CVE-2026-33807

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

Published Apr 15, 2026 CVSS 3.1 · 9.1 Critical EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-hrwm-hgmj-7p9c @fastify/express's middleware path doubling causes authentication bypass in child plugin scopes
History
Type Values Removed Values Added
First Time appeared Fastify fastify\/express
CPEs cpe:2.3:a:fastify:fastify\/express:*:*:*:*:*:node.js:*:*
Vendors & Products Fastify fastify\/express
Type Values Removed Values Added
First Time appeared Fastify
Fastify fastify-express
Vendors & Products Fastify
Fastify fastify-express
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'poc', 'Technical Impact': 'total'}, 'version': '2.0.3'}

Type Values Removed Values Added
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.
Title @fastify/express vulnerable to middleware path doubling causing authentication bypass in child plugin scopes
Weaknesses CWE-436
References
Metrics cvssV3_1

{'score': 9.1, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
openjs
Published
Updated
Apr 15, 2026, 1:09 PM
Reserved
Mar 23, 2026, 7:48 PM
Status
Analyzed
Published
Apr 15, 2026, 10:16 AM
Modified
Oct 26, 2026, 9:10 PM