Message Queue Telemetry Transport (MQTT) is widely accepted as a data exchange protocol in Internet of Things (IoT) environment. For security, MQTT supports Transport Layer Security (MQTT-TLS). However, MQTT-TLS provides thing-to-broker channel encryption only because data can still be exposed after MQTT broker. In addition, ACL becomes impractical due to the increasing number of rules for authorizing massive IoT devices. For solving these problems, we propose MQTT Thing-to-Thing Security (MQTT-TTS) which provides thing-to-thing security which prevents data leak. MQTT-TTS also provides the extensibility to include demanded security mechanisms for various security requirements. Moreover, the transparency of MQTT-TTS lets IoT application developers implementing secure data exchange with less programming efforts. Our MQTT-TTS implementation is available on https://github.com/beebit-sec/beebit-mqttc-sdk for evaluation.