DMM , the cryptocurrency arm of Japanese Internet giant DMM.com, announced on Friday that more than $300 million in bitcoin had been stolen in a major security breach.

The exchange confirmed the incident but did not explain how the assets were stolen. DMM Bitcoin abruptly froze most of its services, including deposits, withdrawals, and trade of almost all cryptocurrencies.

“At approximately 1:26 p.m. on Friday, May 31, 2024, we detected an unauthorized leak of bitcoin from our wallet,” DMM Bitcoin stated. The announcement, originally in Japanese, was translated into English and posted on the company’s website.

The exchange reported that 4,502.9 bitcoin, worth roughly $306 million, had been taken in the breach. DMM Bitcoin assured its clients that their bitcoin deposits would be fully reimbursed. “Please rest assured that all of your bitcoin deposits will be fully guaranteed, as we will procure the equivalent amount of BTC that was leaked with support from our group companies,” the statement read.

In a blog post on its website, DMM Bitcoin reiterated that measures had been taken to prevent further unauthorized outflows. The exchange has restricted all spot purchases on the platform and warned that Japanese yen withdrawals might take longer than usual due to the incident.

In 2018, rival exchange Coincheck suffered one of the biggest hacks in the industry’s history, losing about $534 million worth of (XEM) from its hot wallet. At the time, the Monex Group’s subsidiary spent several weeks trying to identify the attackers and understand how the hack occurred. 

The Coincheck hack had extensive repercussions for both the company and the broader Japanese cryptocurrency industry. Moreover, its reputation was severely damaged due to the hack. Besides the financial losses, the company faced widespread criticism for its inadequate security measures and failure to safeguard user funds.