Cross-window communication

The “Same Origin” (same site) policy limits access of windows and frames to each other.

The idea is that if a user has two pages open: one from john-smith.com, and another one is gmail.com, then they wouldn’t want a script from john-smith.com to read our mail from gmail.com. So, the purpose of the “Same Origin” policy is to protect users from information theft.

Same Origin

Two URLs are said to have the “same origin” if they have the same protocol, domain and port.

These URLs all share the same origin:

  • http://site.com
  • http://site.com/
  • http://site.com/my/page.html

These ones do not:

  • http://www.site.com (another domain: www. matters)
  • http://site.org (another domain: .org matters)
  • https://site.com (another protocol: https)
  • http://site.com:8080 (another port: 8080)

The “Same Origin” policy states that:

  • if we have a reference to another window, e.g. a popup created by window.open or a window inside <iframe>, and that window comes from the same origin, then we have full access to that window.
  • otherwise, if it comes from another origin, then we can’t access the content of that window: variables, document, anything. The only exception is location: we can change it (thus redirecting the user). But we cannot read location (so we can’t see where the user is now, no information leak).

In action: iframe

An <iframe> tag hosts a separate embedded window, with its own separate document and window objects.

We can access them using properties:

  • iframe.contentWindow to get the window inside the <iframe>.
  • iframe.contentDocument to get the document inside the <iframe>, shorthand for iframe.contentWindow.document.

When we access something inside the embedded window, the browser checks if the iframe has the same origin. If that’s not so then the access is denied (writing to location is an exception, it’s still permitted).

For instance, let’s try reading and writing to <iframe> from another origin:

<iframe src="https://example.com" id="iframe"></iframe>

<script>
  iframe.onload = function() {
    // we can get the reference to the inner window
    let iframeWindow = iframe.contentWindow; // OK
    try {
      // ...but not to the document inside it
      let doc = iframe.contentDocument; // ERROR
    } catch(e) {
      alert(e); // Security Error (another origin)
    }

    // also we can't READ the URL of the page in iframe
    try {
      // Can't read URL from the Location object
      let href = iframe.contentWindow.location.href; // ERROR
    } catch(e) {
      alert(e); // Security Error
    }

    // ...we can WRITE into location (and thus load something else into the iframe)!
    iframe.contentWindow.location = '/'; // OK

    iframe.onload = null; // clear the handler, not to run it after the location change
  };
</script>

The code above shows errors for any operations except:

  • Getting the reference to the inner window iframe.contentWindow – that’s allowed.
  • Writing to location.

Contrary to that, if the <iframe> has the same origin, we can do anything with it:

<!-- iframe from the same site -->
<iframe src="/" id="iframe"></iframe>

<script>
  iframe.onload = function() {
    // just do anything
    iframe.contentDocument.body.prepend("Hello, world!");
  };
</script>
iframe.onload vs iframe.contentWindow.onload

The iframe.onload event (on the <iframe> tag) is essentially the same as iframe.contentWindow.onload (on the embedded window object). It triggers when the embedded window fully loads with all resources.

…But we can’t access iframe.contentWindow.onload for an iframe from another origin, so using iframe.onload.

Windows on subdomains: document.domain

By definition, two URLs with different domains have different origins.

But if windows share the same second-level domain, for instance john.site.com, peter.site.com and site.com (so that their common second-level domain is site.com), we can make the browser ignore that difference, so that they can be treated as coming from the “same origin” for the purposes of cross-window communication.

To make it work, each such window should run the code:

document.domain = 'site.com';

That’s all. Now they can interact without limitations. Again, that’s only possible for pages with the same second-level domain.

Iframe: wrong document pitfall

When an iframe comes from the same origin, and we may access its document, there’s a pitfall. It’s not related to cross-origin things, but important to know.

Upon its creation an iframe immediately has a document. But that document is different from the one that loads into it!

So if we do something with the document immediately, that will probably be lost.

Here, look:

<iframe src="/" id="iframe"></iframe>

<script>
  let oldDoc = iframe.contentDocument;
  iframe.onload = function() {
    let newDoc = iframe.contentDocument;
    // the loaded document is not the same as initial!
    alert(oldDoc == newDoc); // false
  };
</script>

We shouldn’t work with the document of a not-yet-loaded iframe, because that’s the wrong document. If we set any event handlers on it, they will be ignored.

How to detect the moment when the document is there?

The right document is definitely at place when iframe.onload triggers. But it only triggers when the whole iframe with all resources is loaded.

We can try to catch the moment earlier using checks in setInterval:

<iframe src="/" id="iframe"></iframe>

<script>
  let oldDoc = iframe.contentDocument;

  // every 100 ms check if the document is the new one
  let timer = setInterval(() => {
    let newDoc = iframe.contentDocument;
    if (newDoc == oldDoc) return;

    alert("New document is here!");

    clearInterval(timer); // cancel setInterval, don't need it any more
  }, 100);
</script>

Collection: window.frames

An alternative way to get a window object for <iframe> – is to get it from the named collection window.frames:

  • By number: window.frames[0] – the window object for the first frame in the document.
  • By name: window.frames.iframeName – the window object for the frame with name="iframeName".

For instance:

<iframe src="/" style="height:80px" name="win" id="iframe"></iframe>

<script>
  alert(iframe.contentWindow == frames[0]); // true
  alert(iframe.contentWindow == frames.win); // true
</script>

An iframe may have other iframes inside. The corresponding window objects form a hierarchy.

Navigation links are:

  • window.frames – the collection of “children” windows (for nested frames).
  • window.parent – the reference to the “parent” (outer) window.
  • window.top – the reference to the topmost parent window.

For instance:

window.frames[0].parent === window; // true

We can use the top property to check if the current document is open inside a frame or not:

if (window == top) { // current window == window.top?
  alert('The script is in the topmost window, not in a frame');
} else {
  alert('The script runs in a frame!');
}

The “sandbox” iframe attribute

The sandbox attribute allows for the exclusion of certain actions inside an <iframe> in order to prevent it executing untrusted code. It “sandboxes” the iframe by treating it as coming from another origin and/or applying other limitations.

There’s a “default set” of restrictions applied for <iframe sandbox src="...">. But it can be relaxed if we provide a space-separated list of restrictions that should not be applied as a value of the attribute, like this: <iframe sandbox="allow-forms allow-popups">.

In other words, an empty "sandbox" attribute puts the strictest limitations possible, but we can put a space-delimited list of those that we want to lift.

Here’s a list of limitations:

allow-same-origin
By default "sandbox" forces the “different origin” policy for the iframe. In other words, it makes the browser to treat the iframe as coming from another origin, even if its src points to the same site. With all implied restrictions for scripts. This option removes that feature.
allow-top-navigation
Allows the iframe to change parent.location.
allow-forms
Allows to submit forms from iframe.
allow-scripts
Allows to run scripts from the iframe.
allow-popups
Allows to window.open popups from the iframe

See the manual for more.

The example below demonstrates a sandboxed iframe with the default set of restrictions: <iframe sandbox src="...">. It has some JavaScript and a form.

Please note that nothing works. So the default set is really harsh:

Result
index.html
sandboxed.html
<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
</head>

<body>

  <div>The iframe below is has <code>sandbox</code> attribute.</div>

  <iframe sandbox src="sandboxed.html" style="height:60px;width:90%"></iframe>

</body>
</html>
<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
</head>

<body>

  <button onclick="alert(123)">Click to run a script (doesn't work)</button>

  <form action="http://google.com">
    <input type="text">
    <input type="submit" value="Submit (doesn't work)">
  </form>

</body>
</html>
Please note:

The purpose of the "sandbox" attribute is only to add more restrictions. It cannot remove them. In particular, it can’t relax same-origin restrictions if the iframe comes from another origin.

Cross-window messaging

The postMessage interface allows windows to talk to each other no matter which origin they are from.

So, it’s a way around the “Same Origin” policy. It allows a window from john-smith.com to talk to gmail.com and exchange information, but only if they both agree and call corresponding JavaScript functions. That makes it safe for users.

The interface has two parts.

postMessage

The window that wants to send a message calls postMessage method of the receiving window. In other words, if we want to send the message to win, we should call win.postMessage(data, targetOrigin).

Arguments:

data
The data to send. Can be any object, the data is cloned using the “structured cloning algorithm”. IE supports only strings, so we should JSON.stringify complex objects to support that browser.
targetOrigin
Specifies the origin for the target window, so that only a window from the given origin will get the message.

The targetOrigin is a safety measure. Remember, if the target window comes from another origin, we can’t read it’s location in the sender window. So we can’t be sure which site is open in the intended window right now: the user could navigate away, and the sender window has no idea about it.

Specifying targetOrigin ensures that the window only receives the data if it’s still at the right site. Important when the data is sensitive.

For instance, here win will only receive the message if it has a document from the origin http://example.com:

<iframe src="http://example.com" name="example">

<script>
  let win = window.frames.example;

  win.postMessage("message", "http://example.com");
</script>

If we don’t want that check, we can set targetOrigin to *.

<iframe src="http://example.com" name="example">

<script>
  let win = window.frames.example;

  win.postMessage("message", "*");
</script>

onmessage

To receive a message, the target window should have a handler on the message event. It triggers when postMessage is called (and targetOrigin check is successful).

The event object has special properties:

data
The data from postMessage.
origin
The origin of the sender, for instance http://javascript.info.
source
The reference to the sender window. We can immediately source.postMessage(...) back if we want.

To assign that handler, we should use addEventListener, a short syntax window.onmessage does not work.

Here’s an example:

window.addEventListener("message", function(event) {
  if (event.origin != 'http://javascript.info') {
    // something from an unknown domain, let's ignore it
    return;
  }

  alert( "received: " + event.data );

  // can message back using event.source.postMessage(...)
});

The full example:

Result
iframe.html
index.html
<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
</head>

<body>

  Receiving iframe.
  <script>
    window.addEventListener('message', function(event) {
      alert(`Received ${event.data} from ${event.origin}`);
    });
  </script>

</body>
</html>
<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
</head>

<body>

  <form id="form">
    <input type="text" placeholder="Enter message" name="message">
    <input type="submit" value="Click to send">
  </form>

  <iframe src="iframe.html" id="iframe" style="display:block;height:60px"></iframe>

  <script>
    form.onsubmit = function() {
      iframe.contentWindow.postMessage(this.message.value, '*');
      return false;
    };
  </script>

</body>
</html>
There’s no delay

There’s totally no delay between postMessage and the message event. The event triggers synchronously, faster than setTimeout(...,0).

Summary

To call methods and access the content of another window, we should first have a reference to it.

For popups we have these references:

  • From the opener window: window.open – opens a new window and returns a reference to it,
  • From the popup: window.opener – is a reference to the opener window from a popup.

For iframes, we can access parent/children windows using:

  • window.frames – a collection of nested window objects,
  • window.parent, window.top are the references to parent and top windows,
  • iframe.contentWindow is the window inside an <iframe> tag.

If windows share the same origin (host, port, protocol), then windows can do whatever they want with each other.

Otherwise, only possible actions are:

  • Change the location of another window (write-only access).
  • Post a message to it.

Exceptions are:

  • Windows that share the same second-level domain: a.site.com and b.site.com. Then setting document.domain='site.com' in both of them puts them into the “same origin” state.
  • If an iframe has a sandbox attribute, it is forcefully put into the “different origin” state, unless the allow-same-origin is specified in the attribute value. That can be used to run untrusted code in iframes from the same site.

The postMessage interface allows two windows with any origins to talk:

  1. The sender calls targetWin.postMessage(data, targetOrigin).

  2. If targetOrigin is not '*', then the browser checks if window targetWin has the origin targetOrigin.

  3. If it is so, then targetWin triggers the message event with special properties:

    • origin – the origin of the sender window (like http://my.site.com)
    • source – the reference to the sender window.
    • data – the data, any object in everywhere except IE that supports only strings.

    We should use addEventListener to set the handler for this event inside the target window.

Tutorial map

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