CORS $.ajax session cookies (access-control-allow-credentials & withCredentials=true) - ajax

Question

I realize this question has been asked a dozen or more times and each response given indicates I am doing it right but perhaps I am missing something.
AJAX serves up CORS request like so...
$.ajax({
url: 'someotherdomain.com',
type: 'post',
data: {key: 'value'},
dataType: 'json',
async: false,
crossDomain: true,
beforeSend: function(xhr){
xhr.withCredentials = true;
},
success: function(x, status, xhr){
},
error: function(xhr, status, error){
}
});
PHP serves up CORS requests like so...
header('Access-Control-Max-Age: 1728000');
header('Access-Control-Allow-Origin: http://someotherdomain.com');
header('Access-Control-Allow-Methods: POST');
header('Access-Control-Allow-Headers: Content-MD5, X-Alt-Referer');
header('Access-Control-Allow-Credentials: true');
header("Content-Type: application/json; charset=utf-8");
According to all documentation as long as the 'Access-Control-Allow-Credentials' server side header, and the 'withCredentials=true' client side header is set session cookie handling between the domains should be transparent. Am I missing something?

Answer 1

async: false
was preventing the session cookie from being sent back to the server on each request. The following fixed it.
async: true
Although this does allow for the session cookie to get set by the browser when making a cross origin request sharing call, I am now experiencing problems regarding the following scenario:
Server A sends response to client
Client using CORS makes request of server B
XMLHttpRequest -> PHP -> Session handler -> MySQL -> Stored Procedure
Due to the MUTEX locks in the PHP session management the asynchronous nature and apparently, requirement may force a work around of manually setting the cookie with a different header option such as XCookie or something similar to keep the servers session and client requests synchronized.
This particular work around does not sit well with me as I believe it would open up an easy lane of travel for session hijacking and session replay attack vectors.
Using an SSL/TLS wrapped connection may assist in preventing the above scenario but in terms of independently providing security measures for the client I do not believe this should suffice.
Anyone with any thoughts on this?

Answer 2

In your example above, you are setting the Access-Control-Allow-Origin header to 'http://someotherdomain.com', which is the same as the url you are requesting from JQuery. The Access-Control-Allow-Origin header should be the value of the domain the request is coming from. As a quick, test, try setting the value of this header to '*' (without the quotes) and see if it works ('*' means all domains are allowed).