Vulnerability in Fastify @Fastify/express
CVE-2026-33807
@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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N.
EPSS exploit prediction
EPSS: 0.005 (42.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 43rd percentile — 42.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.001
Affected products
- Fastify @Fastify/express — versions 0, 4.0.5
- Fastify Fastify\/express
Weakness classification (CWE)
References
- ce714d77-add3-4f53-aff5-83d477b104bb (Exploit, Mitigation, Vendor Advisory)
- ce714d77-add3-4f53-aff5-83d477b104bb (Vendor Advisory)
Frequently asked questions
- What is CVE-2026-33807?
- CVE-2026-33807 is a critical-severity vulnerability in Fastify @Fastify/express, classified under CWE-436. CVSS score: 9.1/10. Published 2026-04-15.
- How severe is CVE-2026-33807?
- Critical severity. CVSS v3 base score is 9.1 out of 10.