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CVE

CVE-2026-33804

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

CVE-2026-33804

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

Impact

@fastify/middie v9.3.1 and earlier does not read the deprecated (but still functional) top-level ignoreDuplicateSlashes option, only reading from routerOptions. This creates a normalization gap: Fastify's router normalizes duplicate slashes but middie does not, allowing middleware bypass via URLs with duplicate leading slashes (e.g., //admin/secret).

This only affects applications using the deprecated top-level configuration style (fastify({ ignoreDuplicateSlashes: true })). Applications using routerOptions: { ignoreDuplicateSlashes: true } are not affected.

This is distinct from GHSA-8p85-9qpw-fwgw (CVE-2026-2880), which was patched in v9.2.0.

Patches

Upgrade to @fastify/middie >= 9.3.2.

Workarounds

Migrate from deprecated top-level ignoreDuplicateSlashes: true to routerOptions: { ignoreDuplicateSlashes: true }.

Package Versions Affected

Package Version
patch Availability
No items found.

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CVSS Version

Severity
Base Score
CVSS Version
Score Vector
C
H
U
7.4
-
3.1
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
C
H
U
0
-
3.1
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
C
H
U
7.4
-
3.1
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Related Resources

No items found.

References

https://github.com/fastify/middie/security/advisories/GHSA-v9ww-2j6r-98q6, https://nvd.nist.gov/vuln/detail/CVE-2026-33804, https://cna.openjsf.org/security-advisories.html, https://github.com/fastify/middie

Severity

7.4

CVSS Score
0
10

Basic Information

Ecosystem
Base CVSS
7.4
EPSS Probability
0.00069%
EPSS Percentile
0.21322%
Introduced Version
0,8.1.0,8.0.0,5.0.0
Fix Available
9.3.2,7.1.0

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