(As of December 25th, 2015) Everyone who uses the online video game store Steam, do not log in! Steam is having some massive caching issues, and logging in could potentially leak some of your private information tied to your account. Please refrain from even launching your games while online.

Valve has not yet commented on the matter. but I will try and keep you updated. Read more here.

Update: Valve has responded to some websites, claiming the error was due to a configuration update. Though some personal info could have been leaked, everything should be fine now.
Sorry for posting this on both the news and the games page, but it seemed important. Having your personal information revealed to strangers is a real problem.
Sources: Gameinformer and Gamespot