Question

The same origin policy

I wanted to make a community wiki regarding HTML/JS same-origin policies to hopefully help anyone searching for this topic. This is one of the most searched-for topics on SO and there is no consolidated wiki for it so here I go 🙂

The same origin policy prevents a
document or script loaded from one
origin from getting or setting
properties of a document from another
origin. This policy dates all the way
back to Netscape Navigator 2.0.

What are some of your favorite ways to go around same-origin policies?

Please keep examples verbose and preferably also link your sources.

Answer 1

The document.domain method

• Method type: iframe.

Note that this is an iframe method that sets the value of document.domain to a suffix of the current domain. If it does so, the shorter domain is used for subsequent origin checks. For example, assume a script in the document at http://store.company.com/dir/other.html executes the following statement:

document.domain = "company.com";

After that statement executes, the page would pass the origin check with http://company.com/dir/page.html. However, by the same reasoning, company.com could not set document.domain to othercompany.com.

With this method, you would be allowed to exectue javascript from an iframe sourced on a subdomain on a page sourced on the main domain. This method is not suited for cross-domain resources as browsers like Firefox will not allow you to change the document.domain to a completely alien domain.

Source: https://developer.mozilla.org/en/Same_origin_policy_for_JavaScript

The Cross-Origin Resource Sharing method

• Method type: AJAX.

Cross-Origin Resource Sharing (CORS) is a W3C Working Draft that defines how the browser and server must communicate when accessing sources across origins. The basic idea behind CORS is to use custom HTTP headers to allow both the browser and the server to know enough about each other to determine if the request or response should succeed or fail.

For a simple request, one that uses either GET or POST with no custom headers and whose body is text/plain, the request is sent with an extra header called Origin. The Origin header contains the origin (protocol, domain name, and port) of the requesting page so that the server can easily determine whether or not it should serve a response. An example Origin header might look like this:

Origin: http://www.stackoverflow.com

If the server decides that the request should be allowed, it sends a Access-Control-Allow-Origin header echoing back the same origin that was sent or * if it’s a public resource. For example:

Access-Control-Allow-Origin: http://www.stackoverflow.com

If this header is missing, or the origins don’t match, then the browser disallows the request. If all is well, then the browser processes the request. Note that neither the requests nor responses include cookie information.

The Mozilla team suggests in their post about CORS that you should check for the existence of the withCredentials property to determine if the browser supports CORS via XHR. You can then couple with the existence of the XDomainRequest object to cover all browsers:

function createCORSRequest(method, url){
    var xhr = new XMLHttpRequest();
    if ("withCredentials" in xhr){
        xhr.open(method, url, true);
    } else if (typeof XDomainRequest != "undefined"){
        xhr = new XDomainRequest();
        xhr.open(method, url);
    } else {
        xhr = null;
    }
    return xhr;
}

var request = createCORSRequest("get", "http://www.stackoverflow.com/");
if (request){
    request.onload = function() {
        // ...
    };
    request.onreadystatechange = handler;
    request.send();
}

Note that for the CORS method to work, you need to have access to any type of server header mechanic and can’t simply access any third-party resource.

Source: http://www.nczonline.net/blog/2010/05/25/cross-domain-ajax-with-cross-origin-resource-sharing/

The window.postMessage method

• Method type: iframe.

window.postMessage, when called, causes a MessageEvent to be dispatched at the target window when any pending script that must be executed completes (e.g. remaining event handlers if window.postMessage is called from an event handler, previously-set pending timeouts, etc.). The MessageEvent has the type message, a data property which is set to the string value of the first argument provided to window.postMessage, an origin property corresponding to the origin of the main document in the window calling window.postMessage at the time window.postMessage was called, and a source property which is the window from which window.postMessage is called.

To use window.postMessage, an event listener must be attached:

    // Internet Explorer
    window.attachEvent('onmessage',receiveMessage);

    // Opera/Mozilla/Webkit
    window.addEventListener("message", receiveMessage, false);

And a receiveMessage function must be declared:

function receiveMessage(event)
{
    // do something with event.data;
}

The off-site iframe must also send events properly via postMessage:

<script>window.parent.postMessage('foo','*')</script>

Any window may access this method on any other window, at any time, regardless of the location of the document in the window, to send it a message. Consequently, any event listener used to receive messages must first check the identity of the sender of the message, using the origin and possibly source properties. This cannot be understated: Failure to check the origin and possibly source properties enables cross-site scripting attacks.

Source: https://developer.mozilla.org/en/DOM/window.postMessage