European Diablo III servers were taken down over the weekend for four hours, with gamers reporting "Error 33" login issues across the board. Blizzard hasn't spoken about the incident, but a significant number of accounts have been reported compromised in the aftermath.
Speculation runs rife that perhaps the downtime was prompted by hackers hijacking session identifiers, taking over accounts without notifying the Blizzard authentication servers. Alternatively, others theorise that it was perhaps an SQL injection attacking the game's servers.
Blizzard has kept mum on the topic, but a number of gamers are reporting items and gold missing from both inventory and stash. Those are the lucky ones - other accounts have been stolen completely, and re-sold through third parties.
Fortunately (or unfortunately), Blizzard has some experience with these things, and is rolling back hacked characters to a point before the accounts were stolen. This - predictably - loses some character progress, but should get all of your items back.
In fact - Blizzard has explained that this isn't exactly unexpected behaviour.
Historically, the release of a new game -- such as a World of Warcraft expansion -- will result in an increase in reports of individual account compromises, and that's exactly what we're seeing now with Diablo 3. We know how frustrating it can be to become the victim of account theft, and as always, we're dedicated to doing everything we can to help our players keep their Battle.net accounts safe -- and we appreciate everyone who's doing their part to help protect their accounts as well.
Reportedly, the accounts are compromised through the use of keyloggers, which is why the developer recommends following its security checklist, including a phishing filter, anti-virus software and using an official Blizzard authenticator.