We can confirm that there was a successful 51% attack on the Ethereum Classic (#ETC) network with multiple 100+ block reorganization. We recommend all services to closely monitored the chain and significantly increase required confirmations.

— Bitfly (@etherchain_org) 2019年1月7日

Regarding the recent mining events. We may have an idea of where the hashrate came from.
ASIC manufacturer Linzhi confirmed testing of new 1,400/Mh ethash machines #projectLavaSnow
- Most likely selfish mining (Not 51% attack)
- Double spends not detected (Miner dumped bocks)

— Ethereum Classic (@eth_classic) 2019年1月7日