Simon Willison’s Weblog

Next.js and the corrupt middleware: the authorizing artifact. Good, detailed write-up of the Next.js vulnerability CVE-2025-29927 by Allam Rachid, one of the researchers who identified the issue.

The vulnerability is best illustrated by 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;
  }
}

This was part of Next.js internals used to help avoid applying middleware recursively to requests that are re-dispatched through the framework.

Unfortunately it also meant that attackers could send a x-middleware-subrequest HTTP header with a colon-separated list of middleware names to skip. If a site used middleware to apply an authentication gate (as suggested in the Next.js documentation) an attacker could bypass that authentication using this trick.

The vulnerability has been fixed in Next.js 15.2.3 - here's the official release announcement talking about the problem.

# 3:20 pm / security, javascript

Semantic Diffusion. I learned about this term today while complaining about how the definition of "vibe coding" is already being distorted to mean "any time an LLM writes code" as opposed to the intended meaning of "code I wrote with an LLM without even reviewing what it wrote".

I posted this salty note:

Feels like I'm losing the battle on this one, I keep seeing people use "vibe coding" to mean any time an LLM is used to write code

I'm particularly frustrated because for a few glorious moments we had the chance at having ONE piece of AI-related terminology with a clear, widely accepted definition!

But it turns out people couldn't be trusted to read all the way to the end of Andrej's tweet, so now we are back to yet another term where different people assume it means different things

Martin Fowler coined Semantic Diffusion in 2006 with this very clear definition:

Semantic diffusion occurs when you have a word that is coined by a person or group, often with a pretty good definition, but then gets spread through the wider community in a way that weakens that definition. This weakening risks losing the definition entirely - and with it any usefulness to the term.

What's happening with vibe coding right now is such a clear example of this effect in action! I've seen the same thing happen to my own coinage prompt injection over the past couple of years.

This kind of dillution of meaning is frustrating, but does appear to be inevitable. As Martin Fowler points out it's most likely to happen to popular terms - the more popular a term is the higher the chance a game of telephone will ensue where misunderstandings flourish as the chain continues to grow.

Andrej Karpathy, who coined vibe coding, posted this just now in reply to my article:

Good post! It will take some time to settle on definitions. Personally I use "vibe coding" when I feel like this dog. My iOS app last night being a good example. But I find that in practice I rarely go full out vibe coding, and more often I still look at the code, I add complexity slowly and I try to learn over time how the pieces work, to ask clarifying questions etc.

I love that vibe coding has an official illustrative GIF now!

# 6:30 pm / language, vibe-coding, andrej-karpathy, martin-fowler