CVE-2026-76169

Description

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.

Weakness classification

• CWE-288: Authentication Bypass Using an Alternate Path or Channel

Primary references

• ce714d77-add3-4f53-aff5-83d477b104bb — Third Party Advisory
• ce714d77-add3-4f53-aff5-83d477b104bb — Mitigation, Vendor Advisory