1. A computer-implemented method comprising: acquiring, by a computing system, historical data including a plurality of features associated with known legitimate activities and with known illegitimate activities; applying, by the computing system, a machine learning technique to the historical data to gain information about the plurality of features associated with the known legitimate activities and with the known illegitimate activities; generating, by the computing system, a decision tree based on at least a portion of the information about the plurality of features; identifying, by the computing system, a node in the decision tree that satisfies specified precision criteria; creating, by the computing system, a rule based on the node; and identifying, by the computing system, one or more illegitimate activities based on the rule.