  As the most popular blockchain that supports smart contracts, there are
already more than 296 thousand kinds of cryptocurrencies built on Ethereum.
However, not all cryptocurrencies can be controlled by users. For example, some
money is permanently locked in wallets' accounts due to attacks. In this paper,
we conduct the first systematic investigation on locked cryptocurrencies in
Ethereum. In particular, we define three categories of accounts with locked
cryptocurrencies and develop a novel tool named CLUE to discover them. Results
show that there are more than 216 million dollars value of cryptocurrencies
locked in Ethereum. We also analyze the reasons (i.e., attacks/behaviors) why
cryptocurrencies are locked. Because the locked cryptocurrencies can never be
controlled by users, avoid interacting with the accounts discovered by CLUE and
repeating the same mistakes again can help users to save money.
