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

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 nuxt@4.4.7 (commit 07e39cd6) and backported to nuxt@3.21.7 (commit 3f3e3fa7). 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.