Ethereum Classic (ETC) has been struck with some kind of block reorganization attack, likely either a 51% or a selfish mining attack.
The Coinbase team released a revealing post on the state of Ethereum Classic, making the claim that the coin had undergone a “deep chain reorganization.”
Coinbase said it had detected a so-called "chain reorganization" in which nearly $500,000 worth of ethereum classic was spent twice.
The twitter post expanded on the issue, saying;
“On 1/5/2019, Coinbase detected a deep chain reorganization of the Ethereum Classic blockchain that included a double spend.”
According to Coinbase, beginning on the 5th they began to see multiple, deep reorganizations to the Ethereum Classic blockchain. So far a total of 88,500 ETC has been double spent under the attack and seeing this is what prompted Coinbase to halt all activity with the coin.
Coinbase went on to state that, in the interest of protecting its userbase, they responded to the suspicious activity by suspending movement of affected funds on the ETC blockchain. The specifics of the post, according to Coinbase security engineer Mark Nesbitt, includes a flaw in ETC’s Proof of Work system which allowed for “repeated” block “reorgs”–or an individual or group of attackers to alter blockchain transactions repeatedly.
Despite the response by Ethereum Classic development team, Coinbase has not resumed withdrawal and deposits on ETC, instead choosing to monitor the coin for activity going forward.