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.

Score

CVSS 3.1 9.1 Critical

Status

Analyzed - ce714d77-add3-4f53-aff5-83d477b104bb

Published

2026-04-16

Last modified

2026-04-16

Weaknesses

CWE-436

Affected products

• Fastify/middie (Fastify)
• Fastifymiddie (Fastifymiddie)

References

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