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Comment by nbpoole | Hacker News Reader

No it does not.

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https://developer.mozilla.org/en/http_access_control#Simple_...

A simple cross-site request is one that:

- Only uses GET or POST. If POST is used to send data to the server, the Content-Type of the data sent to the server with the HTTP POST request is one of application/x-www-form-urlencoded, multipart/form-data, or text/plain.

- Does not set custom headers with the HTTP Request (such as X-Modified, etc.)

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This is actually a big deal, since it means you can send a cross-domain mutlipart-POST with no preflight. That allows for an effective CSRF attack against file upload systems.

And of course, cross-domain POST requests via <form> tags have always worked and will continue to work.

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Am I missing something here?

Let's say you're logged into Gmail and Gmail had no CSRF protection anywhere.

You're logged in while visiting my site. In my site, I include a little bit of JavaScript to make a POST request to Gmail telling it to forward copies of all your incoming email to my email address.

This will not work even without CSRF protection. It would only work if Google sends back the header Access-Control-Allow-Origin: mysite or Access-Control-Allow-Origin: * as noted in the section you linked to.

Of course, I could also try to trick you into filling out a form whose method actually is pointed at Gmail's and include all the hidden input tags to set you up for forwarding emails to me, but you would know something fishy is going on because it would redirect you to Gmail.

"This will not work even without CSRF protection."

It actually will work.

What you're describing is what's known as a "simple" request in XMLHttpRequest terms. That means there is no pre-flight necessary. Your browser will simply make the POST as requested and receive the response. It won't make the response available to you since the Access-Control-Allow-Origin header isn't set, but you're a malicious attacker in this example and you don't care what the response is: you just care that you were able to make the request. ;-)

You could even do this by creating an HTML form that POSTs to the right URL and using JavaScript to submit it automatically when the page loads. Same exact thing: no CORS checks.

If a pre-flight were necessary you would be right. The browser would send an OPTIONS request to the server, the server would respond without the appropriate headers, and the POST request would never be sent.

Let me know if any of this needs further explanation!

Oh, I see now. I had assumed that because I couldn't get the response, that the request itself was blocked.

Thanks!

I would like to know one thing.

Who the hell thought it was a good idea to allow crossdomain XmlHttpRequests? Given that the standard say that post is for modification no other website should ever make thoes requsts.

The CORS standard for 'simple' POSTs is no different than what you can already submit via a form from a technological perspective. In that way, it actually makes a lot of sense.

And the whole point of CORS is that some websites do want to make those requests. ;-)

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