ajax javascript same-origin-policy

Ways to circumvent the same-origin policy


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.


  • 4

    nice idea.. You should put your examples into answer(s) though; as it stands, they make the question rather bulky

    – Shog9

    Jun 19, 2010 at 17:59

  • 1

    You should also add a list of security implications for each approach. JSONP is highly insecure for private data.

    – Erlend

    Nov 11, 2011 at 6:55

  • Why the close? This (wiki) question has been quite useful for the past 2 years. Furthermore, many answers are supported by references. An explanation would be appreciated as a not constructive tag seems utterly inane. Voted for re-open.

    Aug 15, 2012 at 2:17


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 executes the following statement:

document.domain = "";

After that statement executes, the page would pass the origin check with However, by the same reasoning, could not set document.domain to

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.


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:


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:


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){, url, true);
    } else if (typeof XDomainRequest != "undefined"){
        xhr = new XDomainRequest();, url);
    } else {
        xhr = null;
    return xhr;

var request = createCORSRequest("get", "");
if (request){
    request.onload = function() {
        // ...
    request.onreadystatechange = handler;

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.


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

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

And a receiveMessage function must be declared:

function receiveMessage(event)
    // do something with;

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


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.



  • I hope im not too late to get an answer: by only question is, is localhost ALWAYS an exception? is it always not allowed? should I stop testing through my localhost?

    – Ayyash

    Mar 13, 2012 at 15:33

  • 1

    I am not sure why but when I set: Access-Control-Allow-Origin: instead of: Access-Control-Allow-Origin: (slash on the end of url), it does not work in Safari and FF but works in Chrome. Of course without slash works fine in all browsers.

    – mtfk

    May 21, 2012 at 7:27

  • 1

    Might be worth letting people know that the postMessage method only works for browsers that support it, as it is an HTML5 addition. This plugin tries to account for that. Just mentioning it because I’m learning this the hard way.

    Aug 2, 2012 at 19:38


The Reverse Proxy method

  • Method type: Ajax

Setting up a simple reverse proxy on the server, will allow the browser to use relative paths for the Ajax requests, while the server would be acting as a proxy to any remote location.

If using mod_proxy in Apache, the fundamental configuration directive to set up a reverse proxy is the ProxyPass. It is typically used as follows:

ProxyPass     /ajax/

In this case, the browser would be able to request /ajax/web_service.xml as a relative URL, but the server would serve this by acting as a proxy to

One interesting feature of the this method is that the reverse proxy can easily distribute requests towards multiple back-ends, thus acting as a load balancer.



    I use JSONP.

    Basically, you add

    <script src="http://..../someData.js?callback=some_func"/>

    on your page.

    some_func() should get called so that you are notified that the data is in.


    • 7

      JSONP has two problems: a) You are adding a script tag to the target domain. They can send anything back, even regular javascript (XSS attack). So you really have to trust them not to do bad stuff or become hacked b) Any other webpage can add the same script-tag, and steal the data, so never ever use JSONP for private data.

      – Erlend

      Nov 11, 2011 at 7:07

    • 1

      @Erlend: Any information served on the web can be retrieved by anyone (unless proper authentication is required). The exact format of how that information is presented does not make this better or worse, not even if it’s JSONP.

      – T-Bull

      Feb 1, 2012 at 21:45

    • 2

      @T-Bull: The problem is that proper authentication is impossible with JSONP. A user logs in on site A and then goes to site B, which loads data from A using a JSONP script tag. As is well and good. Then the user is tricked into visiting evil site C, which also uses a JSONP script tag to load data from A. So because the user is authenticated with A, the owner of C can now steal the users data from A. And that’s even if the user used two factor authentication to authenticated with A. The problem is that JSONP is highly insecure. And JSONP is not presentation. It’s insecure data transfer.

      – Erlend

      Feb 4, 2012 at 8:22

    • 1

      JSONP only supports HTTP GET.

      – opyate

      Apr 1, 2012 at 13:18

    • What .js file does this represent -> “http://…./someData.js….I’m trying to read the dom from another site client-side, and need to circumvent the same-origin policy.

      – CS_2013

      May 21, 2012 at 21:12