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Comment by bazoom42 | Hacker News Reader

Example: I post “fungame.com” on Show HN, you visit it, and in the background the JavaScript calls Facebook on your behalf (using your Facebook authentication cookie) and adds me as friend.

By default such cross-domain requests from JavaScript are disallowed, but CORS allows it if the server specifically opt-in. But the check happen in the browser, since the purpose is to protect the user of the browser.

There are some weird exceptions to this, for example a client can always GET and POST data to another domain under certain constraints, since this have always been possible using HTML forms. So it is not obvious what is possible and what isnt.

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> Example: I post “fungame.com” on Show HN, you visit it, and in the background the JavaScript calls Facebook on your behalf (using your Facebook authentication cookie) and adds me as friend.

Isn't that what CSRF protections are for, not CORS? There are other (very old) ways to trick a user into doing a POST that wouldn't be blocked by CORS -- and as you say, GET and some POST requests can always be sent but you don't see the response.

My understanding is that the actual protection that it gives in this scenario is that the "fungame.com" JavaScript cannot read your friends list or your list of private messages (basically, blocking GET data that should not be shared to random sites, as it would violate user privacy). You still need CSRF protections regardless of CORS.

Yes, the original CSRF attack using a plain html form does not even require JavaScript. CORS does not address this scenario.

But cross-domain post is only allowed if the payload is form data encoded. A Json payload from JavaScript would be blocked by default, as would other methods beyond get and post. Therefore you usually don’t have to worry about CSRF for a JavaScript API.

CORS is a a way to enable cross-domain calls from JavaScript without introducing the CSRF issue.

> Isn't that what CSRF protections are for, not CORS?

Without the same origin policy CSRF protections would be trivial to circumvent, since you’d be able to read the CSRF token from any page.

Sure, but that falls under the "no unauthorised GET data" thing I talked about...?

Is there a reason this has to happen client side with extra pre-flight requests? Taking your example, why couldn't Facebook's server just check the origin header and then reject all request from unapproved origins server side instead?

It is the difference between opt-in and opt-out.

Servers certainly can (and probably should) check request origin. But it is not something they usually do, since cross-domain requests from JavaScript wasn’t possible before CORS.

If support for cross-domain request were introduced in browsers without requiring opt-in from servers, most sites would not be prepared against this new risk. It would open massive security issues across the web.

It’s mostly for backwards compatibility. Sites don’t always check the request origin, the browser SOP mitigates that problem on behalf of the user.

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