Using the back button after logging out shows you pages from the authenticated user's session
You can implement hook_init() in any of your custom modules

&lt;?php 
/**
* Implements hook_init().
*/
function YOUR_MODULE_NAME_init() {
     drupal_add_http_header('Cache-Control', 'no-cache, no-store, max-age=0, must-revalidate, post-check=0, pre-check=0');
}
?&gt;