Prevent back button after logout

Question

I don\'t want the user to go back to secured pages by clicking back button after logging out. In my logout code, I am unsetting the sessions and redirecting to login page.Bu

Answer 1

Here's an easy and quick solution.

To the login form tag add target="_blank" which displays content in a different window. Then after logout simply close that window and the back button problem (Safari browser) is solved.

Even trying to use the history will not display the page and instead redirect to login page. This is fine for Safari browsers but for others such as Firefox the session_destroy(); takes care of it.

Answer 2

Note that although users can't change anything after resetting session data and/or cookie, they still may see usual information accessible to a logged in user as they appeared on the last visit. That is caused by browser caching the page.

You have to be sure to add the header on every page accessible by a logged in user, telling the browser that the data is sensitive and they should not cache the script result for the back button. It is important to add

header("Cache-Control: no-cache, must-revalidate");

Note that those other elements other than the immediate result of the script under this header, will still be cached and you can benefit from it. See that you gradually load parts of your page and tag sensitive data and the main HTML with this header.

As the answer suggests, unsetting the logged_in portion of $_SESSION global variable can achieve logging out, but be aware that first, you don't need to destroy session as mentioned in the PHP's session_destroy() documentation

Note: You do not have to call session_destroy() from usual code. Cleanup $_SESSION array rather than destroying session data.

And second, you better not to destroy the session at all as the next warning on the documentation explains.

Also, unset() is a lazy function; meaning that it won't apply the effect, until next use of the (part of the) variable in question. It is good practice to use assignment for immediate effect in sensitive cases, mostly global variables that may be used in concurrent requests. I suggest you use this instead:

$_SESSION['logged_in'] = null;

and let the garbage collector collects it, at the same time it is not valid as a logged in user.

Finally, to complete the solution, Here are some functions:

<?php
/*
 * Check the authenticity of the user
 */
function check_auth()
{
   if (empty($_SESSION['logged_in']))
   {
      header('Location: login.php');
      // Immediately exit and send response to the client and do not go furthur in whatever script it is part of.
      exit();
   }
}

/*
 * Logging the user out
 */
function logout()
{
   $_SESSION['logged_in'] = null;
   // empty($null_variable) is true but isset($null_variable) is also true so using unset too as a safeguard for further codes
   unset($_SESSION['logged_in']);
   // Note that the script continues running since it may be a part of an ajax request and the rest handled in the client side.
}

Answer 3

the pages on which you required loged in, use setInterval for every 1000 ms and check wheather user is logged in or not using ajax. if user session is invalid, redirect him to log in page.

Answer 4

I think your only server side option is to disallow caching. This is actually not that bad if you are using a Javascript heavy application as your main HTML might only be a series of JS calls and the Views are then generated on the fly. That way the bulk of the data (JS MVC and core code) is cached but the actual page request isn't.

To add to the comments pasted below I would suggest adding a small AJAX call during load time that fires even for cached pages that goes to your backend and checks the session. If not session is not found it would redirect the user away. This is clientside code and not a secure fix, sure, but looks nicer.

You could get this off your conscience with

A cheap fix if all else fails would be a "Please close this window for security reasons" message on the logged out page. – izb May 9 '12 at 8:36

But like N.B. said

You don't have to disable anything. If they go back, they're served the cached version of the restricted page. If they try to click around it, nothing will work because the appropriate session won't be set. – N.B. May 9 '12 at 7:50

Answer 5

Here's an easy solution which I have used in my application.

Add the below code inside script tag in the login HTML page (or whichever page it redirects to after logout)

<script>
    history.pushState(null, null, null);
    window.addEventListener('popstate', function () {
        history.pushState(null, null, null);
    });
</script>

It will disable the back button. You will not be able to go back by clicking on the back button.

Note: Not tested on Safari.

Answer 6

You could insert a condition/function on each restricted page, checking if the appropriate session variable is set or not. This way, you can print 2 versions of the page (one for the valid users, and one redirecting to the login page)