Window: postMessage() method
The window.postMessage()
method safely enables
cross-origin communication between Window
objects; e.g., between
a page and a pop-up that it spawned, or between a page and an iframe embedded within it.
Normally, scripts on different pages are allowed to access each other if and only if
the pages they originate from share the same protocol, port number, and host (also known
as the "same-origin policy").
window.postMessage()
provides a controlled mechanism to securely circumvent this restriction (if used properly).
Broadly, one window may obtain a reference to another (e.g., via
targetWindow = window.opener
), and then dispatch a
MessageEvent
on it with targetWindow.postMessage()
. The
receiving window is then free to handle this event as needed. The arguments passed to window.postMessage()
(i.e., the "message") are exposed to the receiving window through the event object.
Syntax
js
postMessage(message, targetOrigin)
postMessage(message, targetOrigin, transfer)
Parameters
message
-
Data to be sent to the other window. The data is serialized using the structured clone algorithm. This means you can pass a broad variety of data objects safely to the destination window without having to serialize them yourself.
targetOrigin
-
Specifies what the origin of this window must be for the event to be dispatched, either as the literal string
"*"
(indicating no preference) or as a URI. If at the time the event is scheduled to be dispatched the scheme, hostname, or port of this window's document does not match that provided intargetOrigin
, the event will not be dispatched; only if all three match will the event be dispatched. This mechanism provides control over where messages are sent; for example, ifpostMessage()
was used to transmit a password, it would be absolutely critical that this argument be a URI whose origin is the same as the intended receiver of the message containing the password, to prevent interception of the password by a malicious third party. Always provide a specifictargetOrigin
, not*
, if you know where the other window's document should be located. Failing to provide a specific target discloses the data you send to any interested malicious site. transfer
Optional-
A sequence of transferable objects that are transferred with the message. The ownership of these objects is given to the destination side and they are no longer usable on the sending side.
Return value
None (undefined
).
The dispatched event
A window
can listen for dispatched messages by executing the following
JavaScript:
js
window.addEventListener(
"message",
(event) => {
if (event.origin !== "http://example.org:8080") return;
// …
},
false
);
The properties of the dispatched message are:
data
-
The object passed from the other window.
origin
-
The origin of the window that sent the message at the time
postMessage
was called. This string is the concatenation of the protocol and "://", the host name if one exists, and ":" followed by a port number if a port is present and differs from the default port for the given protocol. Examples of typical origins arehttps://example.org
(implying port443
),http://example.net
(implying port80
), andhttp://example.com:8080
. Note that this origin is not guaranteed to be the current or future origin of that window, which might have been navigated to a different location sincepostMessage
was called. source
-
A reference to the
window
object that sent the message; you can use this to establish two-way communication between two windows with different origins.
Security concerns
If you do not expect to receive messages from other sites, do not add
any event listeners for message
events. This is a completely
foolproof way to avoid security problems.
If you do expect to receive messages from other sites, always verify the
sender's identity using the origin
and possibly
source
properties. Any window (including, for example,
http://evil.example.com
) can send a message to any other window,
and you have no guarantees that an unknown sender will not send malicious messages.
Having verified identity, however, you still should always verify the syntax of
the received message. Otherwise, a security hole in the site you trusted to
send only trusted messages could then open a cross-site scripting hole in your site.
Always specify an exact target origin, not *
, when you use
postMessage
to send data to other windows. A malicious site can
change the location of the window without your knowledge, and therefore it can intercept
the data sent using postMessage
.
Secure shared memory messaging
If postMessage()
throws when used with SharedArrayBuffer
objects, you might need to make sure you cross-site isolated your site properly. Shared
memory is gated behind two HTTP headers:
-
Cross-Origin-Opener-Policy
withsame-origin
as value (protects your origin from attackers) -
Cross-Origin-Embedder-Policy
withrequire-corp
orcredentialless
as value (protects victims from your origin)
http
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
To check if cross origin isolation has been successful, you can test against the
crossOriginIsolated
property available to window and worker contexts:
js
const myWorker = new Worker("worker.js");
if (crossOriginIsolated) {
const buffer = new SharedArrayBuffer(16);
myWorker.postMessage(buffer);
} else {
const buffer = new ArrayBuffer(16);
myWorker.postMessage(buffer);
}
Examples
js
/*
* In window A's scripts, with A being on http://example.com:8080:
*/
const popup = window.open(/* popup details */);
// When the popup has fully loaded, if not blocked by a popup blocker:
// This does nothing, assuming the window hasn't changed its location.
popup.postMessage(
"The user is 'bob' and the password is 'secret'",
"https://secure.example.net"
);
// This will successfully queue a message to be sent to the popup, assuming
// the window hasn't changed its location.
popup.postMessage("hello there!", "http://example.com");
window.addEventListener(
"message",
(event) => {
// Do we trust the sender of this message? (might be
// different from what we originally opened, for example).
if (event.origin !== "http://example.com") return;
// event.source is popup
// event.data is "hi there yourself! the secret response is: rheeeeet!"
},
false
);
js
/*
* In the popup's scripts, running on http://example.com:
*/
// Called sometime after postMessage is called
window.addEventListener("message", (event) => {
// Do we trust the sender of this message?
if (event.origin !== "http://example.com:8080") return;
// event.source is window.opener
// event.data is "hello there!"
// Assuming you've verified the origin of the received message (which
// you must do in any case), a convenient idiom for replying to a
// message is to call postMessage on event.source and provide
// event.origin as the targetOrigin.
event.source.postMessage(
"hi there yourself! the secret response " + "is: rheeeeet!",
event.origin
);
});
Notes
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 overstated: Failure to check the origin
and possibly source
properties enables cross-site scripting
attacks.
As with any asynchronously-dispatched script (timeouts, user-generated events), it is
not possible for the caller of postMessage
to detect when an event handler
listening for events sent by postMessage
throws an exception.
After postMessage()
is called, the MessageEvent
will be
dispatched only after all pending execution contexts have finished.
For example, if postMessage()
is invoked in an event handler, that event
handler will run to completion, as will any remaining handlers for that same event,
before the MessageEvent
is dispatched.
The value of the origin
property of the dispatched event is not affected
by the current value of document.domain
in the calling window.
For IDN host names only, the value of the origin
property is not
consistently Unicode or punycode; for greatest compatibility check for both the IDN and
punycode values when using this property if you expect messages from IDN sites. This
value will eventually be consistently IDN, but for now you should handle both IDN and
punycode forms.
The value of the origin
property when the sending window contains a
javascript:
or data:
URL is the origin of the script that
loaded the URL.
Using window.postMessage in extensions Non-standard
window.postMessage
is available to JavaScript running in chrome code
(e.g., in extensions and privileged code), but the source
property of the
dispatched event is always null
as a security restriction. (The other
properties have their expected values.)
It is not possible for content or web context scripts to specify a
targetOrigin
to communicate directly with an extension (either the
background script or a content script). Web or content scripts can use
window.postMessage
with a targetOrigin
of "*"
to
broadcast to every listener, but this is discouraged, since an extension cannot be
certain the origin of such messages, and other listeners (including those you do not
control) can listen in.
Content scripts should use runtime.sendMessage
to communicate with
the background script. Web context scripts can use custom events to communicate with
content scripts (with randomly generated event names, if needed, to prevent snooping
from the guest page).
Lastly, posting a message to a page at a file:
URL currently requires that
the targetOrigin
argument be "*"
. file://
cannot
be used as a security restriction; this restriction may be modified in the future.
Specifications
Specification |
---|
HTML Standard # dom-window-postmessage-options-dev |
Browser compatibility
BCD tables only load in the browser
See also
Document.domain
CustomEvent
BroadcastChannel
- For same-origin communication.