What’s Vulnerable › Vulnerabilities

CVE-2026-33804: @fastify/middie vulnerable to middleware bypass via deprecated ignoreDuplicateSlashes option

• Severity: critical (CVSS 9.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-16 · updated 2026-06-17
• Weakness: CWE-436

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.

Affected products

• fastify middie: before 9.3.2 — fixed in 9.3.2

How to fix

Upgrade to a fixed version: fastify middie 9.3.2. Check the vendor advisory for mitigations.

References

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

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