An important new vulnerability was discovered and is applicable to several in-vehicle networks including Control Area Network (CAN), the de facto standard in-vehicle network protocol. Specifically, a bus-off attack exploits the safe mode of CAN to disconnect or shut down uncompromised (healthy) ECUs. This is an important attack that must be thwarted, since once the attacker compromises an ECU, it is easy to mount the attack on safety-critical ECUs while its prevention/detection is very difficult. Based on analysis and experimental results, a mechanism to detect and/or prevent a bus-off attack is proposed and evaluated.