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

Question

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!";
    });
});

Answer 1

Same-origin policy

You can't access an <iframe> with different origin using JavaScript, it would be a huge security flaw if you could do it. For the same-origin policy browsers block scripts trying to access a frame with a different origin.

Origin is considered different if at least one of the following parts of the address isn't maintained:

protocol://hostname:port/...

Protocol, hostname and port must be the same of your domain if you want to access a frame.

^{NOTE: Internet Explorer is known to not strictly follow this rule, see here for details.}

Examples

Here's what would happen trying to access the following URLs from http://www.example.com/home/index.html

URL                                             RESULT 
http://www.example.com/home/other.html       -> Success 
http://www.example.com/dir/inner/another.php -> Success 
http://www.example.com:80                    -> Success (default port for HTTP) 
http://www.example.com:2251                  -> Failure: different port 
http://data.example.com/dir/other.html       -> Failure: different hostname 
https://www.example.com/home/index.html:80   -> Failure: different protocol
ftp://www.example.com:21                     -> Failure: different protocol & port 
https://google.com/search?q=james+bond       -> Failure: different protocol, port & hostname 

Workaround

Even though same-origin policy blocks scripts from accessing the content of sites with a different origin, if you own both the pages, you can work around this problem using window.postMessage and its relative message event to send messages between the two pages, like this:

• In your main page:

  const frame = document.getElementById('your-frame-id');
  frame.contentWindow.postMessage(/*any variable or object here*/, 'http://your-second-site.com');
  

  The second argument to postMessage() can be '*' to indicate no preference about the origin of the destination. A target origin should always be provided when possible, to avoid disclosing the data you send to any other site.

• In your <iframe> (contained in the main page):

  window.addEventListener('message', event => {
      // IMPORTANT: check the origin of the data! 
      if (event.origin.startsWith('http://your-first-site.com')) { 
          // The data was sent from your site.
          // Data sent with postMessage is stored in event.data:
          console.log(event.data); 
      } else {
          // The data was NOT sent from your site! 
          // Be careful! Do not use it. This else branch is
          // here just for clarity, you usually shouldn't need it.
          return; 
      } 
  }); 
  

This method can be applied in both directions, creating a listener in the main page too, and receiving responses from the frame. The same logic can also be implemented in pop-ups and basically any new window generated by the main page (e.g. using window.open()) as well, without any difference.

Disabling same-origin policy in your browser

There already are some good answers about this topic (I just found them googling), so, for the browsers where this is possible, I'll link the relative answer. However, please remember that disabling the same-origin policy will only affect your browser. Also, running a browser with same-origin security settings disabled grants any website access to cross-origin resources, so it's very unsafe and should NEVER be done if you do not know exactly what you are doing (e.g. development purposes).

• Google Chrome
• Mozilla Firefox
• Safari
• Opera
• Microsoft Edge: not possible
• Microsoft Internet Explorer