The Sydney Morning Herald.
The newspaper's website says that Neowiz, Mgame, Nexon, and Trion are among the companies affected. Moscow-based security firm Kaspersky Lab is conducting an ongoing investigation into the hacking ring's activities, but has not been given full access to infected servers. Some game companies reported the presence of malicious software which suggests that the hackers manipulated virtual currencies, according to the Herald.
The group, named Winnti by Kaspersky, began its current campaign in 2009 and is still active today. The Herald reports that Neowiz "did not respond to requests for comment, while Trion and Nexon declined to comment. Mgame said it had no immediate comment."