phpBB 2.0.13 - user level exploit

[Some one <someone@xxxxxxxxxxxxxxxx>]

This one goes for all phpBB versions up to 2.0.13. While applying and testing 
the patch for the autologin bug I found that phpBB2 doesn't reset the 
$userdata['user_level'] variable after a failed autologin.

This is the vulvernable code in sessions.php:

        if ( $user_id != ANONYMOUS )
        {
                $auto_login_key = $userdata['user_password'];

                if ( $auto_create )
                {
                        if ( isset($sessiondata['autologinid']) && 
$userdata['user_active'] )
                        {
                                // We have to login automagically
                                if( $sessiondata['autologinid'] === 
$auto_login_key )
                                {
                                        // autologinid matches password
                                        $login = 1;
                                        $enable_autologin = 1;
                                }
                                else
                                {
                                        // No match; don't login, set as 
anonymous user
                                        $login = 0; 
                                        $enable_autologin = 0; 
                                        $user_id = $userdata['user_id'] = 
ANONYMOUS;
                                }
                        }
                        else
                        {
                                // Autologin is not set. Don't login, set as 
anonymous user
                                $login = 0;
                                $enable_autologin = 0;
                                $user_id = $userdata['user_id'] = ANONYMOUS;
                        }
                }
                else
                {
                        $login = 1;
                }
        }
        else
        {
                $login = 0;
                $enable_autologin = 0;
        }

As you can see, if autologin fails it will reset the $userdata['user_id'] value 
to ANONYMOUS, but $userdata['user_level'] stays at the value of the user 
account that failed to login.

Now phpBB only checks for the userlevel in various locations and ignores the 
user_id there. So if you manipulate the "_data" cookie to send the user_id of 
an admin you can see some information that should only be visible to an admin. 
Like hidden users on the "who is online page" or email adresses from users, 
even if the user disallowed that, in their profiles. Maybe you can even use 
some admin functions, I didn't check this in depth.

The cookie manipulation will only work on the first page requestet, as the 
session.php will then overwrite the user_id in it, unless you prevent the 
browser from modifying the cookie of course.

The fix is quite simple, add $userdata['user_level'] = USER; after every 
$userdata['user_id'] = ANONYMOUS; in session.php.