A method described herein involves various operations directed toward network security. The operations include accessing a traffic attribute describing a feature of network traffic. The operations further include determining a baseline distribution for the traffic attribute of a baseline set of transactions involving an online system over a baseline period and, additionally, determining an observed distribution for the traffic attribute of an observed set of transactions involving the online system over an observed period. Using the observed distribution and the baseline distribution, an attribute risk value for the traffic attribute is computed. The operations further include detecting that an anomaly exists in the traffic attribute of the observed set of transactions, based on the attribute risk value. Responsive to detecting the anomaly, an access control is implemented for access to the online system by additional transactions having a particular value in the traffic attribute meeting a pattern of the anomaly.