CVE-2026-71316: Nuxt runtime payload cache discloses another user's SSR data across users and to unauthenticated clients

When a page is covered by routeRules cache / swr / isr, Nuxt enables runtime payload extraction and serves /<page>/_payload.json. On affected versions the renderer stored the SSR payload in the shared cache:nuxt:payload storage under a path-only key (no cookie, authorization, or cache.varies dimension) and, on a later payload request, returned the cached entry before route middleware / page guards ran again.

As a result, once any authenticated user warms a protected, cached page, a subsequent GET /<page>/_payload.json from an unauthenticated client or a different authenticated user receives the first user’s payload: the full SSR data for that route, including anything loaded via useFetch / useAsyncData (for example /api/me: profile, tenant, billing, token-like values). The HTML response stays correctly varied and protected; only the extracted payload leaks. Both cross-user (A warms, B receives A) and unauthenticated disclosure are exploitable. cache.varies does not mitigate it, because the payload cache ignores varies.

Introduced when runtime payload extraction landed for cached routes (#34410); the regression is specific to the 4.x line, where the runtime cache:nuxt:payload storage was added and the import.meta.prerender gate on the payload-cache read/writes was dropped. The 3.x line shipped the same feature with the gate intact and is not affected.