"Even when you are in the business of fun, not every week ends up being fun," Blizzard President Mike Morhaime wrote today as he told the world about a security breach that has compromised a significant amount of users' Battle.net account details.

The breach reportedly includes a list of valid Battle.net e-mail addresses, cryptographically hashed passwords, answers to personal security questions, and information about mobile and dial-in two-factor authentication. Billing information—including credit card numbers, addresses, and real names—appear to be safe at this point in the investigation, the company said. The attack affected Blizzard's North American servers, which are used by players in North America, Latin America, Australia, New Zealand and Southeast Asia.

The passwords collected are protected using the Secure Remote Password protocol, Blizzard says, making it extremely unlikely that they can be used to gain unauthorized account access. These "salted" passwords will have to be deciphered individually to be usable, a painstakingly slow process which makes the passwords much more secure than the "unsalted" passwords revealed in recent high-profile hacks for sites like eHarmony and LinkedIn. Those were protected by simple SHA-1 and MD5 cryptographic hashes, respectively.

Still, Blizzard is recommending that all users change their Battle.net passwords and any similar passwords on outside accounts. Users will also be prompted to update their security question/answer pairs and mobile authenticator software in the coming days (physical two-factor authenticators should still be secure, Blizzard said). In the meantime, customer service representatives have been told to use additional methods to verify player identities for those trying to recover their accounts.

Blizzard says it learned of the breach on August 4, and has been working non-stop to identify the culprits before revealing details of the attack late this evening. "Our first priority was to re-secure our network, and from there we worked simultaneously on the investigation and on informing our global player base," Morhaime wrote. "We wanted to strike a balance between speed and accuracy in our reporting and worked diligently to serve both equally important needs." The security hole that led to the security breach, which was not publicly identified, has since been patched, the company said.

"We take the security of your personal information very seriously, and we are truly sorry that this has happened," Morhaime concluded.

Ahead of the launch for Diablo III's real-money auction house, Blizzard began requiring two-factor authentication for users who wanted to trade in-game items for real-world cash.