A method of securing vehicle electronic control units (100, 105, 110, 115), ECUs, each ECU controlling a vehicle system and being connected to a public Controller Area Network, CAN accessible from outside the vehicle, the method comprising: a) detecting an attempt, by an external device, to access a first ECU via the public CAN using an access event key, and storing the access event key in the first ECU; b) determining when the attempt to access the first ECU is an unauthorized attempt; and c) transmitting a notification message, from the first ECU to a second ECU, of the unauthorized attempt using a private Controller Area Network, CAN, the private CAN operating in addition to the public CAN, being inaccessible from the outside of the vehicle, and using event-based CAN messaging; and d) wherein the transmitting includes transmitting an identification code (120a, 120b, 120c, 102d) in the notification message that uniquely identifies the first ECU; e) storing an encrypted hacking notification key, HNK, in the first ECU and in the second ECU, wherein the HNK is only being used for a notification of the second ECU of a specific reason for the communication which is an unauthorized access event, and is a unique common private key assigned for inter-ECU private communication chosen by the Original Equipment Manager, OEM and stored in all ECU modules; f) wherein the determining includes generating a random number (140a, 140b, 140c, 140d), transmitting the random number to the external device making the access attempt using the public CAN, decrypting a response from the external device, and comparing the decrypted response with a correct value to determine that the response is an authorized response or an unauthorized response; and g) when the response is an unauthorized response, encrypting the access event key as an unauthorized access event key associated with the unauthorized access attempt, and wherein the transmitting includes transmitting the encrypted unauthorized access event key to the second ECU using the private CAN. a) detecting an attempt, by an external device, to access a first ECU via the public CAN using an access event key, and storing the access event key in the first ECU; b) determining when the attempt to access the first ECU is an unauthorized attempt; and c) transmitting a notification message, from the first ECU to a second ECU, of the unauthorized attempt using a private Controller Area Network, CAN, the private CAN operating in addition to the public CAN, being inaccessible from the outside of the vehicle, and using event-based CAN messaging; and d) wherein the transmitting includes transmitting an identification code (120a, 120b, 120c, 102d) in the notification message that uniquely identifies the first ECU; e) storing an encrypted hacking notification key, HNK, in the first ECU and in the second ECU, wherein the HNK is only being used for a notification of the second ECU of a specific reason for the communication which is an unauthorized access event, and is a unique common private key assigned for inter-ECU private communication chosen by the Original Equipment Manager, OEM and stored in all ECU modules; f) wherein the determining includes generating a random number (140a, 140b, 140c, 140d), transmitting the random number to the external device making the access attempt using the public CAN, decrypting a response from the external device, and comparing the decrypted response with a correct value to determine that the response is an authorized response or an unauthorized response; and g) when the response is an unauthorized response, encrypting the access event key as an unauthorized access event key associated with the unauthorized access attempt, and wherein the transmitting includes transmitting the encrypted unauthorized access event key to the second ECU using the private CAN.