Is exposing a session's CSRF-protection token safe?

Question

Django comes with CSRF protection middleware, which generates a unique per-session token for use in forms. It scans all incoming POST requests for the correct token, and rejects the request if the token is missing or invalid.

I'd like to use AJAX for some POST requests, but said requests don't have the CSRF token availabnle. The pages have no <form> elements to hook into and I'd rather not muddy up the markup inserting the token as a hidden value. I figure a good way to do this is to expose a vew like /get-csrf-token/ to return the user's token, relying on browser's cross-site scripting rules to prevent hostile sites from requesting it.

Is this a good idea? Are there better ways to protect against CSRF attacks while still allowing AJAX requests?

Answer 1

UPDATE: The below was true, and should be true if all browsers and plugins were properly implemented. Unfortunately, we now know that they aren't, and that certain combinations of browser plugins and redirects can allow an attacker to provide arbitrary headers on a cross-domain request. Unfortunately, this means that even AJAX requests with the "X-Requested-With: XMLHttpRequest" header must now be CSRF-protected. As a result, Django no longer exempts Ajax requests from CSRF protection.

Original Answer

It's worth mentioning that protecting AJAX requests from CSRF is unnecessary, since browsers do not allow cross-site AJAX requests. In fact, the Django CSRF middleware now automatically exempts AJAX requests from CSRF token scanning.

This is only valid if you are actually checking the X-Requested-With header server-side for the "XMLHttpRequest" value (which Django does), and only exempting real AJAX requests from CSRF scanning.

Answer 2

If you know you're going to need the CSRF token for AJAX requests, you can always embed it in the HTML somewhere; then you can find it through Javascript by traversing the DOM. This way, you'll still have access to the token, but you're not exposing it via an API.

To put it another way: do it through Django's templates -- not through the URL dispatcher. It's much more secure this way.