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CVE-2026-33804

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

Published: Apr 16, 2026Updated: Apr 16, 2026 Sources: CVE List NVD
NVD MITRE
7.4CVSS
HIGH

@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.

EPSS Score
N/A
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
No Fix Available

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

Weaknesses

CWE-436

Affected Products

@fastify/middie
@fastify/middie

Exploitability

No known exploits, KEV entries, or remediation guidance available for this vulnerability yet.

References

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

Timeline

Published
Apr 16, 2026
Last Updated
Apr 16, 2026

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