CANComa is hardware-based software developed to demonstrate how to attack Automotive Microcontrollers in a reliable and stealthy manner by forcing the victim Electronic Control Unit (ECU) into an unexpected shutdown. Once the microcontroller has been deactivated, the attacker, knowing the missing CAN messages, takes its place and injects some undesirable data. By this replacement the attacker can disable safety-critical vehicle functions. The method used to execute the shutdown attack is to modify the messages already present on the CAN bus by destroying their acknowledgement bit, causing a false bus-off reaction freezing the victim ECU. We suppose the attacker only has access to the CAN bus via an onboard diagnostics (OBD-II) reader used by mechanics to retrieve the error memory and data stored on the vehicle’s system. The approach described in this paper was simulated using STMicroelectronics microcontrollers, which are architecturally quite similar to the ECUs found in automobiles.