Machine-to-Machine (M2M) is one of the emergent technologies that has attracted a lot of attention in both industrial and academic sectors and which is expected to grow in the next few years. Indeed, it opens the way to the Internet of Things (IoT), an internet where all devices are connected and communicate without any human intervention. Yet, an important hurdle that may slow down Machine-to-Machine growth and even hinder the massive roll-out of certain applications is security. Even though there have been a lot of research on Machine-to-Machine, nevertheless only few of them focus on the security aspects. As a fusion of heterogeneous networks, Machine-to-Machine poses a number of challenges, particularly to security design. In this paper, we provide a survey on Machine-to-Machine research and development works that mainly addresses security and more precisely the threats that Machine-to-Machine communications have to face. We also tackle the challenges that arise when trying to secure M2M communications along with an investigation of the existing approaches aiming to do so. Finally, we give a general comparison of existing solutions regarding some selected parameters.