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KEY POINTS Cryptocurrency exchange Coinbase says it suspended movement of ethereum classic after it detected a potential attack on the digital coin's underlying technology.

In essence, Coinbase is saying that a malicious party may have been able to wrest control of more than 50 percent of ethereum classic's network, and therefore was able to dominate decisions about what did and didn't belong on the digital coin's blockchain.

Such a "51 percent attack" can allow someone to falsify transactions and spend the same holding more than once, fundamentally damaging the usefulness of a digital payment system — and people's trust in it.

A major cryptocurrency exchange said Monday that it had suspended movement of widely known token ethereum classic after it detected a potential attack on the digital coin's underlying technology. The exchange and wallet firm, Coinbase, said it had detected a so-called "chain reorganization" in which nearly $500,000 worth of ethereum classic was spent twice — an anomaly that is extremely rare for cryptocurrencies, but represents one of the biggest threats to the technology's success. It could have been the result of what is known as a 51 percent attack, exposing a potential flaw in the cryptocurrency's underlying technology known as the blockchain. In essence, Coinbase is saying that a malicious party may have been able to wrest control of more than 50 percent of ethereum classic's network, and therefore was able to dominate decisions about what did and didn't belong on the digital coin's blockchain — the digital ledger that depends on a widely distributed consensus of users to record the asset's history.