-middleware-authorization-bypass

[2] Next.js. (2025). “About React and Next.js.”  Next.js Learn. https://nextjs.org/learn/react-foundations/what-is-react-and-nextjs

[3] Next.js. (2025). “Middleware.” Next.js Docs. https://nextjs.org/docs/app/building-your-application/routing/middleware

[4] Strobes. (2025, March 24). “CVE-2025-29927 – Understanding the Next.js Middleware Vulnerability.” Blog. https://strobes.co/blog/understanding-next-js-vulnerability/

[5] Wilson, J. (2025, March). “Authorization Bypass in Next.js Middleware.” GitHub. https://github.com/vercel/next.js/security/advisories/GHSA-f82v-jwr5-mffw