SecurityError: Blocked a frame with origin from accessing a cross-origin frame

I am loading an <iframe> in my HTML page and trying to access the elements within it using Javascript, but when I try to execute my code, I get the following error:

SecurityError: Blocked a frame with origin "http://www.<domain>.com" from accessing a cross-origin frame.

Can you please help me to find a solution so that I can access the elements in the frame?

I am using this code for testing, but in vain:

$(document).ready(function() {
    var iframeWindow = document.getElementById("my-iframe-id").contentWindow;

    iframeWindow.addEventListener("load", function() {
        var doc = iframe.contentDocument || iframe.contentWindow.document;
        var target = doc.getElementById("my-target-id");

        target.innerHTML = "Found it!";
    });
});