Summary

When experimental.componentIslands is enabled (default in Nuxt 4), any .server.vue file under pages/ is automatically registered as a server island under the key page<routeName> and exposed via the /nuxtisland/:name endpoint. Until this fix, requests through that endpoint rendered the page component directly via the SSR renderer without instantiating Vue Router, which meant route middleware declared on the page (including definePageMeta({ middleware })) did not run.

For Nuxt applications that gate a .server.vue page behind route middleware as their sole auth check, an unauthenticated attacker could bypass that check by requesting /nuxtisland/page<routeName><anyhash> directly and receiving the server-rendered HTML.

Affected configurations

All three conditions must hold for an application to be vulnerable:

1. experimental.componentIslands is enabled (the default in Nuxt 4; opt-in in Nuxt 3). 2. The application defines one or more .server.vue files under pages/, registering them as routed pages. 3. Authentication / authorization for at least one such page is enforced solely via route middleware (middleware/.ts referenced from definePageMeta), without a server-side check inside the page or its data layer.

Applications that enforce auth inside the island's own data layer (server-only API routes, useRequestEvent + manual session checks, etc.) were not affected. The general "route middleware does not run for non-page island components" behaviour is documented and unchanged; this advisory concerns the .server.vue page case specifically, where running middleware is the user's clear expectation.

Details

- Build (packages/nuxt/src/components/templates.ts): .server.vue pages are registered as island components with page prefix, making them addressable through /nuxtisland/page<routeName><hashId>. - Runtime (packages/nitro-server/src/runtime/handlers/island.ts): the handler resolves the requested island component and renders it via renderer.renderToString(ssrContext). The Vue Router plugin previously short-circuited middleware execution whenever ssrContext.islandContext was set. - The two paths interact so that route middleware declared on the source page never runs.

Proof of concept

Given a page app/pages/secret.server.vue:

vue <script setup lang="ts"> definePageMeta({ middleware: 'auth' }) </script>

<template> <h1>SECRET DATA</h1> </template>

with middleware/auth.ts blocking unauthenticated access:

bash Direct page request: blocked by middleware curl -i http://localhost:3000/secret -> 403 / redirect, depending on the middleware

Island request: middleware did not run before this fix curl -i 'http://localhost:3000/nuxtisland/pagesecretanyhash' -> 200 OK, body includes <h1>SECRET DATA</h1>

Patches

Patched in nuxt@4.4.6 and nuxt@3.21.6 by #35092. The Vue Router plugin now runs middleware and redirect handling for page islands (i.e. islands that originate from .server.vue files in pages/). The island handler propagates middleware-issued responses (~renderResponse), and a new beforeResolve guard returns HTTP 400 when the requested page<name> does not match the route component the URL resolves to.

Non-page island components are unaffected - they continue to render without route middleware, by design.

Workarounds

If you cannot upgrade immediately:

- Enforce authentication inside the .server.vue page itself, not via route middleware. Read the session from useRequestEvent() and throw createError({ statusCode: 401 }) (or redirect) before returning data. This is the recommended pattern for islands regardless of this advisory. - Disable experimental.componentIslands if your app does not use the feature. - If your app must keep route-middleware-only auth, gate the /nuxtisland/page URL prefix at your reverse proxy or in a server middleware.

Summary

The /nuxtisland/ endpoint accepts attacker-controlled props query/body parameters and renders any island component without verifying that the URL-resident hash (<Name><hashId>.json) was actually issued for those inputs by <NuxtIsland>. The hash is computed and embedded client-side but never validated server-side, so the same path can return materially different responses depending on the query.

Island components are documented as rendering independently of route context - page middleware does not apply to them, and they are intentionally cacheable as a function of their props. This advisory does not treat that contract as a vulnerability. It treats the absence of a binding between the URL the cache keys on and the response served at that URL as one.

Impact

In applications where a CDN or reverse-proxy in front of the app caches /nuxtisland/ keyed by path only (ignoring query) - a documented misconfiguration class, see GHSA-jvhm-gjrh-3h93 - an attacker can prime the cache for a path with their own choice of props, and subsequent users requesting the same path receive the attacker's rendered HTML rather than the response intended for them. The cache entry persists until normal expiry.

Where the affected island has any prop flowing into an unsafe HTML sink in application code (v-html, innerHTML, a third-party renderer treating a prop as HTML), this becomes stored XSS in the embedding page's origin until the cache entry expires. HttpOnly cookies remain out of reach but anything else in the origin (other cookies, in-origin requests, DOM state) is reachable by the injected script.

Preconditions:

- experimental.componentIslands enabled (or the default 'auto' with at least one server / island component in the app). - A shared intermediary cache (CDN, reverse-proxy, edge cache) keyed on path only. - For the XSS pivot specifically: an application-authored island that puts a prop through an unsafe HTML sink.

Without the second precondition, the response shape is per-request and unaffected. Without the third, the worst case is content-swap / inert HTML injection rather than script execution.

Patches

Patched in nuxt@4.4.6 and nuxt@3.21.6 by #35077. The island handler now recomputes the expected hashId from (name, props, context) using the same ohash function <NuxtIsland> already uses to embed the hash in the URL, and rejects requests (HTTP 400) whose URL-resident hash does not match. The response is now a pure function of the request path: a path-keyed shared cache returns the correct response to every requester for that path, and an attacker cannot synthesise a path whose hash matches arbitrary props.

Workarounds

For users unable to upgrade immediately:

- Ensure any intermediary cache keys /nuxtisland/ on the full query string, not on the path alone. This is the recommended configuration regardless. - Audit application-authored islands for props flowing into v-html / innerHTML / similar HTML sinks; treat island props as untrusted user input.

Note on island authentication

[!IMPORTANT] It's important to remember that route middleware does not run when rendering island components, and islands cannot rely on routing-layer auth. Applications gating sensitive data behind page middleware should enforce that auth inside the island's own data layer (server-only routes, useRequestEvent + manual session checks, etc.) rather than relying on the embedding page's middleware - this was true before this advisory and remains true after it.

A separate advisory addresses .server.vue pages registered as page<routeName> islands, where the documented "middleware doesn't run for islands" contract collides with the page's own definePageMeta({ middleware }) declaration in a way that constitutes a genuine bug rather than documented behaviour.