CVE-2026-33807CWE-436

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

Critical · published April 15, 2026

CVSS v3.1
9.1
EPSS
1%
Percentile
44.2
In the wild
Unconfirmed
What it is

@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.

The record
Technical detail
CVSS v3.1
9.1 · CRITICAL
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
CVSS v4.0
Not supplied
EPSS
0.00534 · 44.2th percentile
Weakness
CWE-436 · Interpretation Conflict
Published
2026-04-15T09:52Z
EPSS history
Timeline
  • 15 APR 09:52Z
    @fastify/express vulnerable to middleware path doubling causing authentication bypass in child plugin scopes
    cvelistv5