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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.

PUBLISHED Reserved 2026-03-23 | Published 2026-04-16 | Updated 2026-04-16 | Assigner openjs




HIGH: 7.4CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Problem types

CWE-436: Interpretation Conflict

Product status

Default status
unaffected

Any version before 9.3.2
affected

9.3.2 (semver)
unaffected

Credits

FredKSchott reporter

mcollina remediation developer

climba03003 remediation reviewer

UlisesGascon remediation reviewer

References

github.com/...middie/security/advisories/GHSA-v9ww-2j6r-98q6

cna.openjsf.org/security-advisories.html

cve.org (CVE-2026-33804)

nvd.nist.gov (CVE-2026-33804)

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