Last week, I wrote about NiceHash in relation to its simplicity to get into mining Bitcoin. The NiceHash website itself still shows a maintenance page.
In a press release on Wednesday, NiceHash confirmed the breach but said that it was yet to work out the precise amount of Bitcoin stolen from customer wallets.
The mining service Nicehash was reportedly hacked, with over $60 million in funds stolen.
Several users pointed to a Bitcoin wallet address that was opened Tuesday at 9:07pm, which went from a balance of 0.01 bitcoin, valued just over $123, to a balance of 4,655.25 bitcoin, worth $57.7 million, after the user made more than 3,000 transactions around 9:17pm.
Update: NiceHash has officially announced they were indeed hacked, with all of the BTC in their wallet gone.
More than 980,000 bitcoins have been stolen, which today would be worth about $4 billion, Reuters found.
In addition to conducting an internal investigation, NiceHash said that they have reported the incident to "relevant authorities and law enforcement" and are "co-operating with them as a matter of urgency".
There have been at least three dozen heists of cryptocurrency exchanges since 2011; numerous hacked exchanges later shut down, most notably Mt. Gox, which was the world's largest bitcoin trading exchange when it went under in 2014.
"Clearly, this is a matter of deep concern and we are working hard to rectify the matter in the coming days", the announcement said. NiceHash says it will relaunch itself with improved security "at the earliest opportunity".
The trouble started on 6 December when users of NiceHash, a service which enables people to exchange computing resources known as hashing power to mine for Bitcoin and other cryptocurrencies, began reporting that they had been kicked offline while mining.