The Internet of things (IoT) has revolutionized the use of connectivity and has given birth to new transmission technologies to satisfy the requirements of diverse IoT applications. Low powered wide area networks (LPWAN) is one of those transmission technologies, and is becoming exceptionally useful for IoT applications. The nodes use energy-efficient mechanisms for long-range data transmission (10–20 km), lasting in hostile environments for years and making them suitable for IoT applications such as environmental monitoring, automated billing systems, smart homes, smart offices, and patient monitoring. However, LPWAN devices have minimal resources, which makes it challenging to provide promising security to devices and data in the network. In this paper, we discuss the security mechanisms used in current LPWAN technologies along with their vulnerabilities and possible attacks on them. A detailed literature review is conducted on existing solutions on the security of constrained IoT networks similar to LPWAN using different networking frameworks. The reviewed literature is then compared based on various network security measures addressed by them. In addition, the emergence of software defined network (SDN) architecture for security in IoT is explained based on literature. Finally, the applicability of SDN in LPWAN security, its opportunities, and challenges in implementation are discussed.