The Internet of Things (IoT) is one of the most novel networking paradigms and there are yet too many technologies defining themselves as IoT complicating the scenario for developing a fully IoT environment. The situation becomes even harder when security and privacy are considered. In this paper, we present a survey on the security aspects of an IoT conformed by wireless sensors communicating through the IEEE 802.15.4 standard. This survey follows a revision of the state of art in a layer-by-layer systematic analysis.