Vulnerability Description
fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The internal not-found router for encapsulated handlers dispatches malformed paths through a single shared handler pointer before URL decoding, ignoring the prefix and skipping the selected handler's normal lifecycle. An unauthenticated attacker can therefore reach an authentication-protected private fallback through an unrelated public prefix and read its full response, bypassing the authentication hook and breaking prefix encapsulation. Users should upgrade to fastify 5.12.2 or later.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Fastify | Fastify | >= 4.0.0, < 5.12.2 |
Related Weaknesses (CWE)
References
- https://cna.openjsf.org/security-advisories.htmlThird Party Advisory
- https://github.com/fastify/fastify/security/advisories/GHSA-p68q-wchp-6fh7MitigationVendor Advisory
FAQ
What is CVE-2026-76169?
CVE-2026-76169 is a vulnerability with a CVSS score of 7.5 (HIGH). fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler h...
How severe is CVE-2026-76169?
CVE-2026-76169 has been rated HIGH with a CVSS base score of 7.5/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-76169?
Check the references section above for vendor advisories and patch information. Affected products include: Fastify Fastify.