As seen on LeakedSource.com:
Dota2 official forums was hacked on July 10th, 2016 (http://dev.dota2.com). This data set contains 1,923,972 records. Each record contains an email address, ip address, username, user identifier, and one password.
Oh, look… the forum runs on vBulletin. Gee, where have we seen something like this before, I wonder….