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CVE

CVE-2026-33808

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

CVE-2026-33808

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

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.

Package Versions Affected

Package Version
patch Availability
No items found.

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CVSS Version

Severity
Base Score
CVSS Version
Score Vector
C
H
U
9.1
-
4.0
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
C
H
U
0
-
C
H
U
-

Related Resources

No items found.

References

https://cna.openjsf.org/security-advisories.html, https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33808.json, https://github.com/fastify/fastify-express/security/advisories/GHSA-6hw5-45gm-fj88, https://nvd.nist.gov/vuln/detail/CVE-2026-33808

Severity

0

CVSS Score
0
10

Basic Information

Base CVSS
0
EPSS Probability
0.00483%
EPSS Percentile
0.39604%
Introduced Version
0
Fix Available
f52f4476cbd874a9526027082b9e075d3bff63ac

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