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 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...

PT-2026-85449

Name of the Vulnerable Software and Affected Versions fastify versions 4.0.0 through 5.12.1 Description An unauthenticated attacker can bypass authentication hooks and break prefix encapsulation by sending a malformed URL. The internal not-found router for encapsulated handlers dispatches malform...