In this paper, a software tool for security analysis of IoT systems is presented. The tool, named ASTo (Apparatus Software Tool) enables the visualization of IoT systems using a domain-specific modeling language. The modeling language provides constructs to express the hardware, software and social concepts of an IoT system along with security concepts. Security issues of IoT systems are identified based on the attributes of the constructs and their relationships. Security analysis is facilitated using the visualization mechanisms of the tool to recognize the secure posture of an IoT system.