Timeline is interesting

Timeline:

02/27/2025: vulnerability reported to the maintainers (specifying that only versions between 12.0.0 and 12.0.7 were vulnerable, which was our understanding at the time)

03/01/2025: second email sent explaining that all versions were ultimately vulnerable, including the latest stable releases

03/05/2025: initial response received from the Vercel team explaining that versions 12.x were no longer supported/maintained (probably hadn’t read the second email/security advisory template indicating that all were vulnerable)

03/05/2025: another email sent so that the team could quickly take a look at the second email/security advisory template

03/11/2025: another email sent to find out whether or not the new information had been taken into account

03/17/2025: email received from the Vercel team confirming that the information had been taken into account

03/18/2025: email received from the Vercel team: the report had been accepted, and the patch was implemented. Version 15.2.3 was released a few hours later, containing the fix (+backports)

03/21/2025: publication of the security advisory

Maybe im misunderstanding how people are building endpoints these days, but every post about this I see how it can bypass auth.

Wouldn’t this bypass auth only for sites where auth is true/false?

I’ve never worked on a site were auth is a boolean. Auth is always a relative. The middleware is only there to identify the user. Then when querying for objects, you query objects related to that user.

Or if you are serving an admin page you check that the user is an admin.

I honestly find it more astounding that people put an admin security check to check the url of a page and redirect away in a middleware and no security on the views themselves.

Is this form of checking paths in middleware officially from NextJS or did people just get lazy? Seems like the worst way to build auth I could ever dream up across any framework or language.

If a middleware is bypassed all endpoints should return empty responses. In my nextjs apps the middleware is simply a convenience method for the user if they are logged out they get redirected to the login page. But all api endpoints check for the active user and serve objects relative to the user.

What surprises me here is that the client side of the request / response is not considered a cunning, bitter enemy, as it should be. Why is x-middleware-subrequest even accepted in production? Why is x-middleware-rewrite even returned? They are instrumental to the attack, and the client has no business accessing them, ever, in my book.

If these headers are only expected to be available within a trusted zone, and some fronting HTTP server should strip them from incoming requests and outgoing responses, why are they named like regular HTTP headers, and not in some scary, easy-to-filter-way, like x-INTERNAL-ONLY-middleware-something?

To my mind, the server should accept the bare minimum of headers needed to serve the request, and issue the minimum amount of headers to provide a well-formed response, while being completely opaque to the client. Any nifty diagnostics like x-middleware-rewrire belong to the logs; correlate by request ID. Any nifty internal processing tweaks in plain text, like x-middleware-subrequest, are, to my mind, bad architecture. If you need to pass such info between HTTP endpoints internally, use something like a JWT.

The vulnerability can be understood through this code snippet:

  const subreq = params.request.headers['x-middleware-subrequest'];
  const subrequests = typeof subreq === 'string' ? subreq.split(':') : [];
  // ...
  for (const middleware of this.middleware || []) {
    // ...
    if (subrequests.includes(middlewareInfo.name)) {
      result = {
        response: NextResponse.next(),
        waitUntil: Promise.resolve(),
      };
      continue;
    }
  }

Pass an x-middleware-subrequest HTTP header with a colon-separated list of middleware names to skip.

https://github.com/vercel/next.js/blob/v12.0.7/packages/next...

Does anyone know which versions of Next.js that is supported?

I don't seem to be able to find a promise from Vercel, but https://endoflife.date/nextjs mentions that 15 and 14 gets security support.

The culture of security within FAANG could not be more opposite than the way that vercel handled this. In big tech, this would have been looked at in 48 hours, and across thousands of systems all oncalls would have been paged to do an emergency deploy. Probably within 5 days, almost the whole company would have deployed the patch.

Vercel to me seems like it is run by hype men, and the CEO is certainly technical, but these people are not in the weeds in the way they come off.

This very recent PR updates the docs to basically remove all common (and previously recommended) middleware use-cases, rendering them almost completely useless:

https://github.com/vercel/next.js/pull/77438

Most don't understand this issue:

Auth middleware is used for _routing_ (e.g. if you're not signed-in, you'll be redirected to the sign-in page).

This just means a 500 is thrown due to the auth() call returning null on the server.

Can someone tl;dr: why there is even logic to bypass middleware in the first-place, I feel like I'm missing something obvious here...

> “Is this form of checking paths in middleware officially from NextJS or did people just get lazy?”

This is a common way to implement auth in many frameworks, not just JS ones. Off the top of my head I know that Laravel (PHP) does it this way. I guess you could call it “being lazy” but most people would refer to it using the age-old software engineering terms “don’t repeat yourself” and “separation of concerns.”

The authentication middleware can choose to redirect or not based on many things: did the user provide the correct credentials, does the user have the necessary permissions to access the requested resource, etc… And you can put all the logic for determining those many things in a single place, so that it can easily be updated. Individual routes can remain as they are, and you don’t have to worry about forgetting to implement some part of the auth checking logic on one of them.

This attack isn't only about auth, auth is just the most drastic obvious example. Middleware is often used for a wide range of sanity checks. You could bypass, say, limit checks and ask the server to return an infinite number of items per page, quickly overloading the server and resulting in DoS.

I don’t know how to put this any other way but my experience with NextJS or just JS-first full stack is that they are still first and foremost a tech stack for frontend devs and the backend piece in that context is an afterthought.

I’ve worked on a few in my time and ‘API routes’ were rarely, if ever, authenticated, and there wouldn’t be a consistent strategy for data access. If anything, everything was built in the context of satisfying a react hook and getting on with the UI.

But I don’t squarely blame developers for it, it’s more like an inverted full stack where the browser is first class and the serverless edge SSR ISG SSG app router component craziness does not help you build out a stable API. Does sell a hell of a lot of SaaS though.

How serious this vulnerability is depends entirely on how the site that's being attacked uses middleware. The auth thing is just the most obvious example of how an attacker can do bad things if they have the ability to selectively disable middleware by passing names as a colon separated list in an HTTP header.

(I've built sites that would have been affected by this in the past, had I used Next and middleware for auth. I've worked on plenty of systems where there are only a small set of users each with the same level of permissions - gating private documentation for example.)

Probably depends on the complexity of the permissions in the application. I'd also expect something more along the lines you described for more complex applications. The middleware would do authentication, but then just attach that information to the request. Later parts would then use the attached information to make decisions about permissions/authorization.

In more complex cases this would be outside middleware, so it should fail as no authentication/authorization information is attached to the request if you skip that middleware.

But putting the security checks into middleware could easily make sense for more rigid or simple cases. In C# for example I can add attributes to the methods that handle each endpoint. So if you need a basic admin/no-admin check you could add a [RequireAdmin] attribute on the relevant endpoints and use a middleware to check that.

I would agree that checking the URL in middleware to make decision about permissions would be a bad idea, it moves this important check to a mostly invisible place.

This probably also allows different attacks, e.g. skipping middleware that does other security-relevant checks (maybe anti-CSRF mechanisms could be vulnerable here).

I don’t think it really matters. All you have to do is write middleware or handlers that assume upstream middleware have run, and then that’s vulnerable to this attack.

For example it’s common to write middleware on /admin so that all of your /admin/* handlers don’t have to repeat the same authz logic. And the platform breaking invariants that you should be able to depend on is why it’s a security bug.