Authentication bypass using an alternate path or channel in Fastify - CVE-2026-76169
Published: September 7, 2026
Vulnerability identifier: #VU147204
CSH Severity: Medium
CVSS v4: 8.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-76169
CWE-ID: CWE-288
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to disclose sensitive information.
The vulnerability exists due to authentication bypass using an alternate path or channel in Fastify\'s internal not-found router when processing malformed request targets for methods without a route in the main router. A remote attacker can send a malformed request to a public plugin prefix to disclose sensitive information.
Applications are affected when private or tenant fallback handlers return protected data.
Affected software
Fastify
How to mitigate CVE-2026-76169
Install security update from vendor's website.
Fastify - update to 5.12.2