After detecting block reorganization history on the Ethereum Classic chain, Coinbase has halted all ETC transactions.
An in-house analyst noticed an unusual hashrate from a single mining pool, causing mass reorganizations of mined blocks. According to a blog post on the Coinbase website from late January 7, 2019, the exchange found 88,500 ETC had been double spent (approx. $460,000 USD). Block explorers Bitfly and Blockscout indicated more than 100 blocks had been found to be reorganized during what appears to be a possible 51% attack.
The ETC team has confirmed the issue by tweeting that they are working with a Chinese security firm, SlowMist, to do investigations. Also, ETC advisor Cody Burns, suggested in an email to Coindesk that the best course of action for exchange and business is to wait for over 400 confirmations. Kraken and Bitfly are following this advice whereas Poloniex has disabled all ETC wallets until further notice.
At the time of writing, the cause of the reorganization has not yet been resolved.