The blockchain technology witnessed a wide adoption and a swift growth in recent years. This ingenious distributed peer-to-peer design attracted several businesses and solicited several communities beyond the financial market. There are also multiple use cases built around its ecosystem. However, this backbone introduced a lot of speculation and has been criticized by several researchers. Moreover, the lack of legislations perceived a lot of attention. In this paper, we are concerned in analyzing blockchain networks and their development, focusing on their security challenges. We took a holistic approach to cover the involved mechanisms and the limitations of Bitcoin, Ethereum and Hyperledger networks. We expose also numerous possible attacks and assess some countermeasures to dissuade vulnerabilities on the network. For occasion, we simulated the majority and the re-entrancy attacks. The purpose of this paper is to evaluate Blockchain security summarizing its current state. Thoroughly showing threatening flaws, we are not concerned with favoring any particular blockchain network.