CVE-2026-33808

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.

CVSS 4.0

9.1CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

CVSS 3.1

9.1CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

0.5% chance of exploitation in the next 30 days; higher than 41% of CVEs (scores of 2026-10-26)

CWE

CWE-436

Published

2026-04-15

Modified

2026-10-26

Record from

the NVD, synced 2026-10-26

Affected

• fastify:fastify\/express

Packages (OSV)

References

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

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