CONSOLE ERROR: Uncaught SecurityError: Failed to read a named property 'document' from 'Window': Blocked a frame with origin "http://127.0.0.1:8000" from accessing a frame with origin "http://localhost:8000". Protocols, domains, and ports must match.
This page opens a window to "", injects malicious code, and then uses window.open.call to set its opener to the victim. The opened window then tries to scripts its opener.
Code injected into window:
<script>function write(target, message) { target.document.body.innerHTML = message; }
setTimeout(function() {write(window.opener.top.frames[0], 'FAIL: XSS was allowed.');}, 100);
setTimeout(function() {write(window.opener.top.frames[1], 'SUCCESS: Window remained in original SecurityOrigin.');}, 200);
setTimeout(function() { window.opener.top.postMessage('done', '*'); }, 300);</script>
PASS: 'window.open' called on another frame threw: SecurityError: Blocked a frame with origin "http://127.0.0.1:8000" from accessing a cross-origin frame.

 

--------
Frame: '<!--framePath //<!--frame0-->-->'
--------
This page doesn't do anything special.

--------
Frame: '<!--framePath //<!--frame1-->-->'
--------
SUCCESS: Window remained in original SecurityOrigin.