Ember JS with remote auth backend (XML response)

Using Ember-Simple-Auth (custom authenticator) and Ember.$.ajax()

Authentication procedure:

1. Link to authsite.com/login, and log in there
2. If OK, external website redirects to http://ift.tt/2fJO3NU
3. I retrieve those parameters
4. I can get a big XML file with a lot of info by making a request to http://ift.tt/2gpnDQ9 if user is authenticated. Auth backend constraint: this request must come from the same IP as step 2.

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I have a problem making this request because:

1. Not allowed by Access-Control-Allow-Origin
2. Not JSONP
3. Cannot enable CORS
4. Setting crossDomain: true and xhr credentials to true doesn't work either when doing Ember.$.ajax()

What are my options if I want to do everything in the front-end?