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

What is CVE-2026-33807?

CVE-2026-33807 is a critical authentication bypass vulnerability in @fastify/express, published on 15 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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.

Weakness: CWE-436

Which versions are affected by CVE-2026-33807?

2 open source packages ship an affected version, including @fastify/express. It affects 1 product.

Products include Fastify Fastify\/Express.

How do I fix CVE-2026-33807?

Upgrade @fastify/express to 4.0.5 or later.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2026-33807 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33807 exists.

Other vulnerabilities in @fastify/express

• CVE-2026-6556 @fastify/express vulnerable to middleware bypass via non-string mount paths in prefixed plugins Critical 9.1
• CVE-2026-33808 @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons) Critical 9.1

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-hrwm-hgmj-7p9c.