Validation bypass via root primitive coercion mismatch

Schema validation bypass in Fastify before 5.12.1 allows attackers to bypass request-body type checks via a crafted root primitive JSON value. When a route schema expects a top-level primitive such as an integer, Ajv coerces a JSON string to the expected type during validation, but Fastify leaves request.body unchanged, so the handler receives the original string rather than the validated primitive. Exposure depends on routes that accept root primitive bodies and then trust the validated type for limits, integrity checks, or access-control decisions; object and array body schemas are not affected.