Spring Integration's JSON to object conversion uses the json__TypeId__ header to choose the deserialization target type, and resolves that header value to a class with ClassUtils.forName and no type/package allow-list. When the header originates from an external producer (it is mapped from the inbound transport, e.g. AMQP/Kafka, into the message), an attacker controls which class the framework loads and which type the JSON payload is bound into.