A security issue allowed some Steam users to randomly see pages generated for other users for a period of less than an hour. Several users online reported to be able to view other users account information. The Valve company that owns the Steam platform confirmed the serious security, it was an internal error that the company has already fixed. The incident was limited to a one-hour period. No official explanation, but one popular theory holds that Steam is incorrectly caching account pages and rendering them for others.”]
Source: https://securityaffairs.co/wordpress/43069/hacking/steam-security-issue.html