Question

I know about all the issues with session fixation and hijacking. My question is really basic: I want to create an authentication system with PHP. For that, after the login, I would just store the user id in the session.

But: I’ve seen some people do weird things like generating a GUID for each user and session and storing that instead of just the user id in the session. Why?

The content of a session cannot be obtained by a client – or can it?

Answer 1

The short answer is that $_SESSION is safe and you do not need to worry about its contents being leaked to a user or attacker.

The content of the session is not normally be accessible to the user. You should be able to store the user’s primary key and you’ll be fine. There are cases where the session can be leaked, on a normal linux system the session folder is in /tmp, however this could be changed in your php.ini to the web root (/var/www/tmp) and then could be accessible. The only other way is if the user is able to get access to the $_SESSION super global by hijacking a call to eval() or by the variable being printed normally.

If you are running on a shared host and using an old version of PHP and/or your server is misconfigured it might be possible for another user on this system to read or even modify a session file stored in /tmp/. I don’t know of a single application that takes this attack into consideration. If this is a problem you can store the information in a session table in the database.