Cross domain file uploads in CKEditor

In version 3.4 of the SimpleUploads plugin I've added support to upload files across domains based on the CORS spec.

If you don't use multiple domains (one server for the uploaded files and another for the editing page) then this change shouldn't affect you. Otherwise you can enable this feature by modifying your uploader to send just two headers in response to an OPTIONS request.
Some simple PHP code:

if (isset($_SERVER["HTTP_ORIGIN"])) {
 // You must verify that the origin domain is on your white-list
 header('Access-Control-Allow-Origin: https://admin.example.com');
 header('Access-Control-Allow-Credentials: true');
}
if ($_SERVER['REQUEST_METHOD']=='OPTIONS')
 exit(0);

• First: check if the browser has sent an Origin header. That means that it's a cross domain request. You can check that domain with the list of domains that you want to allow. The usual behavior is to send back a fixed origin header like
  

  header('Access-Control-Allow-Origin: https://admin.example.com');

• Second: Send an Access-Control-Allow-Credentials header specifying that the browser is allowed to make a request that will use the credentials of the user at this domain. This means that if the user is logged in, the browser will send the cookies required to allow you check his/her identity.
• Last: as the OPTIONS request doesn't require more data in the response you can stop any further processing here. After this first request the browser will upload the file and you must send back again the two Access-Control headers as shown in the sample code.

You must be careful if you want to allow this feature, after all it opens the possibility of another attack vector.
If you're hosting the files in the same physical server, you might be able to get the same functionality by modifying just the URL that it's returned after you upload a file and keeping all your code in the admin.example.com domain.