Abstract: Among the plethora of IoT (Internet of Things) applications, the smart home is one of the fastest-growing. However, the rapid development of the smart home has also made smart home systems a target for attackers. Recently, researchers have made many efforts to investigate and enhance the security of smart home systems. Toward a more secure smart home ecosystem, we present a detailed literature review on the security of smart home systems. Specifically, we categorize smart home systems’ security issues into the platform, device, and communication issues. After exploring the research and specific issues in each of these security areas, we summarize the root causes of the security flaws in today's smart home systems, which include the heterogeneity of internal components of the systems, vendors' customization, the lack of clear responsibility boundaries and the absence of standard security standards. Finally, to better understand the security of smart home systems and potentially provide better protection for smart home systems, we propose research directions, including automated vulnerability mining, vigorous security checking, and data-driven security analysis.