Battle.net is currently being hit by a DDoS attack. The issue has been reported by thousands of users on social media, and has been confirmed on the official @BlizzardCS Twitter account which states the following:
The outage began shortly after 4:00PM PST and has lasted more than an hour. This has affected all of Blizzard's titles, ranging from Hearthstone to Overwatch, across all platforms.
DDoS attacks are malicious in nature and are the consequence of a massive flood of packets being sent to one or more servers. The servers are overwhelmed by the number of requests and are unable to distinguish between real traffic and that of the attackers without the use of an advanced form of security layer.
It is unclear how long the issues will persist. Blizzard has a long history of being able to thwart network issues in a reasonable amount of time, but severe DDoS attacks can result in hours of complications.
