While the team was able to quickly close off access and has already contacted law enforcement and security expert agencies for assistance, the company states that some information was compromised during the breach.
At this time, the company hasn't found any evidence that sensitive financial information such as addresses, names or credit cards have been compromised, but the investigation is ongoing.
As for what was compromised, the company states:
Some data was illegally accessed, including a list of email addresses for global Battle.net users, outside of China. For players on North American servers (which generally includes players from North America, Latin America, Australia, New Zealand, and Southeast Asia) the answer to the personal security question, and information relating to Mobile and Dial-In Authenticators were also accessed. Based on what we currently know, this information alone is NOT enough for anyone to gain access to Battle.net accounts.
In addition to the emails and secret question answers, Blizzard states that cryptographically scrambled Battle.net passwords were also taken. But the company says Battle.net passwords feature an additional layer of security using Secure Remote Password protocol (SRP), making it difficult for hackers to extract the actual passwords without having to decipher each password individually.
Regardless, it is still suggested that players on North American servers change their passwords on Battle.net and any other accounts with the same password. As the investigation continues, Blizzard will prompt its North American players to change their secret questions and answers through an automated process.