UNAUTHORIZED COMMUNICATION DETECTION METHOD, UNAUTHORIZED COMMUNICATION DETECTION SYSTEM, AND NON-TRANSITORY COMPUTER-READABLE RECORDING MEDIUM STORING A PROGRAM

An unauthorized activity detection method in an onboard network system. The detection method includes determining whether or not a message sent out onto the network is an attack message, saving information relating to the attack message in at least one memory in a case where the message is an attack message, identifying a communication pattern from information relating to the attack message, and determining whether or not the message matches a communication pattern. The determination of whether an attack message and determination of whether matching a communication pattern are executed on each of a plurality of messages received from the network. In the determining of whether an attack message executed on a message received after executing of determining of whether matching a communication pattern, results of the determination of whether an attack message that has already be executed are used.