Session in classic ASP

You should implement a session timeout of your own. Both options mentioned by others (session.gc_maxlifetime and session.cookie_lifetime) are not reliable. I'll explain the reasons for that.

First:

session.gc_maxlifetime
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and cleaned up. Garbage collection occurs during session start.

But the garbage collector is only started with a probability of session.gc_probability divided by session.gc_divisor. And using the default values for those options (1 and 100 respectively), the chance is only at 1%.

Well, you could simply adjust these values so that the garbage collector is started more often. But when the garbage collector is started, it will check the validity for every registered session. And that is cost-intensive.

Furthermore, when using PHP's default session.save_handler files, the session data is stored in files in a path specified in session.save_path. With that session handler, the age of the session data is calculated on the file's last modification date and not the last access date:

Note: If you are using the default file-based session handler, your filesystem must keep track of access times (atime). Windows FAT does not so you will have to come up with another way to handle garbage collecting your session if you are stuck with a FAT filesystem or any other filesystem where atime tracking is not available. Since PHP 4.2.3 it has used mtime (modified date) instead of atime. So, you won't have problems with filesystems where atime tracking is not available.

So it additionally might occur that a session data file is deleted while the session itself is still considered as valid because the session data was not updated recently.

And second:

session.cookie_lifetime
session.cookie_lifetime specifies the lifetime of the cookie in seconds which is sent to the browser. […]

Yes, that's right. This only affects the cookie lifetime and the session itself may still be valid. But it's the server's task to invalidate a session, not the client. So this doesn't help anything. In fact, having session.cookie_lifetime set to 0 would make the session’s cookie a real session cookie that is only valid until the browser is closed.

Conclusion / best solution:

The best solution is to implement a session timeout of your own. Use a simple time stamp that denotes the time of the last activity (i.e. request) and update it with every request:

if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > 1800)) {
    // last request was more than 30 minutes ago
    session_unset();     // unset $_SESSION variable for the run-time 
    session_destroy();   // destroy session data in storage
}
$_SESSION['LAST_ACTIVITY'] = time(); // update last activity time stamp

Updating the session data with every request also changes the session file's modification date so that the session is not removed by the garbage collector prematurely.

You can also use an additional time stamp to regenerate the session ID periodically to avoid attacks on sessions like session fixation:

if (!isset($_SESSION['CREATED'])) {
    $_SESSION['CREATED'] = time();
} else if (time() - $_SESSION['CREATED'] > 1800) {
    // session started more than 30 minutes ago
    session_regenerate_id(true);    // change session ID for the current session and invalidate old session ID
    $_SESSION['CREATED'] = time();  // update creation time
}

Notes:

• session.gc_maxlifetime should be at least equal to the lifetime of this custom expiration handler (1800 in this example);
• if you want to expire the session after 30 minutes of activity instead of after 30 minutes since start, you'll also need to use setcookie with an expire of time()+60*30 to keep the session cookie active.

I too have been researching this question, and while none of the ideas below are complete solutions, they might help others rule out ideas, or provide further ones.

1) Simply remove the token from the client

Obviously this does nothing for server side security, but it does stop an attacker by removing the token from existence (ie. they would have to have stolen the token prior to logout).

2) Create a token blocklist

You could store the invalid tokens until their initial expiry date, and compare them against incoming requests. This seems to negate the reason for going fully token based in the first place though, as you would need to touch the database for every request. The storage size would likely be lower though, as you would only need to store tokens that were between logout & expiry time (this is a gut feeling, and is definitely dependent on context).

3) Just keep token expiry times short and rotate them often

If you keep the token expiry times at short enough intervals, and have the running client keep track and request updates when necessary, number 1 would effectively work as a complete logout system. The problem with this method, is that it makes it impossible to keep the user logged in between closes of the client code (depending on how long you make the expiry interval).

Contingency Plans

If there ever was an emergency, or a user token was compromised, one thing you could do is allow the user to change an underlying user lookup ID with their login credentials. This would render all associated tokens invalid, as the associated user would no longer be able to be found.

I also wanted to note that it is a good idea to include the last login date with the token, so that you are able to enforce a relogin after some distant period of time.

In terms of similarities/differences with regards to attacks using tokens, this post addresses the question: https://github.com/dentarg/blog/blob/master/_posts/2014-01-07-angularjs-authentication-with-cookies-vs-token.markdown