The emergence of in-vehicle networks, which are composed of controller area network (CAN) buses and a great number of ECUs, significantly reduces the difficulty of vehicle designing, repairing and refitting. While owing to the intrinsic vulnerabilities of in-vehicle networks and the increasingly rich interfaces to connect in-vehicle networks to the outside, adversarial attacks can be easily implemented on in-vehicle networks. Such attacks cause serious threats to automobile security and drivers' privacy. In light of this, we provide in this article a detailed guidance, explain the basic concepts, introduce the vulnerabilities of in-vehicle networks, and summarize the attacking methodologies. We also provide countermeasures for in-vehicle networks and point out challenges and future directions.