HTML: The Living Standard

Edition for Web Developers — Last Updated 30 November 2023

1. 1. 9.3 Cross-document messaging
      1. 9.3.1 Introduction
      2. 9.3.2 Security
      3. 9.3.3 Posting messages
   2. 9.4 Channel messaging
      1. 9.4.1 Introduction
         1. 9.4.1.1 Examples
         2. 9.4.1.2 Ports as the basis of an object-capability model on the web
         3. 9.4.1.3 Ports as the basis of abstracting out service implementations
      2. 9.4.2 Message channels
      3. 9.4.3 Message ports
      4. 9.4.4 Broadcasting to many ports
      5. 9.4.5 Ports and garbage collection
   3. 9.5 Broadcasting to other browsing contexts

9.3 Cross-document messaging

Web browsers, for security and privacy reasons, prevent documents in different domains from affecting each other; that is, cross-site scripting is disallowed.

While this is an important security feature, it prevents pages from different domains from communicating even when those pages are not hostile. This section introduces a messaging system that allows documents to communicate with each other regardless of their source domain, in a way designed to not enable cross-site scripting attacks.

The postMessage() API can be used as a tracking vector.

9.3.1 Introduction

var o = document.getElementsByTagName('iframe')[0];
o.contentWindow.postMessage('Hello world', 'https://b.example.org/');

window.addEventListener('message', receiver, false);
function receiver(e) {
  if (e.origin == 'https://example.com') {
    if (e.data == 'Hello world') {
      e.source.postMessage('Hello', e.origin);
    } else {
      alert(e.data);
    }
  }
}

9.3.2 Security

Use of this API requires extra care to protect users from hostile entities abusing a site for their own purposes.

Authors should check the origin attribute to ensure that messages are only accepted from domains that they expect to receive messages from. Otherwise, bugs in the author's message handling code could be exploited by hostile sites.

Furthermore, even after checking the origin attribute, authors should also check that the data in question is of the expected format. Otherwise, if the source of the event has been attacked using a cross-site scripting flaw, further unchecked processing of information sent using the postMessage() method could result in the attack being propagated into the receiver.

Authors should not use the wildcard keyword (*) in the targetOrigin argument in messages that contain any confidential information, as otherwise there is no way to guarantee that the message is only delivered to the recipient to which it was intended.

Authors who accept messages from any origin are encouraged to consider the risks of a denial-of-service attack. An attacker could send a high volume of messages; if the receiving page performs expensive computation or causes network traffic to be sent for each such message, the attacker's message could be multiplied into a denial-of-service attack. Authors are encouraged to employ rate limiting (only accepting a certain number of messages per minute) to make such attacks impractical.

9.3.3 Posting messages

window.postMessage(message [, options ])

Posts a message to the given window. Messages can be structured objects, e.g. nested objects and arrays, can contain JavaScript values (strings, numbers, Date objects, etc.), and can contain certain data objects such as File Blob, FileList, and ArrayBuffer objects.

Objects listed in the transfer member of options are transferred, not just cloned, meaning that they are no longer usable on the sending side.

A target origin can be specified using the targetOrigin member of options. If not provided, it defaults to "/". This default restricts the message to same-origin targets only.

If the origin of the target window doesn't match the given target origin, the message is discarded, to avoid information leakage. To send the message to the target regardless of origin, set the target origin to "*".

Throws a "DataCloneError" DOMException if transfer array contains duplicate objects or if message could not be cloned.

window.postMessage(message, targetOrigin [, transfer ])

This is an alternate version of postMessage() where the target origin is specified as a parameter. Calling window.postMessage(message, target, transfer) is equivalent to window.postMessage(message, {targetOrigin, transfer}).

When posting a message to a Window of a browsing context that has just been navigated to a new Document is likely to result in the message not receiving its intended recipient: the scripts in the target browsing context have to have had time to set up listeners for the messages. Thus, for instance, in situations where a message is to be sent to the Window of newly created child iframe, authors are advised to have the child Document post a message to their parent announcing their readiness to receive messages, and for the parent to wait for this message before beginning posting messages.

9.4 Channel messaging

9.4.1 Introduction

To enable independent pieces of code (e.g. running in different browsing contexts) to communicate directly, authors can use channel messaging.

Communication channels in this mechanism are implemented as two-ways pipes, with a port at each end. Messages sent in one port are delivered at the other port, and vice-versa. Messages are delivered as DOM events, without interrupting or blocking running tasks.

To create a connection (two "entangled" ports), the MessageChannel() constructor is called:

var channel = new MessageChannel();

One of the ports is kept as the local port, and the other port is sent to the remote code, e.g. using postMessage():

otherWindow.postMessage('hello', 'https://example.com', [channel.port2]);

To send messages, the postMessage() method on the port is used:

channel.port1.postMessage('hello');

To receive messages, one listens to message events:

channel.port1.onmessage = handleMessage;
function handleMessage(event) {
  // message is in event.data
  // ...
}

Data sent on a port can be structured data; for example here an array of strings is passed on a MessagePort:

port1.postMessage(['hello', 'world']);

9.4.1.1 Examples

<script src="contacts.js"></script> <!-- exposes a contacts object -->
<script src="compose-mail.js"></script> <!-- exposes a composer object -->
<script>
 var channel = new MessageChannel();
 composer.addContactsProvider(channel.port1);
 contacts.registerConsumer(channel.port2);
</script>

function addContactsProvider(port) {
  port.onmessage = function (event) {
    switch (event.data.messageType) {
      case 'search-result': handleSearchResult(event.data.results); break;
      case 'search-done': handleSearchDone(); break;
      case 'search-error': handleSearchError(event.data.message); break;
      // ...
    }
  };
};

function addContactsProvider(port) {
  port.addEventListener('message', function (event) {
    if (event.data.messageType == 'search-result')
      handleSearchResult(event.data.results);
  });
  port.addEventListener('message', function (event) {
    if (event.data.messageType == 'search-done')
      handleSearchDone();
  });
  port.addEventListener('message', function (event) {
    if (event.data.messageType == 'search-error')
      handleSearchError(event.data.message);
  });
  // ...
  port.start();
};

9.4.1.2 Ports as the basis of an object-capability model on the web

Ports can be viewed as a way to expose limited capabilities (in the object-capability model sense) to other actors in the system. This can either be a weak capability system, where the ports are merely used as a convenient model within a particular origin, or as a strong capability model, where they are provided by one origin provider as the only mechanism by which another origin consumer can effect change in or obtain information from provider.

For example, consider a situation in which a social web site embeds in one iframe the user's email contacts provider (an address book site, from a second origin), and in a second iframe a game (from a third origin). The outer social site and the game in the second iframe cannot access anything inside the first iframe; together they can only:

• Navigate the iframe to a new URL, such as the same URL but with a different fragment, causing the Window in the iframe to receive a hashchange event.
• Resize the iframe, causing the Window in the iframe to receive a resize event.
• Send a message event to the Window in the iframe using the window.postMessage() API.

The contacts provider can use these methods, most particularly the third one, to provide an API that can be accessed by other origins to manipulate the user's address book. For example, it could respond to a message "add-contact Guillaume Tell <tell@pomme.example.net>" by adding the given person and email address to the user's address book.

To avoid any site on the web being able to manipulate the user's contacts, the contacts provider might only allow certain trusted sites, such as the social site, to do this.

Now suppose the game wanted to add a contact to the user's address book, and that the social site was willing to allow it to do so on its behalf, essentially "sharing" the trust that the contacts provider had with the social site. There are several ways it could do this; most simply, it could just proxy messages between the game site and the contacts site. However, this solution has a number of difficulties: it requires the social site to either completely trust the game site not to abuse the privilege, or it requires that the social site verify each request to make sure it's not a request that it doesn't want to allow (such as adding multiple contacts, reading the contacts, or deleting them); it also requires some additional complexity if there's ever the possibility of multiple games simultaneously trying to interact with the contacts provider.

Using message channels and MessagePort objects, however, all of these problems can go away. When the game tells the social site that it wants to add a contact, the social site can ask the contacts provider not for it to add a contact, but for the capability to add a single contact. The contacts provider then creates a pair of MessagePort objects, and sends one of them back to the social site, who forwards it on to the game. The game and the contacts provider then have a direct connection, and the contacts provider knows to only honor a single "add contact" request, nothing else. In other words, the game has been granted the capability to add a single contact.

9.4.1.3 Ports as the basis of abstracting out service implementations

Continuing the example from the previous section, consider the contacts provider in particular. While an initial implementation might have simply used XMLHttpRequest objects in the service's iframe, an evolution of the service might instead want to use a shared worker with a single WebSocket connection.

If the initial design used MessagePort objects to grant capabilities, or even just to allow multiple simultaneous independent sessions, the service implementation can switch from the XMLHttpRequests-in-each-iframe model to the shared-WebSocket model without changing the API at all: the ports on the service provider side can all be forwarded to the shared worker without it affecting the users of the API in the slightest.

9.4.2 Message channels

channel = new MessageChannel()

Returns a new MessageChannel object with two new MessagePort objects.

channel.port1

Returns the first MessagePort object.

channel.port2

Returns the second MessagePort object.

9.4.3 Message ports

Each channel has two message ports. Data sent through one port is received by the other port, and vice versa.

port.postMessage(message [, transfer])

port.postMessage(message [, { transfer }])

Posts a message through the channel. Objects listed in transfer are transferred, not just cloned, meaning that they are no longer usable on the sending side.

Throws a "DataCloneError" DOMException if transfer contains duplicate objects or port, or if message could not be cloned.

port.start()

Begins dispatching messages received on the port.

port.close()

Disconnects the port, so that it is no longer active.

9.4.4 Broadcasting to many ports

Broadcasting to many ports is in principle relatively simple: keep an array of MessagePort objects to send messages to, and iterate through the array to send a message. However, this has one rather unfortunate effect: it prevents the ports from being garbage collected, even if the other side has gone away. To avoid this problem, implement a simple protocol whereby the other side acknowledges it still exists. If it doesn't do so after a certain amount of time, assume it's gone, close the MessagePort object, and let it be garbage collected.

9.4.5 Ports and garbage collection

Authors are strongly encouraged to explicitly close MessagePort objects to disentangle them, so that their resources can be recollected. Creating many MessagePort objects and discarding them without closing them can lead to high transient memory usage since garbage collection is not necessarily performed promptly, especially for MessagePorts where garbage collection can involve cross-process coordination.

9.5 Broadcasting to other browsing contexts

Pages on a single origin opened by the same user in the same user agent but in different unrelated browsing contexts sometimes need to send notifications to each other, for example "hey, the user logged in over here, check your credentials again".

For elaborate cases, e.g. to manage locking of shared state, to manage synchronization of resources between a server and multiple local clients, to share a WebSocket connection with a remote host, and so forth, shared workers are the most appropriate solution.

For simple cases, though, where a shared worker would be an unreasonable overhead, authors can use the simple channel-based broadcast mechanism described in this section.

broadcastChannel = new BroadcastChannel(name)

Returns a new BroadcastChannel object via which messages for the given channel name can be sent and received.

broadcastChannel.name

Returns the channel name (as passed to the constructor).

broadcastChannel.postMessage(message)

Sends the given message to other BroadcastChannel objects set up for this channel. Messages can be structured objects, e.g. nested objects and arrays.

broadcastChannel.close()

Closes the BroadcastChannel object, opening it up to garbage collection.

Authors are strongly encouraged to explicitly close BroadcastChannel objects when they are no longer needed, so that they can be garbage collected. Creating many BroadcastChannel objects and discarding them while leaving them with an event listener and without closing them can lead to an apparent memory leak, since the objects will continue to live for as long as they have an event listener (or until their page or worker is closed).

var authChannel = new BroadcastChannel('auth');
authChannel.onmessage = function (event) {
  if (event.data == 'logout')
    showLogout();
}

function logoutRequested() {
  // called when the user asks us to log them out
  doLogout();
  showLogout();
  authChannel.postMessage('logout');
}

function doLogout() {
  // actually log the user out (e.g. clearing cookies)
  // ...
}

function showLogout() {
  // update the UI to indicate we're logged out
  // ...
}