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.

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