DMM announced an unauthorized outflow of $300 million in Bitcoin. 

Some engineers investigated this issue and reported the details. Only one cold wallet was used by DMM, whereas other virtual currency exchanges have many. 

We can infer from this that the hacker could easily find the key since large amounts of money were transferred daily. 

The hacker used the same key, which starts with 1B6r and ends with 7P, to carry out the scam.  This incident was inevitable. As a result, DMM paid a substantial price for neglecting security.

DMM hasn't announced the details or the solution yet. I'm planning to open a new account with Rakuten Wallet.

https://www.google.com/amp/s/www.elliptic.co/blog/dmm-bitcoin-loses-308-million-in-unauthorized-leak%3fhs_amp=true