VEHICLE-MOUNTED NETWORK SYSTEM, ABNORMALITY DETECTION ELECTRONIC CONTROL UNIT AND ABNORMALITY DETECTION METHOD

In a fraud-detection method for use in an in-vehicle network system including a plurality of electronic control units (ECUs) that exchange messages on a bus and a fraud-detection ECU connected to the bus, the fraud-detection ECU includes a memory that stores rule information indicating a rule regarding transmission of a message to be transmitted on the bus, the fraud-detection ECU determines whether or not a message transmitted on the bus is malicious by using the rule information, and, in a case where the message is malicious, transmits an error message including a message identifier of the malicious message. The fraud-detection ECU acquires updated rule information transmitted from an external server, and updates the rule information by using the updated rule information.