To detect identity spray attacks, a machine learning model classifies account access attempts as authorized or unauthorized, based on dozens of different pieces of information (machine learning model features). Boosted tree, neural net, and other machine learning model technologies may be employed. Model training data may include user agent reputation data, IP address reputation data, device or agent or location familiarity indications, protocol identifications, aggregate values, and other data. Account credential hash sets or hash lists may serve as model inputs. Hashes may be truncated to further protect user privacy. Classifying an access attempt as unauthorized may trigger application of multifactor authentication, password change requirements, account suspension, or other security enhancements. Statistical or heuristic detections may supplement the model. However, the model may adapt to changed attacker behavior through retraining with updated data, making the model-based approach more effective over time than rigid statistical or heuristic detection approaches.