ZooRank: Ranking Suspicious Entities in Time-Evolving Tensors

Most user-based websites such as social networks (Twitter, Facebook) and e-commerce websites (Amazon) have been targets of group fraud (multiple users working together for malicious purposes). How can we better rank malicious entities in such cases of group-fraud? Most of the existing work in group anomaly detection detects lock-step behavior by detecting dense blocks in matrices, and recently, in tensors. However, there is no principled way of scoring the users based on their participation in these dense blocks. In addition, existing methods do not take into account temporal features while detecting dense blocks, which are crucial to uncover bot-like behaviors. In this paper (a) we propose a systematic way of handling temporal information; (b) we give a list of axioms that any individual suspiciousness metric should satisfy; (c) we propose zooRank, an algorithm that finds and ranks suspicious entities (users, targeted products, days, etc.) effectively in real-world datasets. Experimental results on multiple real-world datasets show that zooRank detected and ranked the suspicious entities with high accuracy, while outperforming the baseline approach.