Anomaly Detection in Blockchain Networks: A Comprehensive Survey

Over the past decade, blockchain technology has attracted a huge attention from both industry and academia because it can be integrated with a large number of everyday applications working over features of modern information and communication technologies (ICT). Peer-to-peer (P2) architecture of blockchain enhances these applications by providing strong security and trust-oriented guarantees, such as immutability, verifiability, and decentralization. Despite these incredible features} that blockchain technology brings to these ICT applications, \rev{modern research has indicated that these strong} guarantees are not sufficient enough and blockchain \rev{networks may still be prone to various security, privacy, and reliability related issues. In order to overcome these issues, it is important to identify the anomalous behaviour within the actionable time frame. In this article, we provide an in-depth survey regarding integration of anomaly detection models in blockchain technology. For this, we first discuss how anomaly detection can aid in ensuring security of blockchain based applications. Then, we demonstrate certain fundamental evaluation matrices and key requirements that can play a critical role while developing anomaly detection models for blockchain. Afterwards, we present a thorough survey of various anomaly detection models from the perspective of each layer of blockchain} to provide readers an in-depth overview of integration that has been carried out till date. Finally, we conclude the article by highlighting certain \rev{important challenges alongside discussing how they can serve as future research directions for new researchers in the field.