== Why is this an issue?

Browsers https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage[allow message exchanges] between Window objects of different origins. 

Because any window can send or receive messages from another window, it is important to verify the sender's/receiver's identity:

* When sending a message with the postMessage method, the identity's receiver should be defined (the wildcard keyword (``++*++``) should not be used).
* When receiving a message with a message event, the sender's identity should be verified using the origin and possibly source properties.


=== Noncompliant code example

When sending a message:

[source,javascript]
----
var iframe = document.getElementById("testiframe");
iframe.contentWindow.postMessage("secret", "*"); // Noncompliant: * is used
----
When receiving a message:

[source,javascript]
----
window.addEventListener("message", function(event) { // Noncompliant: no checks are done on the origin property.
      console.log(event.data);
 }); 
----


=== Compliant solution

When sending a message:

[source,javascript]
----
var iframe = document.getElementById("testsecureiframe");
iframe.contentWindow.postMessage("hello", "https://secure.example.com"); // Compliant
----
When receiving a message:

[source,javascript]
----
window.addEventListener("message", function(event) {

  if (event.origin !== "http://example.org") // Compliant
    return;

  console.log(event.data)
}); 
----


== Resources

* https://owasp.org/Top10/A01_2021-Broken_Access_Control/[OWASP Top 10 2021 Category A1] - Broken Access Control
* https://owasp.org/www-project-top-ten/2017/A2_2017-Broken_Authentication[OWASP Top 10 2017 Category A2] - Broken Authentication
* https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage[developer.mozilla.org] - postMessage API
* https://cwe.mitre.org/data/definitions/345.html[MITRE, CWE-345] - Insufficient Verification of Data Authenticity



ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

include::message.adoc[]

'''
== Comments And Links
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include::comments-and-links.adoc[]
endif::env-github,rspecator-view[]