The running joke was that Xbox Live would suffer some hideous outage today, Christmas Day, but alas it's actually Steam that takes the crown this year. A major security issue is currently in play (no pun intended) whereby users are being given gratuitous access to accounts which are not their own and a smattering of Russian for good measure.
We've not personally investigated, because frankly, we're staying clear of this hot mess. However numerous outlets online are reporting the troubles. From PC Gamer:
"Something has gone very wrong, and the Steam Store is showing users the private information of other accounts, including partial credit card numbers, email addresses, balance and purchase history."
Furthermore it doesn't appear to matter if you have Steam Guard enabled or not, all accounts are seemingly affected. For now it seems the Steam Store is down, which is probably good, since you can't really use it, and the official support channels are being somewhat quiet on what's actually going on.
via PC Gamer