It's Christmas Day and while people were watching Xbox Live and PlayStation Network for any possible issues, it appears that the biggest problem today is with Steam. It's coming in the form of a bug and quite a nasty one.
It's been reported on Reddit, as well as from our own Chatty community, that attempting to log into Steam and access account information will cause the account information for a different user to pop up. This is allowing people to see other customers' account information, buying history, Wallets, and even credit card numbers (albeit blocked out).
An official cause has not been issued, but speculation (including from the diligent Valve followers at Steam Database) is pointing to a caching issue gone horribly wrong.
By the way, this is not a security breach. This is page caching gone rogue. Most likely not respecting Cache-Control headers.— Steam Database (@SteamDB) December 25, 2015
Logins have been disabled for the time being, but the damage is already being done. Shacknews is reaching out to Valve for comment and any additional instructions, but in the meantime, users are advised to brace for the worst. Those that have not visited Steam today, do NOT visit Steam until this is all sorted, or else you could be caught in the caching web.
Update (2:33PM PT): There is more speculation coming in from the community-run, unofficial site, Steam Database, offering further insight into what might have happened, in regards to the rogue caching issue. More importantly, the site is offering sound advice on unlinking your PayPal information from your Steam account, if absolutely necessary. There is still no official word from Valve at this time, but Shacknews will continue monitoring this situation.
Update (3:40PM PT): There is a sense of cautious optimism that the issue has been resolved, with no issues indicated on the unofficial Steam Database. However, there is still no official statement from Valve in regards to this issue, so be careful out there.
Update (5:33PM PT): Valve has issued the following statement to Shacknews:
Steam is back up and running without any known issues. As a result of a configuration change earlier today, a caching issue allowed some users to randomly see pages generated for other users for a period of less than an hour. This issue has since been resolved. We believe no unauthorized actions were allowed on accounts beyond the viewing of cached page information and no additional action is required by users.