An apparent malfunction is hitting Steam right now, giving players across the world access to the wallets and buying history of other people's accounts. It's not yet clear how this happened.
Various players across the world are reporting logging into the Steam client and finding that their homepage has changed to Russian or other random languages. When they check the "account info" section of Steam, they find that they have access to another user's account, complete with email addresses, buying history, and other private information.
Based on some rudimentary testing I've done on my own Steam client, it seems like trying to view purchase histories and licenses will switch to other random accounts, too.
We've reached out to Valve for more information and will keep updating you guys as we learn more.