A private controller area network (CAN) is provided in a vehicle to notify non-targeted electronic control units (ECUs) of unauthorized attempts to access a target ECU. Each ECU on the private CAN stores a common encrypted hacking notification key (HNK) and a unique identification code. When a first vehicle system ECU detects an unauthorized access attempt via the public CAN, it sends a warning message to the other ECUs via the closed, private CAN.