Nothing is secure in today's digital world. You can implement the most powerful security that money can buy, and somebody will still find a way around it. Blizzard probably thought they had some of the best security in the world, but hackers made short work of it this week.
Blizzard's Mike Morhaime announced late last night that Blizzard and it's Battle.net servers were hacked. Thankfully, no financial information seems to have been stolen in the attack. The hackers did manage to make it out with some info that might lead to compromised accounts.
Morhaime says that the hackers were able to extract email addresses for global Battle.net users. Outside of the email addresses, it seems that North American players were specifically targeted as their security question answers, mobile authenticators and cryptographically scrambled passwords were also taken. Morhaime notes that the passwords are extremely tough to crack due to their encryption, but you should still change your password nonetheless.
As Blizzard works through this mess, they will be sending out notifications to players over the coming week to change their security questions. They will also update the mobile authenticator software so that it can't be accessed by would be hackers.
As Morhaime suggested, you're going to want to change your password. While the change of your account getting hacked is low, the chance is still there. It's always better to be safe than be sorry over your account getting billed for purchases you didn't make.
Like with all other hack attacks, Blizzard is now working with the authorities to find out who was behind it. They may never find out who did it, but we'll keep a look out for any groups that claim responsibility.