Although Machine to Machine (M2M) networks allow the development of new promising applications, the restricted resources of machines and devices in the M2M networks bring several constraints including energy, bandwidth, storage, and computation. Such constraints pose several challenges in the design of M2M networks. Furthermore, some elements that contributed to the rise of M2M applications have caused several new security threats and risks, typically due to the advancements in technology, increasing computing power, declining hardware costs, and freely available software tools. Due to the restricted capabilities of M2M devices, most of the recent research efforts on M2M have focused on computing, resource management, sensing, congestion control and controlling technologies. However, there are few studies on security aspects and there is a need to introduce the threats existing in M2M systems and corresponding solutions. Accordingly, in this paper, after presenting an overview of potential M2M applications, we present a survey of security threats against M2M networks and solutions to prevent or reduce their impact. Then, we investigate security-related challenges and open research issues in M2M networks to provide an insight for future research opportunities. Moreover, we discuss the oneM2M standard, one of the prominent standard initiatives for more secure and smoother M2M networks and the Internet of Things. (C) 2017 Elsevier Inc. All rights reserved.