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 navigates its opener to the victim. The opened window then tries to scripts its opener after document.writeing a new document.
Code injected into window:
<script>document.write('<script>function write(target, message) { target.document.body.innerHTML = message; }setTimeout(function() {write(window.opener, \'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>');</script>

 

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

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