Current Wireless Sensor Networks (WSNs) approaches do not provide an efficient and secure code dissemination function due to emerging issues of IoT applications. In this work, we adopt a multicast approach instead of the existing end-to- end or epidemic approaches. In order to enable the multicast approach, we propose an efficient/robust group key distribution scheme. We will evaluate and quantify the performance of our prototype implementation in a public testbed, while emulating several practical IoT settings, and show our security measures against known attack models.