Blockchain is an innovative technology that gives built-in security to any software or application. There is a wide range of applications for blockchain, from risk management to financial services, crypto-currencies and the Internet of Things (IoT). This innovation is based on transparency, immutability, security, efficiency and decentralization. It is a trending topic since cryptocurrencies are a hot topic in the market. Blockchain is a combination of mathematics, cryptography, algorithms and models. In this paper, we present a general overview of the security aspects of blockchain technology.