Credit: http://www.nczonline.net/blog/2010/09/07/learning-from-xauth-cross-domain-localstorage/

gistfile1.txt

(function(){

    //allowed domains
    var whitelist = ["foo.example.com", "www.example.com"];

    function verifyOrigin(origin){
        var domain = origin.replace(/^https?:\/\/|:\d{1,4}$/g, "").toLowerCase(),
            i = 0,
            len = whitelist.length;

        while(i < len){
            if (whitelist[i] == domain){
                return true;
            }
            i++;
        }

        return false;
    }

    function handleRequest(event){
        if (verifyOrigin(event.origin)){
            var data = JSON.parse(event.data),
                value = localStorage.getItem(data.key);
            event.source.postMessage(JSON.stringify({id: data.id, key:data.key, value: value}), event.origin);
        }
    }

    if(window.addEventListener){
        window.addEventListener("message", handleRequest, false);
    } else if (window.attachEvent){
        window.attachEvent("onmessage", handleRequest);
    }
})();