A leading South Korean cryptocurrency exchange has lost virtual currencies valued at $31 million in a series of hacks. Bithumb, the country’s second-largest exchange, said cyberattacks overnight Wednesday led to the loss of 35 billion won worth of cryptocurrencies. Trading had been suspended and the company promised it would compensate for the loss. This month, a smaller exchange in South Korea called Coinrail was hacked and reportedly lost coins valued at $37 million, leading the value of vitcoin to tumble by 7 percent. Bloomberg News put the wider crypto losses in market value at more than $42 billion. The latest hack highlights the security risks and weak regulation of global crypto markets.