MQTT is a Client Server publish/subscribe messaging transport protocol. It is lightweight, open, simple, and designed to be easy to implement. These characteristics make it ideal for use in many situations, including constrained environments such as for communication in Machine to Machine (M2M) and Internet of Things (IoT) contexts where a small code footprint is required and/or network bandwidth is at a premium. MQTT Client Test Suite is designed to act as a malicious MQTT Server. It can be used to test MQTT clients to find security flaws and robustness problems. The test suite contains test cases for testing MQTT protocol specific operations. Additionally, the test suite can be configured to test MQTT application layer specific payload which goes over the MQTT protocol.