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 request target reaches Fastify's internal not-found router before URL decoding and is dispatched through a single shared handler pointer, regardless of prefix and without the normal request lifecycle. An unauthenticated request to a public prefix can therefore reach an authentication-protected not-found handler registered under a different prefix and receive its full response, breaking prefix encapsulation and bypassing the authentication hook. Applications whose private or tenant fallbacks return protected data from a not-found handler are affected.

Patches

Patched in fastify 5.12.2. Malformed URLs are now routed through the configured onBadUrl and onMaxParamLength handlers so they fail closed before any application not-found handler runs, and the shared not-found handler pointer has been removed.

Workarounds

Reject malformed request targets before they reach the application, for example at an upstream proxy or gateway, and do not rely on a not-found handler to serve protected data. A global onRequest authentication hook does not mitigate this, because the malformed-URL path skips it.

Fastify is a web framework with minimal overhead and plugin architecture. The attacker can use the incorrect Content-Type to bypass the Pre-Flight checking of fetch. fetch() requests with Content-Type’s essence as "application/x-www-form-urlencoded", "multipart/form-data", or "text/plain", could potentially be used to invoke routes that only accepts application/json content type, thus bypassing any CORS protection, and therefore they could lead to a Cross-Site Request Forgery attack. This issue has been patched in version 4.10.2 and 3.29.4. As a workaround, implement Cross-Site Request Forgery protection using @fastify/csrf'.