CVE-2026-76169

Описание

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 malformed paths through a single shared handler pointer before URL decoding, ignoring the prefix and skipping the selected handler's normal lifecycle. An unauthenticated attacker can therefore reach an authentication-protected private fallback through an unrelated public prefix and read its full response, bypassing the authentication hook and breaking prefix encapsulation. Users should upgrade to fastify 5.12.2 or later.

A flaw was found in fastify. Malformed URLs can be routed to a custom not-found handler of a different plugin, bypassing the intended security checks. This allows an unauthenticated attacker to access and read the full response of an authentication-protected private fallback, leading to information disclosure and an authentication bypass.