I realize this question is old, but I figured I’d throw in on it anyhow. In the case of CORS requests, the browser preflights them. What this means is – in spite of whatever $.ajax() method you are using, an OPTIONS request is sent to the server.
What this preflighted OPTIONS request is actually doing is saying:
“Hey there, foreign-server-from-some-other-domain, I want to send you a not-simple request (simple req’s are not preflighted). My not-simple request going to have these kinds of headers and content type and so on. Can you let me know if this is okay?”
Then the server will do whatever it does (probably check some configuration or database) and respond with the allowable origin(s), the allowable header(s), and/or the allowable method(s).
Finally – if that preflight OPTIONS request has received response that allows the actual $.ajax() method to go – it goes.
CORS is not the same as JSONP.
All that said – while withCredentials preflight success requires the response to carry a Access-Control-Allow-Credentials header (as stated in the question), that is IN ADDITION to Access-Control-Allow-Origins AND Access-Control-Allow-Methods values, which must include the facets of the intended request.
For example – if you are making a CORS POST request from origin http://foo-domain.com with headers somevalue to http://bar-domain.com, a preflight OPTIONS request would go out and in order for the actual post request to be made to http://bar-domain.com, the OPTIONS request would need to receive a response with an Access-Control-Allow-Origins value that included http://foo-domain.com. This could be the origin name itself or *. The response would also need to have an Access-Control-Allow-Methods value that included POST. This may also be *. And Finally if we want our somevalue header to be allowed, the response must contain a Access-Control-Allow-Headers value that includes our somevalue header key or *.
To circle back – if you can’t control the server, or have no way to allow the server to allow your CORS requests, you could always use JSONP or some urlEncoded datatype and/or make simple requests without custom headers. GET, HEAD, and full POST requests are usually simple requests.