And we discovered the cause. Apparently there's a worm spreading on 4chan which steals your private keys and sends them to the attacker. We've studied this worm's behavior. The short story is we figured out how it propagates and we've given the worm a name. The worm only spreads to you when you reply to a post made from a computer/device which was infected. So you can avoid it by not responding to "infected posts". How to know which posts are infected? Well, you usually can't.
However, due to what's most likely a flaw in the worm's coding, something which can give the worm away is that it causes various combinations of letters written in a post to become corrupted in such a manner that they're changed after the post is published. Thanks to this, it's possible to occasionally see some of the infected posts. And also it's possible to test if you yourself are infected. READ THE PIC FOR MORE INFO ON THIS.
I'm not sure how we can put a stop to the worm, but spreading awareness of it certainly is a good start. So please save the pic and share it with fellow anons across the boards to spread awareness of the worm.
NOTE: don't reply directly to this post, because my computer might already be infected