Crash Reproduction Series: IE Developer Console UAF

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During a DFIR investigation, using ZecOps Crash Forensics on a developer’s computer we encountered a consistent crash on Internet Explorer 11. The TL;DR is that albeit this bug is not exploitable, it presents an interesting expansion to the attack surface through the Developer Consoles on browsers.

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While examining the stack trace, we noticed a JavaScript engine failure. The type of the exception was a null pointer dereference, which is typically not alarming. We investigated further to understand whether this event can be exploited.

We examined the stack trace below: 

58c0cdba     mshtml!CDiagnosticsElementEventHelper::OnDOMEventListenerRemoved2+0xb
584d6ebc     mshtml!CDomEventRegistrationCallback2<CDiagnosticsElementEventHelper>::OnDOMEventListenerRemoved2+0x1a
584d8a1c     mshtml!DOMEventDebug::InvokeUnregisterCallbacks+0x100
58489f85     mshtml!CListenerAry::ReleaseAndDelete+0x42
582f6d3a     mshtml!CBase::RemoveEventListenerInternal+0x75
5848a9f7     mshtml!COmWindowProxy::RemoveEventListenerInternal+0x1a
582fb8b9     mshtml!CBase::removeEventListener+0x57
587bf1a5     mshtml!COmWindowProxy::removeEventListener+0x29
57584dae     mshtml!CFastDOM::CWindow::Trampoline_removeEventListener+0xb5
57583bb3     jscript9!Js::JavascriptExternalFunction::ExternalFunctionThunk+0x1de
574d4492     jscript9!Js::JavascriptFunction::CallFunction<1>+0x93
[...more jscript9 functions]
581b0838     jscript9!ScriptEngineBase::Execute+0x9d
580b3207     mshtml!CJScript9Holder::ExecuteCallback+0x48
580b2fd3     mshtml!CListenerDispatch::InvokeVar+0x227
57fe5ad1     mshtml!CListenerDispatch::Invoke+0x6d
58194d17     mshtml!CEventMgr::_InvokeListeners+0x1ea
58055473     mshtml!CEventMgr::_DispatchBubblePhase+0x32
584d48aa     mshtml!CEventMgr::Dispatch+0x41e
584d387d     mshtml!CEventMgr::DispatchPointerEvent+0x1b0
5835f332     mshtml!CEventMgr::DispatchClickEvent+0x2c3
5835ce15     mshtml!CElement::Fire_onclick+0x37
583baa8e     mshtml!CElement::DoClick+0xd5
[...]

and noticed that the flow that led to the crash was:

  • An onclick handler fired due to a user input
  • The onclick handler was executed
  • removeEventListener was called

The crash happened at:

mshtml!CDiagnosticsElementEventHelper::OnDOMEventListenerRemoved2+0xb:

58c0cdcd 8b9004010000    mov     edx,dword ptr [eax+104h] ds:002b:00000104=????????

Relevant commands leading to a crash:

58c0cdc7 8b411c       mov     eax, dword ptr [ecx+1Ch]
58c0cdca 8b401c       mov     eax, dword ptr [eax+1Ch]
58c0cdcd 8b9004010000 mov     edx, dword ptr [eax+104h]

Initially ecx is the “this” pointer of the called member function’s class. On the first dereference we get a zeroed region, on the second dereference we get NULL, and on the third one we crash.

Reproduction

We tried to reproduce a legit call to mshtml!CDiagnosticsElementEventHelper::OnDOMEventListenerRemoved2 to see how it looks in a non-crashing scenario. We came to the conclusion that the event is called only when the IE Developer Tools window is open with the Events tab.

We found out that when the dev tools Events tab is opened, it subscribes to events for added and removed event listeners. When the dev tools window is closed, the event consumer is freed without unsubscribing, causing a use-after-free bug which results in a null dereference crash.

Summary

Tools such as Developer Options dynamically add additional complexity to the process and may open up additional attack surfaces.

Exploitation

Even though Use-After-Free (UAF) bugs can often be exploited for arbitrary code execution, this bug is not exploitable due to MemGC mitigation. The freed memory block is zeroed, but not deallocated while other valid objects still point to it. As a result, the referenced pointer is always a NULL pointer, leading to a non-exploitable crash.

Responsible Disclosure

We reported this issue to Microsoft, that decided to not fix this UAF issue.

POC

Below is a small HTML page that demonstrates the concept and leads to a crash.
Tested IE11 version: 11.592.18362.0
Update Versions: 11.0.170 (KB4534251)

<!DOCTYPE html>
<html>
<body>
<pre>
1. Open dev tools
2. Go to Events tab
3. Close dev tools
4. Click on Enable
</pre>
<button onclick="setHandler()">Enable</button>
<button onclick="removeHandler()">Disable</button>
<p id="demo"></p>
<script>
function myFunction() {
    document.getElementById("demo").innerHTML = Math.random();
}
function setHandler() {
    document.body.addEventListener("mousemove", myFunction);
}
function removeHandler() {
    document.body.removeEventListener("mousemove", myFunction);
}
</script>
</body>
</html>

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