In this section, we'll explain what web-message manipulation vulnerabilities are and suggest ways to reduce your exposure to them.
Web-message vulnerabilities arise when a script sends attacker-controllable data as a web message to another document within the browser. An attacker may be able to use the web-message data as a source by constructing a web page that, if visited by a user, will cause the user's browser to send a web message containing data that is under the attacker's control. For more information about the using web messages as a source, please refer to the Controlling the web-message source page.
postMessage() method for sending web messages can lead to vulnerabilities if the event listener for receiving messages handles the incoming data in an unsafe way.
In addition to the general measures described on the DOM-based vulnerabilities page, you should avoid sending web messages that contain data that originated from any untrusted source. When sending cross-origin messages, you should always explicitly specify the target window. Most importantly, make sure to implement robust measures to verify the origin of any incoming messages.