Making a Same Domain iframe Secure

Question

tl;dr Can I execute un-trusted scripts on an iframe safely?

Back story:

I'm trying to make secure JSONP requests. A lot of older browsers do not support Web Workers which means that the current solution I came up with is not optimal.

I figured I could create an <iframe> and load a script inside it. That script would perform a JSONP request (creating a script tag), which would post a message to the main page. The main page would get the message, execute the callback and destroy the iframe. I've managed to do this sort of thing.

function jsonp(url, data, callback) {
    var iframe = document.createElement("iframe");
    iframe.style.display = "none";
    document.body.appendChild(iframe);

    var iframedoc = iframe.contentDocument || iframe.contentWindow.document;
    sc = document.createElement("script");

    sc.textContent = "(function(p){ cb = function(result){p.postMessage(result,'http://fiddle.jshell.net');};})(parent);";
    //sc.textContent += "alert(cb)";
    iframedoc.body.appendChild(sc);
    var jr = document.createElement("script");

    var getParams = ""; // serialize the GET parameters
    for (var i in data) {
        getParams += "&" + i + "=" + data[i];
    }

    jr.src = url + "?callback=cb" + getParams;
    iframedoc.body.appendChild(jr);
    window.onmessage = function (e) {
        callback(e.data);
        document.body.removeChild(iframe);
    }

}

jsonp("http://jsfiddle.net/echo/jsonp/", {
    foo: "bar"
}, function (result) {
    alert("Result: " + JSON.stringify(result));
});

The problem is that since the iframes are on the same domain, the injected script still has access to the external scope through .top or .parent and such.

Is there any way to create an iframe that can not access data on the parent scope?

I want to create an iframe where scripts added through script tags will not be able to access variables on the parent window (and the DOM). I tried stuff like top=parent=null but I'm really not sure that's enough, there might be other workarounds. I tried running a for... in loop, but my function stopped working and I was unable to find out why.

NOTE:

I know optimally WebWorkers are a better isolated environment. I know JSONP is a "bad" technique (I even had some random guy tell me he'd never use it today). I'm trying to create a secure environment for scenarios where you have to perform JSONP queries.

Answer 1

You can't really delete the references, setting null will just silently fail and there is always a way to get the reference to the parent dom.

References like frameElement and frameElement.defaultView etc. cannot be deleted. Attempting to do so will either silently fail or throw exception depending on browser.

You could look into Caja/Cajita though.

Answer 2

tl;dr no

Any untrusted script can steal cookies (like a session id!) or read information from the DOM like the value of a credit card input field.

JavaScript relies on the security model that all code is trusted code. Any attempts at access from another domain requires explicit whitelisting.

If you want to sandbox your iframe you can serve the page from another domain. This does mean that you can't share a session or do any kind of communication because it can be abused. It's just like including an unrelated website. Even then there are possibilities for abuse if you allow untrusted JavaScript. You can for instance do: window.top.location.href = 'http://my.phishing.domain/';, the user might not notice the redirect.