Description
Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware when Fastify router normalization options are enabled. This allows complete bypass of path-scoped authentication middleware via duplicate slashes when ignoreDuplicateSlashes is enabled, or via semicolon delimiters when useSemicolonDelimiter is enabled. In both cases, Fastify router normalizes the URL and matches the route, but @fastify/express passes the original un-normalized URL to Express middleware, which fails to match and is skipped. An unauthenticated attacker can access protected routes by manipulating the URL path.

PatchesUpgrade to @fastify/express v4.0.5 or later.
Published: 2026-04-15
Score: 9.1 Critical
EPSS: < 1% Very Low
KEV: No
Impact: Authentication Bypass
Action: Apply Patch
AI Analysis

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Remediation

No vendor fix or workaround currently provided.

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Tracking

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Advisories
Source ID Title
Github GHSA Github GHSA GHSA-6hw5-45gm-fj88 @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)
History

Wed, 15 Apr 2026 15:15:00 +0000

Type Values Removed Values Added
First Time appeared Fastify
Fastify fastify-express
Vendors & Products Fastify
Fastify fastify-express

Wed, 15 Apr 2026 14:15:00 +0000

Type Values Removed Values Added
Metrics ssvc

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


Wed, 15 Apr 2026 10:00:00 +0000

Type Values Removed Values Added
Description Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware when Fastify router normalization options are enabled. This allows complete bypass of path-scoped authentication middleware via duplicate slashes when ignoreDuplicateSlashes is enabled, or via semicolon delimiters when useSemicolonDelimiter is enabled. In both cases, Fastify router normalizes the URL and matches the route, but @fastify/express passes the original un-normalized URL to Express middleware, which fails to match and is skipped. An unauthenticated attacker can access protected routes by manipulating the URL path. PatchesUpgrade to @fastify/express v4.0.5 or later.
Title @fastify/express vulnerable to middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)
Weaknesses CWE-436
References
Metrics cvssV4_0

{'score': 9.1, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N'}


Subscriptions

Fastify Fastify-express
cve-icon MITRE

Status: PUBLISHED

Assigner: openjs

Published:

Updated: 2026-04-15T13:10:24.054Z

Reserved: 2026-03-23T19:48:48.715Z

Link: CVE-2026-33808

cve-icon Vulnrichment

Updated: 2026-04-15T13:10:10.057Z

cve-icon NVD

Status : Awaiting Analysis

Published: 2026-04-15T10:16:48.453

Modified: 2026-04-17T15:38:09.243

Link: CVE-2026-33808

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-04-15T14:53:06Z

Weaknesses