GAD-EBM: Graph Anomaly Detection using Energy-Based Models

Graph Anomaly Detection (GAD) is essential in fields ranging from network security, bioinformatics to finance. Previous works often adopt auto-encoders to compute reconstruction errors for anomaly detection: anomalies are hard to be reconstructed. In this work, we revisit the first principle for anomaly detection, i.e., the Neyman-Pearson rule, where the optimal anomaly detector is based on the likelihood of a data point given the normal distribution of data. However, in practice, the distribution is often unknown and the estimation of the distribution of graph-structured data may be hard. Moreover, the likelihood computation of a graph-structured data point may be challenging as well. In this paper, we propose a novel approach GAD-EBM that can estimate the distribution of graphs and compute likelihoods efficiently by using Energy-Based Models (EBMs) over graphs. GAD-EBM approaches the likelihood of a rooted subgraph of node v, and further can leverage the likelihood to accurately identify whether node v is anomalous or not. Traditional score matching for training EBMs may not be used to apply EBMs that model the distribution of graphs because of complicated discreteness and multi-modality of graph data. We propose a Subgraph Score Matching (SSM) approach, which is specifically designed for graph data based on a novel framework of neighborhood state-space graphs. Experimentation conducted on six real-world datasets validates the effectiveness and efficiency of GAD-EBM and the source code for GAD-EBM is openly available.