presenting a session id to the server is causing session fixation

Consider my code below:

<?php
session_start();

if(!isset($_SESSION['count'])) $_SESSION['count']=0;
else ++$_SESSION['count'];

echo $_SESSION['count'];
?>

when i call it up on my browser http://localhost/user_login.php?PHPSESSID=1234

when i press reload a few times I see that my counter its increasing, however when i type

http://localhost/user_login.php?PHPSESSID=5678

and reaload a few times I see that it count up again from 0.

When i leave the counter on a different number than the first url and then go back to the first url i see that the number changes back again!! It seems that I have created two different sessions and I could even create more this way !!! Is there any way to prevent from happeneing ?

Solution

This approach it's very dangerous the attacker would be able to take over any sessions that has not been deleted or expired.

To prevent this, add a simple check to change the session ID using session_regenerate_id.

This function keeps all current session variable values, but replaces the session ID with a new one that an attacker can not know.

To do this, check for a special session variable that you arbitrarily invent. If it doesnt exists, you know that this is a new session, so you simply change the session ID and set the special session variable to note the change

Code:

    <?php
session_start();

if(!isset($_SESSION['initiated'])){
    session_regenerate_id();
    $_SESSION['initiated']=1;
}
if(!isset($_SESSION['count'])) $_SESSION['count']=0;
else ++$_SESSION['count'];

echo $_SESSION['count'];
?>

If you want to be ultra paranoid you can even regenerate the session ID on each request.