1) are you also including the “Access-Control-Allow-Credentials: true” header? This is needed for passing cookie credentials (and the corresponding XHR client must set .withCredentials = true)
2) Have you tried the suggestion from your link and only include the origin for the current request. For example, if a request comes in with the header “Origin: http://blog.example.com”, you would respond with “Access-Control-Allow-Origin: http://blog.example.com”, and not a list of origins. This requires a little more work on your server side implementation.
3) One other thought, you mention that you have a single login form that must be shared by various domains. Well, if it is a standard HTML form, you can do a regular form-post across domains. You don’t need to use CORS. Just set the “action” property of the form to the url you wish to post to. For example:
<form name="login" action="http://login.example.com/doLogin">