SteamDB currently reports heavy caching-Problems at Steam.
This leads to some information disclosure, like users seeing profiles of other users (incl. all games and personal data).

There is currently only one solution: DO NOT USE STEAM, and don't try to change personal data (adress, paying informations aso)!

UPDATE: according to SteamDB the problems have been fixed. Here is a article from SteamDB about the latest findings.

More links und information:

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

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

Don't do anything. Do not visit any Steam Store url. This include using store in the Steam client.

By the way, this is not a security breach. This is page caching gone rogue. Most likely not respecting Cache-Control headers.

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