This paper addresses the issue of security in blockchain systems, with a focus on attacks targeting the peer-to-peer architecture. The peer-to-peer nature of blockchain is fundamental to many of the benefits promised by blockchain applications. We detail various attacks affecting this architecture, including network attacks, eclipse attack, majority attacks, selfish mining attack, block-withholding attacks, and time-jacking attack. This paper provides a significant contribution in three parts: firstly, it offers a comprehensive description of several attacks targeting this architecture. Secondly, it examines the necessary conditions for the effectiveness of these attacks and, thirdly, it presents a qualitative overview of the defense strategies identified in the existing literature to deal with these threats. © IFIP International Federation for Information Processing 2024.