Vulnerability 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.
CVSS Score
CRITICAL
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Fastify | Fastify\/Express | < 4.0.5 |
Related Weaknesses (CWE)
References
- https://cna.openjsf.org/security-advisories.htmlVendor Advisory
- https://github.com/fastify/fastify-express/security/advisories/GHSA-6hw5-45gm-fjExploitMitigationVendor Advisory
- https://github.com/fastify/fastify-express/security/advisories/GHSA-6hw5-45gm-fjExploitMitigationVendor Advisory
FAQ
What is CVE-2026-33808?
CVE-2026-33808 is a vulnerability with a CVSS score of 9.1 (CRITICAL). 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-s...
How severe is CVE-2026-33808?
CVE-2026-33808 has been rated CRITICAL with a CVSS base score of 9.1/10. This is considered a critical vulnerability requiring immediate attention.
Is there a patch for CVE-2026-33808?
Check the references section above for vendor advisories and patch information. Affected products include: Fastify Fastify\/Express.