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CVE-2026-33808

@fastify/express vulnerable to middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

Published: Apr 15, 2026Updated: Jun 1, 2026 Sources: CVE List NVD BDUCWE-436
9.1CRITICAL
NVD MITRE
EPSS Score
0.5%
Top 62.2%
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
Patch Available

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.

No summary for this CVE yet.

CVSS Vector Breakdown

AV:NAC:LPR:NUI:NS:UC:HI:HA:N
Exploitability
AV:NAttack Vector
Network
AC:LAttack Complexity
Low
PR:NPrivileges Required
None
UI:NUser Interaction
None
Scope
S:UScope
Unchanged
Impact
C:HConfidentiality
High
I:HIntegrity
High
A:NAvailability
None
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Weaknesses

CWE-436

Affected Products

FastifyMatteo Collina
Library
OSS Libraries / web-framework
@fastify/expressfastify
Library
OSS Libraries / web-framework
fastify\/expressfastify
Library
OSS Libraries / web-framework
matteo collinaindividual-devOSS Libraries
fastifyoss-projectOSS Librariesaka fastify/middie, fastify-multipart

Exploitability

Official Patch Available
Workaround Available
No detection template yetWe can run the check for you — a free external exposure review, no install. Check my exposure

References

https://github.com/fastify/fastify-express/security/advisories/GHSA-6hw5-45gm-fj88
github.com
https://cna.openjsf.org/security-advisories.html
cna.openjsf.org
https://github.com/fastify/fastify-express/security/advisories/GHSA-6hw5-45gm-fj88
github.com

Timeline

Published
Apr 15, 2026
Last Updated
Jun 1, 2026

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