Input validation error in Fastify - CVE-2026-84504
Published: September 7, 2026
Vulnerability identifier: #VU147205
CSH Severity: Medium
CVSS v4: 8.6 [CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-84504
CWE-ID: CWE-20
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote user to perform unauthorized state changes or disclose information.
The vulnerability exists due to improper input validation in Fastify asynchronous request schema validation when handling a request body validated by an `$async` schema. A remote user can send a crafted request body containing a `value` or `error` property to perform unauthorized state changes or disclose information.
Exploitation depends on the application dispatching operations from the schema-validated request body.
Affected software
Fastify
How to mitigate CVE-2026-84504
Install security update from vendor's website.
Fastify - update to 5.12.2