CVE-2026-33804@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option

Description

@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.

Data: CVE.org, NVD, CISA KEV, FIRST EPSS, Ubuntu. Published as the sources state it, in English.

Affected products and versions

References and patches

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