From the article:
Joe Stewart, director of malware research at SecureWorks, said the hacker targeted firms that hosted servers generating virtual currencies such as Bitcoin
Can an expert explain, how this hack really worked? The technical information in this article is useless. As I understand the Bitcoin protocol the block reward is just a transfer like any other but with no input. Since only the block reward was stolen, it can't be a weakness of the protocol. The hacker must have gained access to the private keys of the miner. But why would a miner store the reward in a hot wallet with the private keys revealed and not notice the theft over a period of 4 months?