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{{HTML}}
'''Web Messaging''', or '''cross-document messaging''', is an [[API]] introduced in the [[WHATWG]] [[HTML5]] draft specification, allowing documents to communicate with one another across different origins, or source domains<ref name="WHATWG"/> while rendered in a [[web browser]]. Prior to HTML5, web browsers disallowed [[cross-site scripting]], to protect against security attacks. This practice barred communication between non-hostile pages as well, making document interaction of any kind difficult.<ref name="WHATWG"/><ref name="WebKit"/> Cross-document messaging allows scripts to interact across these boundaries, while providing a rudimentary level of security.
==Requirements and attributes==
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* the window which opened the sender document
The message <code>event</code> being received has the following attributes:
* <code>data</code>
* <code>origin</code>
* <code>source</code>
<code>postMessage</code> is not a blocking call; messages are processed asynchronously.<ref>{{Cite web|url=http://www.w3.org/TR/2010/WD-webmessaging-20101118/#web-messaging|title = HTML5 Web Messaging}}</ref>
==Example==
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window.addEventListener('message', receiver, false);
</syntaxhighlight>
An [[event listener]] is set up to receive messages from document A. Using the <code>origin</code> property, it then checks that the domain of the sender is the expected domain. Document B then looks at the message, either displaying it to the user, or responding in turn with a message of its own for document A.<ref name="WHATWG"/>
==Security==
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* [[Cross-site scripting]]
* [[Cross-site request forgery]]
* [[Same
* [[Cross-origin resource sharing]]
* [[JSONP]]
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<ref name="WHATWG">[http://www.whatwg.org/specs/web-apps/current-work/multipage/comms.html#web-messaging ''Cross-Document Messaging – HTML Draft Standard'']</ref>
<ref name="INTRODUCING">Remy, Sharp, ''Messages, Workers, and Sockets'', ''Introducing HTML5'', New Riders, 2011, p. 197-199</ref>
<ref name="WebKit">[https://developer.apple.com/library/mac/#documentation/AppleApplications/Conceptual/SafariJSProgTopics/Articles/Cross-documentmessaging.html ''WebKit DOM Programming Topics - Cross Document Messaging'']{{dead link|date=October 2020}}</ref>
<ref name="Security">[http://www.veracode.com/blog/2010/05/html5-security-in-a-nutshell/ ''HTML5 Security in a Nutshell'']</ref>
<ref name="canIuse">[http://caniuse.com/x-doc-messaging ''When can I use Cross-Document Messaging?'']</ref>
<ref name="SUPPORT">[http://molly.com/html5/html5-0709.html ''A Selection of Supported Features in HTML5'']{{dead link|date=October 2020}}</ref>}}
==External links==
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* [http://dev.w3.org/html5/postmsg/ HTML5 Web Messaging specification]
* {{citation |url=http://www.whatwg.org/specs/web-apps/current-work/multipage/comms.html#web-messaging |title=''Cross-Document Messaging – HTML Draft Standard'' |publisher=[[Web Hypertext Application Technology Working Group|WHATWG]]}}
* {{citation |url=https://developer.apple.com/library/mac/#documentation/AppleApplications/Conceptual/SafariJSProgTopics/Articles/Cross-documentmessaging.html |title=''WebKit DOM Programming Topics - Cross Document Messaging'' |publisher=Apple Developer Library
* {{citation |url=http://www.veracode.com/blog/2010/05/html5-security-in-a-nutshell/ |title=''HTML5 Security in a Nutshell'' |date=2010-05-17 |first=Chris |last=Eng |publisher=Veracode}}
* {{citation |url=http://caniuse.com/x-doc-messaging |title=''When can I use Cross-Document Messaging?'' |publisher=CanIUse}}
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