AIKIDO-2026-349253
fastify is vulnerable to Authentication Bypass
75
High Risk
This Affects:
fastifyTL;DR
Fastify's internal not-found router can dispatch a malformed URL under one plugin prefix to the custom not-found handler registered by a sibling plugin while skipping that handler's preHandler. An unauthenticated request to a public prefix can therefore reach an authentication-protected fallback under another prefix and receive protected data. The fix routes malformed URLs through the configured bad-URL handlers before any application not-found handler runs and removes the shared handler pointer.
Who does this affect?
You are affected if you are using a version that falls within the vulnerable range and protected data is returned from custom not-found handlers registered under plugin prefixes.
Background info
fastify is vulnerable to Authentication Bypass in versions 4.0.0 - 5.12.1.
How to fix this
Upgrade the fastify library to the patch version.
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