CVE-2026-33807

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

@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

fastify · @fastify/express

CVSS v3.1 CNA

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

EPSS

0.53% chance of exploitation in the next 30 days, 44th percentile

Published

2026-04-15

Proof-of-concept exploits (1)

• advisories/GHSA-hrwm-hgmj-7p9c

Security advisories

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

References

• CVE Record
• NVD entry