ENISA issues report on blockchain technology and security

[...] ENISA analysed the technology and identified security benefits, challenges and good practices. The report [Distributed Ledger Technology & Cybersecurity; pdf] identifies that some principles used in the security of traditional systems and in blockchain, such as key management and encryption, are still largely the same.  There are however new challenges that the technology brings, like consensus hijacking and smart contract management. Additionally, it highlights that public and private ledger implementations will face different sets of challenges. [...]
Also worth a look: Security of Mobile Payments and Digital Wallets                
Source: ENISA