The report starts with an overview of the blockchain security system and then highlights the specific security threats and summarizes them. We review with some comments and possible research direction. This survey, we examines the security issues of blockchain model related technologies and their applications. The blockchain is considered a still growing like the internet in 1990. It has the potential to disrupt so many technology areas in the future. But as a new underdeveloped field, it is suffering many setbacks mostly resulting from the security area. Its security concerns coming not only from distributed/decentralized computing issue or Cryptography algorithm issue, from some unexpected field too. Here, in this paper, we tried to classify the security concerns for the blockchain based on our survey from recent research papers. We also tried to show which way blockchain development trends are going.