CVE-2026-33804 — @fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option
@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…
Published: 2026-04-16 · CVSS: 7.4 · CVSS 3.1
@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.
Source: CVE Program · Revision 8b16575c39f1e1810c5e863010da073d2d3285e7