What’s Vulnerable › Vulnerabilities

CVE-2026-33807: @fastify/express vulnerable to middleware path doubling causing authentication bypass in child plugin scopes

• Severity: critical (CVSS 9.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-04-15 · updated 2026-10-26
• Weakness: CWE-436

Description

@fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware security controls, including authentication, authorization, and rate limiting, for all routes defined within affected child plugin scopes. No special configuration or request crafting is required. Upgrade to @fastify/express v4.0.5 or later.

Affected products

• Fastify/Express: before 4.0.5 — fixed in 4.0.5

How to fix

Upgrade to a fixed version: Fastify/Express 4.0.5. Check the vendor advisory for mitigations.

References

• https://cna.openjsf.org/security-advisories.html
• https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c
• https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c

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