TI-2026-26176 Aliases CVE-2026-33804 GHSA-v9ww-2j6r-98q6

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

MEDIUM 41CWE-436Published 2026-04-16 · Updated 2026-04-16

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: Low
Impact type: —
Confidentiality · integrity · availability: High · High · None
Fix status: Fixed

Attack profile from CVSS 3.1 · NVD. Fix status from CNA and GitHub.

Summary

@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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00278 20th percentileScored 2026-11-07

Fix Fixed @fastify/middie (@fastify/middie): 9.3.2@fastify/middie (npm): 9.3.2

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: No
Technical impact: Total

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
fastify	@fastify/middie	before 9.3.2	9.3.2	Patch available	CNA
—	@fastify/middie (npm)	before 9.3.2	9.3.2	Patch available	GHSA-v9ww-2j6r-98q6

@fastify/middie Vendorfastify Affected before 9.3.2 Fixed in 9.3.2 Patch available CNA
@fastify/middie (npm) Vendor— Affected before 9.3.2 Fixed in 9.3.2 Patch available GHSA-v9ww-2j6r-98q6

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:fastify:fastify\/middie::::::node.js::*	before 9.3.2	Vulnerable	fastify

cpe:2.3:a:fastify:fastify\/middie:*:*:*:*:*:node.js:*:* Versions before 9.3.2 Vulnerable ·fastify

Remediation

Fix · CNA @fastify/middie (@fastify/middie) Fixed in 9.3.2
Fix · GHSA-v9ww-2j6r-98q6 @fastify/middie (npm) Fixed in 9.3.2 Advisory →

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

Release · 2026-04-16
@fastify/middie 9.3.2
2 records Highest priority Priority HIGH 1 HIGH, 1 MEDIUM
Other records, highest priority first
- TI-2026-05498 @fastify/middie vulnerable to middleware authentication bypass in child plugin scopesPriority HIGH 74
Show all 2 records →

Timeline

2026-10-02 Threatist record created from CVE-2026-33804
2026-10-03 Alias added: GHSA-v9ww-2j6r-98q6

References

Advisory 3

Other 1

https://github.com/fastify/middie

TI-2026-05498 @fastify/middie vulnerable to middleware authentication bypass in child plugin scopes CVSS 3.1CRITICAL 9.1
TI-2026-17725 Fastify Middie Middleware Path Bypass CVSS 3.1HIGH 8.8
TI-2026-58517 @fastify/middie vulnerable to path-scoped middleware bypass via absolute-form request target CVSS 3.1CRITICAL 9.1
TI-2026-12029 @fastify/middie vulnerable to authorization bypass via encoded slash in path parameter values CVSS 3.1CRITICAL 9.1
TI-2026-12013 @fastify/middie standalone engine vulnerable to Denial of Service via malformed percent-encoded paths CVSS 3.1HIGH 7.5
TI-2026-02474 @fastify/middie has an improper path normalization vulnerability CVSS 4.0HIGH 8.2

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences