Abstract: The blockchain satisfies not only the cryptographic aspect of tamper resistance, but also the network aspect of high availability, and furthermore, it can directly handle money, which is called a cryptoasset. On the other hand, it has been observed that attacks have actually been carried out using the properties of tamper resistance and high availability. Because of the direct handling of money, attacks on blockchains are very active. In this talk, we present recent researches on security risks related to blockchain and cryptoassets, including blockchain poisoning attacks, unauthorized access to cryptoasset nodes, and the risk of worthless cryptoassets.