The Ethereum Classic (ETC) blockchain has suffered a second 51% in the last week, with the attackers double-spending well over 800,000 ETC in the first one.
The most recent 51% attack was spotted by Ethermine’s parent firm Bitfly, which quickly warned the cryptocurrency community of the incident and advised miners that until further notice Ethereum Classic payouts would be disabled.
Shortly after, cryptocurrency exchange Binance confirmed the attack occurred and halted ETC deposits and withdrawals.
The 51% attack saw the malicious miner(s) reorganize over 4,000 blocks on the Ethereum Classic blockchain and occurred at block height 10,935,622. In a tweet, the developers behind the cryptocurrency advised cryptocurrency exchanges and mining pools to “significantly raise confirmation times on all deposits and incoming transactions,” over the “recent network attacks.”
A chain reorganization occurs when an entity gains more hashing power than the rest of the network – 51% of the total hashrate – and uses it to rewrite the blockchain’s history and double-spend its cryptocurrency, which in this case is ETC.
The co-founder of Ethereum, Vitalik Buterin, responded in a tweet to the recent 51% attack by suggesting that ETC could move to a Proof-of-Stake (PoS) consensus algorithm from its current Proof-of-Work (PoW) one.
Between July 29 and August 1 Ethereum Classic suffered another 51% attack, hat according to blockchain data firm Bitquery saw the malicious miners double spend 807,260 ETC tokens, worth over $5.5 million, thanks to a reorganization of over 3,000 blocks.
Last year, the cryptocurrency, which came to be after the Ethereum blockchain split over a controversial decision following the Decentralized Autonomous Organization (DAO) hack, was also hit with a 51% attack. At the time, it was estimated $460,000 worth of ETC were double-spent from cryptocurrency exchanges, with YoBit and Gate.io being the main targets.