Once hailed as unhackable, blockchains are now getting hacked

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More and more security holes are appearing in cryptocurrency and smart contract platforms, and some are fundamental to the way they were built.

Early last month, the security team at Coinbase noticed something strange going on in Ethereum Classic, one of the cryptocurrencies people can buy and sell using Coinbase’s popular exchange platform. Its blockchain, the history of all its transactions, was under attack.

An attacker had somehow gained control of more than half of the network’s computing power and was using it to rewrite the transaction history. That made it possible to spend the same cryptocurrency more than once—known as “double spends.” The attacker was spotted pulling this off to the tune of $1.1 million. Coinbase claims that no currency was actually stolen from any of its accounts. But a second popular exchange, Gate.io, has admitted it wasn’t so lucky, losing around $200,000 to the attacker (who, strangely, returned half of it days later).

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