fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers

Impact Fastify routes a malformed URL under one plugin prefix to the custom not-found handler of a different sibling plugin, invoking the handler registered last and skipping the preHandler declared in its setNotFoundHandler. When the request method has no route in the main router, a malformed...

CVE-2026-76169 fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers

fastify versions = 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The internal not-found router for encapsulated handlers dispatches...

CVE-2026-76169: Authentication Bypass Using an Alternate Path or Channel

fastify versions = 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The internal not-found router for encapsulated handlers dispatches...