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window.postMessage is a browser API that allows sending structured data between two domains with with signature: postMessage(message: any, targetOrigin: string)

The parameter in question is targetOrigin as it is the URL of the window that the message is being sent to. The protocol, port and hostname of the target window must match this parameter for the message to be sent. It is possible to specify "*" as the targetOrigin parameter to match any URL however. This however, is strongly discouraged as it opens a vector for an attacker to send sensitive information from the browser to an origin of their choice.


Insecure Example

const myWindow = document.getElementById("myIFrame").contentWindow;

myWindow.postMessage(message, "*");

Secure Example

The secure way is to be very explicit about the target origin. As most code is deployed and tested on different environments (dev, staging, production, etc) it is more practical to employ a configuration file for the value targetOrigin as opposed to hard coded literal as described below.

const myWindow = document.getElementById('myIFrame').contentWindow;

myWindow.postMessage(message, "");