5G networks are currently being developed rapidly and are dedicated to connect all things of the Internet with new technologies, components and services. Due to the crucial role of 5G, as well as new architectures and designs, 5G faces a great number of security threats and demands suitable security technologies. This article proposes an automated attack and defense framework toward 5G security with the aim of providing exploratory guidance for 5G security research. We first review the security challenges of attack/defense objects from the perspective of a hierarchical structure, and then propose a hierarchical security model that supports both single-layer security and cross-layer security. According to the hierarchical model, an automated attack and defense framework based on a security knowledge graph is proposed for both known and unknown security threats of 5G and provides possible directions toward 5G security automation.