Nuxt: Route-rule middleware bypass via case-sensitivity mismatch between vue-router and the routeRules matcherCVE-2026-53721

## Impact Nuxt looks up `routeRules` for the current navigation by calling `getRouteRules({ path: to.path })` from the page-router plugin and the no-pages router plugin. The compiled `routeRules` matcher (built on `rou3`) performs case-sensitive matching, while vue-router is configured with its default `sensitive: false` and matches paths case-insensitively. The two routers therefore disagree on which rules apply to a given request path: vue-router still matches the page record for `/Admin/dashboard`, but the `routeRules` lookup for the same path returns no match. Any `appMiddleware` declared via `routeRules` is never added to the middleware set and never runs, on both SSR and client navigations. The same path skips other path-keyed route rules in the same way (`ssr`, `redirect`, `appLayout`, and the prerender / payload hints used client-side). For applications using `routeRules` with `appMiddleware` as an authorization gate (a documented pattern), an attacker can flip the case of any static segment in a protected URL (for example `/Admin/dashboard` instead of `/admin/dashboard`) to render the protected page with the middleware skipped. The server returns the fully server-rendered page including any `useFetch` / `useAsyncData` results captured during SSR. This is an instance of CWE-178 (Improper Handling of Case Sensitivity) leading to CWE-863 (Incorrect Authorization) for apps that treat `appMiddleware` as an authorization boundary. ## Mitigating factors - Only affects apps that use `routeRules.appMiddleware`. The more idiomatic `definePageMeta({ middleware })` is bound to the matched route record and is unaffected. - Nuxt route middleware is documented as an app-layer concern, not a server-side auth boundary; well-built apps enforce authorization again at the API / data-fetching layer. - Apps that explicitly set `router.options.sensitive = true` are not affected. ## Patches Fixed in `[email protected]` (commit [`07e39cd6`](https://github.com/nuxt/nuxt/commit/07e39cd6f26e407b4192b7865bd17bc44536b9bb)) and backported to `[email protected]` (commit [`3f3e3fa7`](https://github.com/nuxt/nuxt/commit/3f3e3fa7b5eec8e495f4f8ce0a54813a8875a11e)). The fix normalizes the path used for `routeRules` lookups so it matches vue-router's default case-insensitive semantics. ## Workarounds Until you can upgrade, you can mitigate by either: 1. Setting `router.options.sensitive = true` so vue-router matches case-sensitively (this changes route-matching behaviour app-wide). 2. Moving security-critical middleware off `routeRules.appMiddleware` and onto `definePageMeta({ middleware: [...] })` on the protected page components, which is bound to the matched record. 3. Enforcing authorization at the API / data-fetching layer (which you should be doing in any case). ## Credit Reported by Anthropic / Claude through Anthropic's coordinated vulnerability disclosure process. Reference: ANT-2026-9FSEBYMC.

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