In-Vehicle Network (IVN) is composed of many communication nodes and Electronic Control Units (ECUs). The complex and interactive hybrid IVN expose more vulnerabilities of the system as it connects to the external network environment and opens up more attacking surface. In this paper, we first show a complete structure of the in-vehicle hybrid network. Then, we propose a three-layer network structure model, analyze the security threats of each layer, and compare the state-of-the-art countermeasures in detail. Finally, we identify the challenges and future research directions for the security of the in-vehicle hybrid network.