Abstract: The described technology relates to a method for detecting abnormal transaction data between computing devices in a communication network using valid transaction patterns and a server for performing the same. In one aspect, the method includes receiving, at a data collection processor, from an institution server computer, a plurality of pieces of transaction data communicated between two or more of the plurality of user devices, and determining, at a data processor, at least one piece of valid transaction data from the plurality of pieces of transaction data. The method may also include generating, at the data collection processor, a valid transaction pattern based on the determined valid transaction data. The method may further include detecting, at an abnormal transaction detection processor, an existence of an abnormal transaction data between two or more of the plurality of user devices based on the generated valid transaction pattern.