The Ethereum Classic (ETC) blockchain has experienced a second 51% attack in a week.
Bitfly, the parent company of Ethermine mining pool, reported the attack on Thursday morning. Binance also confirmed the reorganization of more than 4,000 blocks on ETC at block number 10935622. As a result, both Bitfly and Binance have halted ETC payouts, withdrawals and deposits.
A 51% attack or reorganization of a blockchain occurs when a group of miners controls more than 50% of the network's hash rate or computing power. This allows them to prevent new transactions from gaining confirmations, halt payments between users, as well as reverse transactions that were completed, meaning they could double-spend coins.
Today's attack is the second 51% attack on ETC in just five days, followed by the one that occurred on August 1. In that attack, 807,260 ETC (worth about $5.6 million) was double spent, according to blockchain data company Bitquery. The value of today's attack is not immediately apparent. The Block will update this story as it develops.
Vitalik Buterin, co-founder of Ethereum, today tweeted that ETC should move to a proof-of-stake (PoS) consensus algorithm from the current proof-of-work (PoW) mechanism. "Even given its risk-averse culture, at this point making the jump seems lower-risk than not making it," said Buterin.