Smash-and-grab. Berliner Volksbank via Getty Images One of the largest bitcoin exchanges in the world has been hacked, and 30,000 customers' data has been compromised, according to a report from Yonhap News.

Bithumb is based in South Korea, and the country's watchdog Internet and Security Agency is investigating after some customers said they lost money in the attack.

Bithumb did not immediately respond to Business Insider's request for comment, though it says users' passwords were not stolen, according to Yonhap News.

It's one of the busiest exchanges in the world. On Tuesday, Motherboard reported it was the fourth-largest globally in terms of volumes of all cryptocurrencies traded daily; by Wednesday, it had jumped to first place.

In a statement, Bithumb said (via the cryptocurrency news site BraveNewCoin) that an employee computer was compromised, rather than its core servers:

"The employee PC, not the head office server, was hacked. Personal information such as mobile phone and email address of some users were leaked. However, some customers were found to have been stolen from because of the disposable password used in electronic financial transactions."

BraveNewCoin reports "billions" of won has been stolen. For context, 1 billion won is about $870,000, or £670,000. As such, it would be markedly smaller than some previous hacks of bitcoin exchanges, such as Mt. Gox, where $460 million in bitcoin (at then current prices) disappeared in 2014.

Some Bithumb users were victims of "voice phishing," where someone phoned them up saying they worked for Bithumb and scammed them out of funds, according to BraveNewCoin.

The incident underscores the fact that it is not just investors and digital-currency enthusiasts who are excited by the current surge of interest in cryptocurrencies — criminals are also eyeing up businesses that hold bitcoin and its sister currencies, and no one can promise absolute security.

There has been a wild surge of interest in bitcoin, Ethereum, and other digital currencies over the past month, with prices surging to all-time highs. Bitcoin is currently worth about $2,600 per coin, with a total global market cap of $43 billion, while Ethereum trades at about $260 with a market cap of $24 billion, according to data from CoinDesk.