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

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.

Answer 2

Cancel that, I was wrong. (See comments.) You can prevent the exploit by ensuring your JSON follows the spec: Always make sure you return an object literal as the top-level object. (I can't guarantee there won't be further exploits. Imagine a browser providing access to the failed code in its window.onerror events!)

You can't rely on cross-site-scripting rules to keep AJAX responses private. For example, if you return the CSRF token as JSON, a malicious site could redefine the String or Array constructor and request the resource.

bigmattyh is correct: You need to embed the token somewhere in the markup. Alternatively, you could reject any POSTs that do have a referer that doesn't match. That way, only people with overzealous software firewalls will be vulnerable to CSRF.

Answer 3

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.