CVE-2025-29927 Explained: How a Header Bypass Flaw Affected Next.js Middleware

Last month March 2025, a critical security vulnerability was disclosed in Next.js CVE-2025-29927. This bug allowed attackers to bypass middleware logic, which, for many apps, meant skipping over crucial authentication and security checks. The issue impacted Next.js versions from 11.1.4 all the way to 15.2.2.

If you’re using middleware in your Next.js app to handle anything sensitive auth redirects, token validation, setting headers like CSP or HSTS this vulnerability meant a user could potentially skip all of it. The bug wasn’t in your code, but in how the framework itself trusted part of the request it never should have.

Let’s walk through what happened, why it was a problem, how it was fixed, and what you should do to protect your apps both now and going forward.

What Actually Went Wrong

To understand this bug, we first need to understand how middleware works in Next.js.

In Next.js, middleware runs before a request hits your route handler or page. It’s super useful: you can block unauthenticated users, rewrite URLs, add security headers, or redirect based on geolocation. It runs on the edge (in most cases), and it’s supposed to be the first gate before anything else happens.

But here’s the issue: to prevent infinite loops like middleware calling itself through a rewrite Next.js uses an internal header called x-middleware-subrequest. Each time a middleware runs, the framework appends a value to that header to track the call depth.

For example:

x-middleware-subrequest: middleware

gets longer with each nested call:

x-middleware-subrequest: middleware:middleware

And so on. Eventually, if it gets too long (five entries), Next.js assumes there’s a loop and stops running middleware. This was originally a safeguard to avoid stack overflows or runaway recursion. Fair enough.

But here’s where it gets dangerous: the framework never validated whether this header was coming from the system or from the user.

That means an attacker could send a request with this header already filled in:

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

Next.js would see that and go, “Ah, looks like we’re five levels deep better skip all middleware from here.”

That’s it. The attacker bypasses everything you put in middleware. No login redirect, no geolocation block, no custom headers. They’re into your protected route like nothing ever happened.

Why This Is a Big Deal

In real-world apps, middleware is often used for access control and security logic. For example:

• You check if a user is authenticated before serving a dashboard.

• You add security headers like Content-Security-Policy and Strict-Transport-Security.

• You redirect traffic from restricted regions.

If middleware gets skipped, none of that runs. So the user gets straight to the protected content. And unless you have server-side checks after middleware (which many people don’t), the request goes through unhindered.

That’s why this bug wasn’t just a small edge-case issue it undermined one of the most common patterns developers rely on.

How It Was Fixed

The Next.js team fixed this in multiple versions:

• 15.2.3

• 14.2.25

• 13.5.9

• 12.3.5

The patch ensures that if the x-middleware-subrequest header is included in the incoming request from the browser or client it’s either stripped or ignored. Only internal processes are now allowed to modify and trust this header.

So, even if someone tries to spoof it, it won’t affect the middleware chain.

How You Might Have Been Affected

If your application uses middleware to protect routes, or you rely on it for any kind of security logic, you should assume it was potentially bypassable. That includes:

• Redirecting users who aren’t logged in

• Validating sessions or tokens in edge middleware

• Blocking access by region or IP

• Applying CSP, HSTS, or other headers

The scary part is this would not necessarily leave any trace in logs especially if your logging happens inside middleware.

Takeaways and What You Should Do

1. Upgrade immediately. This one’s obvious. If you haven’t already, get your project on one of the patched versions listed above.

2. Audit your assumptions about middleware. If you’re using middleware for anything critical, think about whether that logic should also exist deeper in your stack like inside API routes, server actions, or database access permissions.

3. Filter client headers. Whether you’re using a reverse proxy like NGINX or just a small Express server in front, drop any incoming headers that shouldn’t be set by the client.

4. Follow security advisories. Frameworks like Next.js move fast. It’s important to keep track of official changelogs, GitHub security alerts, and even community discussions. Even a small header misuse like this can turn into a critical bug.

A Word on Keeping Dependencies Updated

This vulnerability is a textbook example of why keeping dependencies up to date is so important. It’s not just about new features or performance framework updates often include essential security patches like this one.

In any team or company, you need a process for regular dependency audits. That might mean:

• Running npm audit or similar tools on a schedule

• Subscribing to GitHub security alerts

• Having a clear policy for upgrading frameworks, even if you’re not actively working on a project

Ignoring updates because “everything works fine right now” is how vulnerabilities like this sit unnoticed and exploitable in production for months.

Final Thoughts

CVE-2025-29927 is a clear reminder that even trusted frameworks can have serious flaws. Middleware is a great feature but it shouldn’t be your only line of defense.

Never trust anything the client can control, even if it seems internal. Always think in layers: frontend checks, middleware, backend validation, and database permissions all play a role in keeping things secure.

Stay patched. Stay alert. And treat security as something continuous not something you do once and forget.

- Jagadhiswaran Devaraj

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