A method, system, and medium used in unauthorized communication detection in an onboard network system having electronic control units connected to a network include: identifying, from information relating to an attack message on the onboard network system, a communication pattern indicating features of the attack message; determining whether a candidate reference message matches the communication pattern; and determining a reference message used as a reference in determining whether or not a message sent out onto the network is an attack message, using results of the determining of whether or not the candidate reference message matches the communication pattern identified in the identifying operation.