@nuxt/webpack-builder and @nuxt/rspack-builder dev server same-origin check bypassed when Sec-Fetch-Site, Origin, and Referer are all absent (incomplete fix for GHSA-6m52-m754-pw2g)CVE-2026-49993

### Summary This is an incomplete fix for [GHSA-6m52-m754-pw2g](https://github.com/nuxt/nuxt/security/advisories/GHSA-6m52-m754-pw2g). Source code may still be stolen during dev when using the webpack / rspack builder if the dev server is bound to a non-loopback address (e.g. `nuxt dev --host`) and the developer opens a malicious site on the same network. ### Details The fix for [GHSA-6m52-m754-pw2g](https://github.com/nuxt/nuxt/security/advisories/GHSA-6m52-m754-pw2g) added an `Origin` / `Referer` fallback to the dev-middleware same-origin check, with a `return true` branch when neither header is present so that non-browser clients (curl, the HMR client, address-bar navigation) keep working. That fallback is bypassed when a cross-origin attacker request reaches the dev server with all three signal headers absent: - `Sec-Fetch-Site` is [not sent by browsers to non-potentially-trustworthy destinations](https://w3c.github.io/webappsec-fetch-metadata/#sec-fetch-site-header) (HTTP on a non-loopback address). - `Origin` is not sent on non-CORS subresource fetches (a bare `<script>` with no `crossorigin`). - `Referer` can be suppressed by the attacker page with `<meta name="referrer" content="no-referrer">` or `referrerpolicy="no-referrer"` on the `<script>` element. A classic `<script src="http://VICTIM_LAN_IP:3000/_nuxt/app.js" referrerpolicy="no-referrer">` from a non-trustworthy attacker origin produces exactly that header set, the request is allowed, and the attacker page can read the built source out of `window.webpackChunk*` via `Function.prototype.toString()`. Since the attack requires the dev server to be reachable via a non-potentially-trustworthy origin, only apps using `--host` (or `--host 0.0.0.0`) are affected. Chrome 142+ users are also protected by [Local Network Access restrictions](https://developer.chrome.com/release-notes/142#local_network_access_restrictions). ### PoC 1. Create a Nuxt project with the webpack / rspack builder. 1. Run `npm run dev -- --host 0.0.0.0`. 1. Open `http://localhost:3000` on the developer machine. 1. From a different LAN host, serve the page below and open it in the same browser. 1. The compiled module source is exfiltrable from `window.webpackChunknuxt_<projectname>`. ```html <!doctype html> <meta name="referrer" content="no-referrer"> <script> ['/_nuxt/runtime.js', '/_nuxt/app.js'].forEach(p => { const s = document.createElement('script') s.src = 'http://VICTIM_LAN_IP:3000' + p s.referrerPolicy = 'no-referrer' document.head.appendChild(s) }) setTimeout(() => { const key = Object.keys(window).find(k => k.startsWith('webpackChunk')) for (const [, mods] of window[key]) { for (const id in mods) { console.log(id, mods[id].toString()) } } }, 1500) </script> ``` ### Impact Users using the webpack / rspack builder with `nuxt dev --host` may get the built source code read by malicious websites on the same network, including module identifiers, the developer's local filesystem path, and any developer-controlled strings inlined into the bundle. This vulnerability does not affect Chrome 142+ (and other Chromium-based browsers) users due to [Local Network Access restrictions](https://developer.chrome.com/release-notes/142#local_network_access_restrictions). The default Vite builder is not affected. ### Patches Fixed in `@nuxt/[email protected]` / `@nuxt/[email protected]` and backported to `@nuxt/[email protected]` / `@nuxt/[email protected]` by [#35200](https://github.com/nuxt/nuxt/pull/35200) (4.x: commit [`e351de94`](https://github.com/nuxt/nuxt/commit/e351de943e82db16970618b60dc7fdbaa58630f3); 3.x: commit [`77187ee4`](https://github.com/nuxt/nuxt/commit/77187ee4015e9267fb464951542a3e09e8b5fa05)). The dev-middleware same-origin check now treats a request with no `Sec-Fetch-Site`, no `Origin`, and no `Referer` as same-origin only when the dev server is loopback-bound, closing the header-suppression bypass. The fix only ships for the `@nuxt/webpack-builder` and `@nuxt/rspack-builder` packages. The default Vite builder was not affected. ### Workarounds If you cannot upgrade immediately: - Don't use `nuxt dev --host`. Bind the dev server to `localhost` (the default) and tunnel from other devices via SSH or a reverse proxy that enforces same-origin checks. - Use Chrome 142+ or another Chromium-based browser that enforces [Local Network Access restrictions](https://developer.chrome.com/release-notes/142#local_network_access_restrictions). - Switch to the Vite builder for development. ### Credit Reported by Berkan SAL ([@Uhudsavasindankacanokcu2](https://github.com/Uhudsavasindankacanokcu2)) via the Vercel Open Source HackerOne program. Independently reported by [@DavidCarliez](https://github.com/DavidCarliez) via GitHub's coordinated disclosure flow (`GHSA-xw96-2f5x-v9pv`), closed as a duplicate of this advisory.

Quoted source text, attributed separately from HOL analysis.