CVE-2026-33804

Name: CVE-2026-33804
Creator: NVD / NIST
Published: 2026-04-16T15:17:34.633+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.1/10EPSS 0.28%

Last modified Jun 17, 2026

CVE-2026-33804 is a critical-severity vulnerability rated 9.1/10 on the CVSS scale. @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. EPSS estimates a 0.28% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

9.1/10

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

EPSS Probability

0.28%

19.5th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-436

Affected Software

Vendor	Product	Versions
Fastify	Fastify\/Middie	< 9.3.2

References

https://cna.openjsf.org/security-advisories.htmlVendor Advisory
https://github.com/fastify/middie/security/advisories/GHSA-v9ww-2j6r-98q6Mitigation, Vendor Advisory

Timeline

Published: Apr 16, 2026
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-33804?

How severe is CVE-2026-33804?

CVE-2026-33804 has a CVSS score of 9.1/10 (CRITICAL severity). The EPSS model estimates a 0.28% probability of exploitation in the next 30 days.

How do I fix CVE-2026-33804?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2026-33804?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST