Caching Pages that Contain CSRF Protected Forms
Caching Pages that Contain CSRF Protected Forms¶
CSRF tokens are meant to be different for every user. This is why you need to be cautious if you try to cache pages with forms including them.
For more information about how CSRF protection works in Symfony, please check CSRF Protection.
Why Caching Pages with a CSRF token is Problematic¶
Typically, each user is assigned a unique CSRF token, which is stored in the session for validation. This means that if you do cache a page with a form containing a CSRF token, you’ll cache the CSRF token of the first user only. When a user submits the form, the token won’t match the token stored in the session and all users (except for the first) will fail CSRF validation when submitting the form.
In fact, many reverse proxies (like Varnish) will refuse to cache a page with a CSRF token. This is because a cookie is sent in order to preserve the PHP session open and Varnish’s default behavior is to not cache HTTP requests with cookies.
How to Cache Most of the Page and still be able to Use CSRF Protection¶
To cache a page that contains a CSRF token, you can use more advanced caching techniques like ESI fragments, where you cache the full page and embedding the form inside an ESI tag with no cache at all.
Another option would be to load the form via an uncached AJAX request, but cache the rest of the HTML response.
Or you can even load just the CSRF token with an AJAX request and replace the form field value with it.
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.