CVE-2026-33807

The record

@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 packages

Related vulnerabilities

• GHSA-hrwm-hgmj-7p9c @fastify/express's middleware path doubling causes authentication bypass in child plugin… Same vulnerability
• GHSA-6hw5-45gm-fj88 @fastify/express has a middleware authentication bypass via URL normalization gaps… Also affects @fastify/express
• CVE-2026-33808 Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to… Also affects @fastify/express
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