CVE-2026-84469: Fastify

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

fastify versions before 5.12.2 decide whether to compile a request schema based on JavaScript truthiness, but JSON Schema Draft 7 defines the boolean false as a valid schema that rejects every instance. When an application assigns false to a route's body, querystring, params, or headers schema to deny all input, fastify treats it as a missing schema, compiles no validator, and runs the route handler on any request. An unauthenticated remote client can therefore reach a handler that a valid deny-all schema was intended to make unreachable, a complete validation bypass that can lead to unauthorized state changes or execution of disabled operations. Users should upgrade to fastify 5.12.2 or later.

Affected products

• Fastify Fastify: before 5.12.2 (fixed in 5.12.2)

Published 2026-09-04. Last modified 2026-09-15.