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.

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 middlewa…

EPSS: 0.000 (11.4th percentile) — read the EPSS interpretation.

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.

Affected products

Fastify @Fastify/express — versions 0, 4.0.5
Fastify Fastify\/express

Weakness classification (CWE)

CWE-436

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.