Fifth-generation technology [5G] services commercialized in 2019 have not only provided voice and data communication but also undergone significant structural changes in mobile networks to accommodate Internet-of-things devices, sensitive to latency and reliability, by adopting the latest ICT technologies, such as software-defined networking/network function virtualization, multi-access edge computing, and network slicing. However, this technological evolution poses new security challenges, such as creation of new access paths, owing to its complex inter-operation structures, security downgrading, and limitations in security visibility. To address these issues, research on 5G security threats and security architecture has been actively underway at international standards organizations, communication carriers, and universities. However, security researchers find it difficult to conduct studies on 5G security technology design and application methods owing to the relatively unknown nature of the mobile carrier network. Therefore, in this paper, we analyzed five new security issues for each 5G section, relative to 5G technical advantages, by reviewing previous studies. In addition, we classified cyber attacks against nine network protocols primarily used in the 5G core network. The result of this study is expected to be used as basic data for modeling 5G security threats.