Sources including Coinbase say that top 20 cryptocurrency (by market cap) Ethereum Classic (ETC) was hit by a 51% attack on 5th January 2019.
51% attacks occur when a bad actor gains enough hash power to control 51% of a particular blockchain network. At that point, they can maliciously amend or delete previously-mined blocks and divert funds as they wish.
A major strength of the Bitcoin network is the 44 million terrahash per second it would take to mount a 51% attack. For comparison, it would only take 8 terrahash per second to successfully attack the ETC network. (Figures at time of publication according to https://www.crypto51.app/ ) Or to use a better comparison, according to Litecoin’s Charlie Lee, “ETC has less than 5% of the total Ethash hashrate and is 98% NiceHash-able. 1-hr attack costs $5k”.
Initially, ETC denied the attack completely, saying on twitter: “There have been rumors of a possible chain reorganization or double spend attack. From what we can tell the ETC network is operating normally.”
However, as events unfolded ETC sprung into action, asking exchanges and pools to “significantly increase block confirmation times” in order to mitigate the attacks.
Coinbase, on the other hand was quick to announce in a blog that they had “detected a deep chain reorganization of the Ethereum Classic blockchain that included a double spend. In order to protect customer funds, we immediately paused interactions with the ETC blockchain”.
Coinbase say they first detected the attack on 5th Jan, much to the annoyance of ETC who claimed Coinbase “unfortunately did not connect with ETC personnel regarding the attack”. ETC began addressing the alleged attack via social media on the afternoon of January 6th.
As the debate rumbled on, Coinbase updated their blog on 7th January after more double-spends were found. The attack appeared worse than originally feared; “Subsequent to this event, we detected 12 additional reorganizations that included double spends, totaling 219,500 ETC (~$1.1M)” They said.
At the time of publishing, ETC was still investigating the activity without confirming the validity of the 51% attack claims, hinting in a deleted tweet that the problem could instead have been caused by “selfish mining”.
“To be clear we are making no attempt to hide or downplay recent events. Facts are facts and as the situation develops we’ll soon get a full picture of what actually took place. Linzhi is testing ASICS [Miners]. Coinbase reported double spends; both may be true. In time we will see”. They said, In their last significant tweet on the subject on January 7th.