Blizzard servers are generally very secure but even the most secure servers are vulnerable to DDOS attacks because of the nature of the attacks. And at the time of writing this article, Blizzard is facing such an attack which is why almost all the Blizzard game servers are down. There is speculation in the community that this attack was caused by members of a Private World of Warcraft server [Nostalrius] that Blizzard had closed down recently but the claim doesn’t have substantial evidence to back it up, other than the coincidental timing of both situations.
However, in this particular attack, email IDs of several Blizzard employees have been compromised as well and their personal information have been revealed over Twitter and other social media websites.
You can follow the current situation of what’s going on over at Blizzard by visiting the forums in the link right here. The Blizzard community managers have been very active in updating the community on what was happening over at their end which is more than appreciated.