Update: Valve released the following statement to Gamespot. It seems like it's safe to use Steam again, now.

　　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.

　　Original story: If you opened Steam or visited the store on a browser, you probably noticed there's something very wrong about it.

　　You may end up getting a store page in a completely different language, but that's the very least of the current issues. In fact, it seems like there is a caching issue allowing some users to access others' information such as email address, phone number and even credit card.

　　Here's what Steam Spy and SteamDB, the leading sources of information on the most popular online games store, found out.

　　I can confirm that: Steam gave me access to another person's account with credit card info and purchase history pic.twitter.com/IzhE4M5sme

　　— Steam Spy (@Steam_Spy) December 25, 2015

　　Looks like it is a cache problem, so you can't change anything on another user's profile. Accounts with Steam Guard are also affected.

　　— Steam Spy (@Steam_Spy) December 25, 2015

　　Valve is having caching issues allowing users to view things such as account information of other users. Don't use Store for now.

　　— Steam Database (@SteamDB) December 25, 2015

　　.@ZylusDota The best way to protect yourself is to completely avoid Steam websites for now.

　　— Steam Database (@SteamDB) December 25, 2015

　　They also stressed that this is not technically a security breach, though it's still quite the inconvenience for those who get their account information viewed by someone else.

　　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

　　The recommendation is clearly not to use Steam in any way until the issue is resolved.

　　To repeat, do NOT visit any Steam Store links. Doesn't matter what you want to do, do not visit any of them.

　　— Steam Database (@SteamDB) December 25, 2015

　　It is also recommended not to unlink PayPal via Steam right now, though it's fine to do it via PayPal.com.

　　Do NOT attempt to unlink PayPal, remove your credit card details or anything else. Doing so will put you at risk instead.

　　— Steam Database (@SteamDB) December 25, 2015

　　@SteamDB It is fine to unlink Steam using https://t.co/zlBegj03Gp itself. Just don't do it via the Steam store.

　　— Steam Database (@SteamDB) December 25, 2015

　　Obviously, there is a ton of people who would like to buy games today. Still, you really are better off waiting until Valve sorts this out, just in case.