UPDATE 2: The Steam Store currently appears to be back up with no word yet from Valve. The unaffiliated Steam Database says the issue is "highly likely to be a misconfiguration in one of Valve’s caching layers." You can get more info there (opens in new tab) if you have any concerns.
UPDATE 1: Valve has shut down the Steam store completely.
Heads up if you have a Steam account. Valve's market place is currently behaving VERY strangely, granting people random access to accounts. Your best course of action right now is to do nothing though as trying to sign in could make it worse.
It doesn't seem to be a hack, instead some sort of caching issue that's presenting people with whatever the hell Steam seems to have to hand when they try to interact.
People trying to log in are currently able to see account details, last card numbers, emails and other private stuff.
So far no one seems to be reporting unauthorised purchases but obviously something has gone very wrong with the overall opinion being TURN IT OFF TURN IT ALL OFF. General advice at the moment seems to be to not interact with it all. We'll keep you updated.
Seen something newsworthy? Tell us!