An user can view info of another users

Hi! I' ve a seriuos problem: my users can see info of another users. How? I don't know, but I suspect that session variables in my server are the problem. How do I test it? 

Situation is next:

User A is logued in app with his credentials. All info (username, avatar, id, permission) are stored in session variables using codeigniter. Suddenly, he can see the avatar, username and info of another user of the application... what can I do!?

Please, help me! 

Thanks!