Ethereum Classic Suffers Reorganization That Resembles 51% Attack Amid Miner Complications

Ethereum Classic suffered a 3,693-blockchain reorganization early Saturday morning, an event first thought to be a possible 51% attack, after a miner used old software after having been offline, according to Terry Culver, CEO of Ethereum Classic Labs.

• The reorganization caused all state-pruned nodes to stop syncing and is “likely caused by a 51% attack,” cryptocurrency miner Bitfly initially wrote on Twitter.
• In a later report, Ethereum Classic developers said the reorganization instead could have resulted from "the offending miner [having] lost access to internet access for a while when mining," a scenario later confirmed by Culver.
• While this is getting sorted out, "exchanges need to pause [ETC] deposits and withdrawals," said Hudson Jameson, developer at the Ethereum Foundation.
• Saturday’s reorganization lasted for roughly 15.4 hours worth of blocks, assuming the protocol's target 15-second block times. 
• A reorganization happens when two versions of a blockchain compete for validity from nodes in the network. Eventually, one chain of blocks will gain a majority of the mining hash power and "win," leaving the competing version to be "orphaned" or abandoned. In this case, an old version of ETC is vying to replace the most current version with itself.
• So far, the ETC market seems unaffected by the news, having gained more than 1% over the past 24 hours, according to Messari.

Update (August 1, 15:17 UTC): This article has been updated with an estimate of the attack cost.
Update (August 1, 16:28 UTC): This article has been further updated to reflect that the event was likely not a malicious act, but an accidental one in which an offline miner came back online using an old version of ETC. That event then mimicked a 51% attack.