  Since its inception, the blockchain technology has shown promising
application prospects. From the initial cryptocurrency to the current smart
contract, blockchain has been applied to many fields. Although there are some
studies on the security and privacy issues of blockchain, there lacks a
systematic examination on the security of blockchain systems. In this paper, we
conduct a systematic study on the security threats to blockchain and survey the
corresponding real attacks by examining popular blockchain systems. We also
review the security enhancement solutions for blockchain, which could be used
in the development of various blockchain systems, and suggest some future
directions to stir research efforts into this area.
