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.

A critical validation bypass vulnerability exists in Fastify versions before 5.12.2. The framework incorrectly evaluates JSON Schema Draft 7 boolean false schemas using JavaScript truthiness, causing it to treat a false schema (intended to deny all input) as a missing schema. As a result, no validator is compiled and the route handler executes on any incoming request. An unauthenticated remote attacker can exploit this to reach handlers that should be completely unreachable, potentially causing unauthorized state changes or execution of disabled operations. The root cause is a mismatch between JavaScript's falsy evaluation and JSON Schema's definition of false as a valid schema that rejects all instances. All four schema types are affected: body, querystring, params, and headers. The fix is to upgrade to Fastify 5.12.2 or later.