Ethereum Classic experiences 51% attack and 3,000 block reorg

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Ethereum Classic suffered a 51% attack, which saw a 3,000 block reorganization taking place at block height 10,904,146.

Today the #ETC chain experienced a chain reorg of 3693 blocks at block 10904146. This caused all state pruned nodes to stop syncing. It is likely caused by a 51% attack and all exchanges are advised to halt deposits & withdrawals immediately and investigate all recent tx. pic.twitter.com/lUNtifaBWT

— Bitfly (@etherchain_org) August 1, 2020

Blockchain firm Bitfly was the first to detect the reorg and encouraged all digital currency exchanges to halt ETC deposits and withdrawals immediately.

How it happened

Yaz Khoury, head developer relations at the Ethereum Classic Cooperative, wrote in his analysis that 2miner—one of the largest ETC mining pools—went offline for maintenance on August 1. At the time, a significant amount of hash dropped out of the Ethereum Classic network.

While the pool was offline, an unknown miner was able to add 3,000 blocks to the ETC blockchain, and by the time the 2miner pool went back online, the version of the ETC blockchain that experienced the 3,000 block reorg had the most PoW behind it. As a result, the reorg chain became the longest blockchain, and the chain that ETC miners are continuing to mine.

This isn’t the first time that Ethereum Classic experienced a 51% attack. In January 2019, ETC experienced a 100+ block reorg as a result of a 51% attack. 

Because of the currently ongoing potential 51% attack on #ETC we have temporarily increased block confirmations on our ETC mining pool. We will notify you once the network has stabilized again

— Bitfly (@etherchain_org) January 7, 2019

In addition to the 2019 block reorg and 51% attack, a double-spend attack took place in which the attacker was able to double spend 88,500 ETC worth approximately $460,000 at the time of the attack. 

Fool me once shame on you, fool me twice….

Considering that two block reorgs and 51% attacks have taken place on the ETC blockchain within the past two years, it is clear that Ethereum Classic has major issues. As you can see from the recent ETC attack, it does not take much for the ETC blockchain to be compromised; one major mining pool went offline and the whole ETC network was upended for over 15 hours.

That being said, supporting a blockchain like ETC comes with significant risks, to add insult to injury, nobody uses Ethereum Classic and there is virtually no development activity taking place on the Ethereum Classic network.