CSRF

CSRF (cross-site request forgery) are hacks where a user on one system is tricked into doing something on that system while browsing another system.

Example

Let’s say you are logged into http://yoursite.example.com/ as an admin, and you can easily delete an object by clicking a link that sends a request to http://yoursite.example.com/a/delete.php?object=1.

You take a break and go read some websites.

Now, let’s say that one of those websites has had a little piece of code attached

<img src="http://yoursite.example.com/a/delete.php?object=1" style="display:none"/>

Other readers of the same site will not notice anything – they’re not logged into your site, and so have no delete rights. But, you are!

This vulnerability is called “CSRF” because the hack happens on a different website than your own, taking advantage of the fact that you are logged in, to delete stuff (or move money, etc).

Solution

On the server, you should create a CSRF token, send it to the client, and make sure that all actions that are requested include that token.

To set the token, just create a random string and save it to your session:

<?php
if (!isset($_SESSION['csrf'])) {
  $_SESSION['csrf']=md5(mt_rand().time());
}

Then, whenever an action is performed, make sure that the request includes that token before the action is performed.

<?php
if (!((isset($_REQUEST['_csrf']) && $_REQUEST['_csrf']==$_SESSION['csrf'])
  || apache_request_headers()['X-CSRF']==$_SESSION['csrf'])
) {
  header('Content-type: text/json');
  echo json_encode(array(
    'error'=>'CSRF violation'
  ));
  exit;
}

Note that my code above allows two ways to send the CSRF – as a request variable (GET/PUT/POST), or as a header.

For HTML forms, make sure that each form includes the CSRF token:

<input name="_csrf" value="<?=$_SESSION['csrf'];?>"/>

And finally, for AJAX, make sure that the token is included by default. Personally I use jQuery, so this does that:

  $.ajaxSetup({
    'beforeSend': function(xhr) {
      xhr.setRequestHeader('X-CSRF', window.csrf);
    }
  });

(make sure that window.csrf is set as inline javascript in the page)

Conclusion

Now what happens is that each time a request is made to the server, the CSRF token that’s sent is checked against the session’s CSRF token, and if they don’t match (or no token is sent), then the action is ignored.

It is not possible for any website to guess your CSRF token (we set it to a random MD5), so you are safe.

About Kae Verens

owner of KV Sites, author of two books, father of two children.

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