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CVE-2026-33804

@fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option

Published: Apr 16, 2026Updated: May 14, 2026 Sources: CVE List NVDCWE-436
NVD MITRE
7.4CVSSHIGH
EPSS Score
0.3%
Top 80.7%
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
Patch Available

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.

CVSS Vector Breakdown

AV:NAC:HPR:NUI:NS:UC:HI:HA:N
Exploitability
AV:NAttack Vector
Network
AC:HAttack Complexity
High
PR:NPrivileges Required
None
UI:NUser Interaction
None
Scope
S:UScope
Unchanged
Impact
C:HConfidentiality
High
I:HIntegrity
High
A:NAvailability
None
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Weaknesses

CWE-436

Affected Products

fastify\/middiefastify
Library
OSS Libraries / web-framework
@fastify/middie@fastify/middie
Library
OSS Libraries / web-framework
fastifyoss-projectOSS Librariesaka fastify/middie, fastify-multipart
@fastify/middieoss-projectUSOSS Librariesaka @fastify/middie, fastify-middie

Exploitability

Official Patch Available
Workaround Available

References

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

Timeline

Published
Apr 16, 2026
Last Updated
May 14, 2026

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