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…

PriorityP261critical9.1CVSS 3.1

AVNACLPRNUINSUCHIHAN

EPSS

0.28%

19.5th percentile

@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.

Affected

3 ranges

Vendor	Product	Version range	Fixed in
fastify	fastify_middie	< 9.3.2	9.3.2
fastify	middie	>= 0 < 9.3.2	9.3.2
fastify	middie_fastify_middie	< 9.3.2	9.3.2

CVSS provenance

nvdv3.19.1CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

ghsa8.2HIGH

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