CVE-2025-29927: Authorization Bypass in Next.js Middleware, What You Need to Know

A critical vulnerability—CVE-2025-29927—has been identified in Next.js, affecting versions 11.1.4 and up. This flaw enables authorization bypass in applications that use middleware with matchers. Attackers can manipulate internal headers to skip authentication checks and gain unauthorized access to protected routes.

This vulnerability is particularly dangerous because it compromises one of the most fundamental layers of web application security: access control. Exploiting it could lead to data breaches, account takeovers, or full system compromise.

What Makes This Vulnerability So Risky?

The root of the issue lies in the mishandling of the <-middleware-subrequest header. When improperly validated, this header allows attackers to bypass middleware-based authentication and authorization logic. The vulnerability doesn’t affect applications deployed using next export or hosted on Vercel or Netlify, but self-hosted apps or those deployed on custom infrastructure remain at risk.

Who’s Affected?

If you're using any version of Next.js from 11.1.4 and your application uses middleware with a matcher, you are potentially affected—especially if you're still running:

• Next.js 11: End-of-life since January 2022
• Next.js 12: End-of-life since November 2022
• Next.js 13: End-of-life as of December 2024

While recent patches were backported to 12, 13, 14, and 15, those relying on older middleware implementations in unsupported versions are still at risk.

How HeroDevs Fixes This with NES for Next.js

HeroDevs’ Never-Ending Support (NES) for Next.js includes a patch for CVE-2025-29927, ensuring that organizations still relying on Next.js 11 or 12 can remain secure without migrating immediately to version 14 or later.

With Next.js NES, you get:

• Security patches for critical vulnerabilities like CVE-2025-29927
• Compliance assurance for frameworks no longer supported by Vercel
• Drop-in replacement that requires no code rewrites
• Support from Next.js and React specialists who understand legacy environments

What Should You Do Now?

If you're running an EOL version of Next.js and relying on middleware authentication, you have two options:

1. Migrate to a supported version (14 or 15), which may require significant application refactoring.
2. Secure your current version with Next.js NES from HeroDevs—a drop-in, production-grade solution that keeps your application protected and compliant.

Final Thoughts

Authorization bypass vulnerabilities are among the most critical threats in modern web applications. CVE-2025-29927 reminds us that even mature, stable frameworks like Next.js can introduce severe risks when support ends.

If you’re not ready to migrate, HeroDevs has your back. With Never-Ending Support for Next.js, your apps stay secure, compliant, and reliable—even in end-of-life environments.

👉 Contact HeroDevs to get protected today.

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