CVE-2026-33804

@fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications using the deprecated ignoreDuplicateSlashes option. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds other than disabling the ignoreDuplicateSlashes option.

The following products are affected by CVE-2026-33804 vulnerability.

Here, you will find a curated list of external links that provide in-depth information to CVE-2026-33804.

URL	Resource
https://cna.openjsf.org/security-advisories.html
https://github.com/fastify/middie/security/advisories/GHSA-v9ww-2j6r-98q6