How to handle an expired CSRF token after a page is left open

I’m using CodeIgniter 2 along with the Ion Auth authorization system by Ben Edmunds.

After creating my project, I would sometimes get a CodeIgniter error upon certain login attempts but this error was intermittent.

The action you have requested is not allowed.

After some troubleshooting, it became apparent this error was caused by an invalid CSRF token. Why is the token invalid? Well, in CodeIgniter’s configuration file, it’s set to expire in 4 hours. So if you load your login page and allow it to sit there for 4 hours before attempting a login, the CSRF tokens will expire and this will generate the error message as above. Simply reloading the login page avoids any issues.

You can verify this error message for yourself by deleting the CSRF cookie after you load the login page.

A cleaner solution would be to redirect to a custom error page or to display a flash message. However, this solution is not as simple as it sounds because when you extend the CodeIgniter Security class, certain hook-points are not available and you cannot yet access CodeIgniter’s Super Object using get_instance().

So when you extend the Security class, you’re limited to standard PHP. In this case, I’m using PHP header() to redirect the offending login page (or any form page) back to itself.

This works fine except that the user gets a screen refresh without any indication why they have to enter their login credentials a seconds time.

I decided to make this a bit more user-friendly by adding another function into a Controller, in my case, the Ion Auth controller…

As you can see, this function sets a flash message telling the user what happened and then redirects them to a fresh instance of the login page.

Session cookie automatically reset due to expired browser session. Please try again.

Instead of using PHP header() to redirect a page refresh, redirect to this new Ion Auth controller function at /auth/csrf_redirect.

The minor downside to this method is that you are always redirected back to the login page rather than a refresh of whatever page/form you’re trying to submit. However, that should be a moot point, since the session cookie expires at nearly the same time as the CSRF cookie, you’d be redirected back to the same login page regardless. You may also not be requiring the user be logged in for your particular form, so please be aware and re-direct accordingly.

When CodeIgniter’s CSRF Protection breaks your Ajax

CSRF stands for “Cross Site Request Forgery” and if you’re using forms on your site, you’ll probably want to protect yourself and users against this kind of attack.

You just finished your latest PHP project using the CodeIgniter framework and decide to enable the CSRF protection option in your config.php file.

Enabling it within config.php is not enough. You also need to use the form helper form_open() function to construct the form’s HTML markup. This function constructs the form so that it contains a <input type="hidden"> element containing the CSRF token value. If the submitted form data is missing this token, it will not submit.

Now CSRF is working but you discover that your jQuery ajax requests are all suddenly failing with a type 500 server error. This is a direct result of activating the CSRF Protection option in CodeIgniter. As just explained, the submitted form data must contain the CSRF token, but it’s missing from your ajax requests.

The solution is simple. You need to make sure that your ajax requests simulate a regular form submission by including the CSRF token value within the submitted data.

There are two types of solutions:

Solution #1:

This only works if your ajax requests occur when a form is already constructed on the page, such as when doing remote validation to check to see if a password or username already exists.

You’ll need to copy the value from the hidden field called csrf_token (the name is exactly as per your $config['csrf_token_name'] option setting) and send this along with your ajax request.

Solution #2:

This works for all ajax requests, even when you do not have a form on the page, such as remotely loading some content.

In this case, you can’t get the CSRF token from a hidden field, since there is no form. You must retrieve it from the CSRF cookie. I’m using a jQuery cookie plugin.

Notice how the ajax in both solutions is sending the token with the same name, that’s your name as per your configuration, csrf_token. Only the source of the token value is different… Solution #1 gets the token value from the hidden field, where Solution #2 gets the same token value from the cookie.

You can only use Solution #1 when you have a form on the page constructed with the form_open() function. However, you can use Solution #2 with or without a form, in all cases.

NOTES:

I have CodeIgniter v2.1.4 and by default, the $config['csrf_token_name'] option is set to csrf_test_name. This mismatch might get a little confusing, but you can use whatever naming convention you wish. In my solution above, I changed it to csrf_token.

  • To retrieve the current token from the hidden input, use the name assigned to the $config['csrf_token_name'] option.
  • To retrieve the current token from the cookie, use the name assigned to the $config['csrf_cookie_name'] option.

No matter how you retrieve the token value, the important thing to remember is to always send the token value along with whatever name you’ve assigned to the $config['csrf_token_name'] option.