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CVE-2026-6270 is an authentication and authorization bypass vulnerability in @fastify/middie affecting versions 9.3.1 and earlier. The flaw occurs because inherited middleware registered in a parent Fastify scope is not directly registered on child plugin engine instances. In affected applications, when authentication middleware is defined in a parent scope and routes are later exposed from child plugins using @fastify/middie, the child scope may fail to inherit and execute the parent middleware. The issue is described as stemming from incorrect middleware path prefix propagation during child plugin registration, causing the effective middleware path to no longer match incoming requests. As a result, middleware that developers expect to protect child-scope routes silently never runs, allowing requests to reach those routes without the intended authentication or authorization checks.
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An authentication and authorization bypass vulnerability in @fastify/middie where inherited middleware is not registered on child plugin engine instances, allowing unauthenticated access to routes in child plugin scopes.
An authentication bypass vulnerability in @fastify/middie where incorrect middleware path prefix propagation in child plugin scopes can cause parent-scope authentication middleware to silently not execute, leaving routes unprotected.
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