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 M2M growth and even hinder the massive roll-out of certain applications is security. Even though there has been a lot of research on M2M, nevertheless only few focus on the security aspects. As a fusion of heterogeneous networks, M2M poses a number of challenges, particularly to security design. In this paper, we provide a survey on M2M research and development works that mainly addresses security and more precisely the threats that M2M 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.