Late last night on Twitter, the official Ethereum Classic team noted that possible chain reorganization and double spend attacks are ongoing. They subsequently asked crypto exchanges and ETC mining pools to require as many as 400 confirmations for withdrawals and deposits.
Chain Reorganizations and Double-Spends Afoot?
A chain reorganization is when a client in most blockchain designs discovers a longer chain which disregards blocks it had previously processed. “The longest chain wins” is one of the rules of decentralized networks. “Longest chain” is defined by various metrics including difficulty (the most miners were mining this version).
51% attacks are based on the concept of the longest chain. If a miner has enough hash power, they can essentially rewrite a blockchain to suit their needs. They can pay for goods, services, or even other cryptocurrencies and then rewrite the history of the blockchain so that these transactions never actually happened.