Hypergraph-Based Anomaly Detection in Very Large Networks

This paper addresses the problem of detecting anomalous interactions or traffic within a very large network using a limited number of unlabeled observations. In particular, consider n recorded interactions among p nodes, where p may be very large relative to n. A novel method based on using a hypergraph representation of the data is proposed to deal with this very high-dimensional, “big p, small n” problem. Hypergraphs constitute an important extension of graphs which allows edges to connect more than two vertices simultaneously. An algorithm for detecting anomalies directly on the corresponding discrete space, without any feature selection or dimensionality reduction, is presented. The algorithm has O(np) computational complexity, making it ideally suited for very large networks, and requires no tuning, bandwidth or regularization parameters. The distribution of the data is modeled as a two-component mixture, consisting of a “nominal” and an “anomalous” component. The deviance of each observation from nominal behavior, as well as the mixture parameters, are learned using ExpectationMaximization (EM), assuming a multivariate Bernoulli variational approximation. This approach is related to probability mass function level set estimation and is shown to allow False Discovery Rate control. The identifiability of the underlying distribution, the local consistency of the EM algorithm, and the avoidance of singular solutions are proved. The proposed approach is validated on high-dimensional synthetic data and it is shown that, for a useful class of data distributions, it can outperform other state-of-the-art methods.