  Due to their interesting features, blockchains have become popular in recent
years. They are full-stack systems where security is a critical factor for
their success. The main focus of this work is to systematize knowledge about
security and privacy issues of blockchains. To this end, we propose a security
reference architecture based on models that demonstrate the stacked hierarchy
of various threats (similar to the ISO/OSI hierarchy) as well as threat-risk
assessment using ISO/IEC 15408. In contrast to the previous surveys, we focus
on the categorization of security incidents based on their origins and using
the proposed architecture we present existing prevention and mitigation
techniques. The scope of our work mainly covers aspects related to the
decentralized nature of blockchains, while we mention common operational
security issues and countermeasures only tangentially.
