Internet of Things (IoT) has been acknowledged as a novel transformation technology because of its wide range of applications in various domains, namely connected agriculture, industrial control, smart buildings and home automation. It promises innovative business models and improved user experience. However, the devices are prone to failures and malicious attacks on account of their resource-constrained characteristics. In this paper, we present a framework for security monitoring of IoT systems. It is based on MMT-IoT, which is a reactive monitoring tool to be deployed in a running IoT environment to address malicious behaviors, failures and attacks. In this paper we also present the experiments conducted on two practical IoT-6LoWPAN testbeds. The preliminary results confirmed the efficiency of the proposed solution.