Tuesday, December 6, 2016

Javascript inside svg?

Readers might or might not be aware that an SVG file is really just an XML document. And yes, as you suspected, it allows javascript and viewers are expected to support it, as if that was a sane thing to do. I'm sure this kind of thing is just about as smart as it sounds, in fact I believe I first discovered this foolhardy feature in a forum post displaying some obfuscated javascript inside an svg tag and it was in reference to a XSS attack that was leveraging Facebook.

I had to run a quick test, to see if it was true. I typed the following text to a file with the extension .svg, and the file opened up in internet explorer:

<svg version="1.1" xmlns="http://www.w3.org/2000/svg">
  <script type="text/javascript">alert('Hax!');</script>

Now, I use chrome and keep my internet explorer locked down. I don't know what comes up on other people's machines, but at least mine prompted me, asking if I wanted to allow blocked ActiveX controls. However, chrome runs it no problem and without bothering to prompt me. Joy.

I don't know about you, but I don't want my images to be able to prompt me. It is not hard to imagine a scenario where an svg being served up by a server could contain javascript attempting to access the user's cookies for that server.

While the black hats of the world are busy thinking of new (ab)uses of this technology, it is interesting to consider the creative aspects. If an SVG contains only a JavaScript loop that draws the image, can the image be said to draw itself? Clearly, not in the most literal sense, as rather the image is interpreted, and it is the interpreter that does the drawing. Yet in some sense it does draw itself. At any rate, it can certainly create some intense images in only 1 kB or so.

Naturally, my mind jumped to the idea of making a prime number sieve, as a way to make a complex drawing with only a single loop. After a little bit of playing, I had something that looks a lot like Sieves of Chaos. After a little more tweaking, I had something that looks almost identical to Sieves of Chaos.

I encourage you to try it out for yourself; just copy and paste the below code to Notepad++, and save with a .SVG extension. You will notice the image is 8096 units wide. If your machine can handle it, there is no reason the width couldn't be extended. For some machines, this may already be a crippling number. The effect is very nice, and I even created a spanning desktop background out of it.

<svg version="1.1" xmlns="http://www.w3.org/2000/svg">
 <script type="text/javascript"><![CDATA[
  var width=8096;
  var height=900;
  var ns="http://www.w3.org/2000/svg";
  var svg = document.getElementsByTagName('svg')[0];
  svg.setAttribute("width", width);
  svg.setAttribute("height", height);
  var rect = document.createElementNS(ns, 'rect');
  rect.setAttribute("height", height);
  rect.setAttribute("width", width);
  rect.style.fill = "black";

  for (var b = 2; b < width; b += 1)
   for (var a = 1; a < width; a += b*2)
    var cir = document.createElementNS(ns, 'circle');
    cir.setAttribute("shape-rendering", "geometricPrecision");
    cir.setAttribute("stroke-opacity", "0.07");
    cir.setAttribute("r",  b);
    cir.setAttribute("cx", a+b);
    cir.setAttribute("cy", height/2);
    cir.style.stroke = "red";
    cir.style.strokeWidth = "1";
    cir.style.fill = "none";

Blogger is wise, and does not accept .svg files. There is hope. Here is the .png image that I am using for my background (below). Feel free to use that or render your own with the above code. Be warned, the below png file is 3 MB.


carluchoparis said...

Holy cow! I would LOVE to have a high res version of that image! I'm the creator of sievesofchaos by the way...

I love the fact that the view is completely engulfed in the circles!

Let me know if this is something you'd be willing to share with me.


Carlos Paris

Adam White said...

You can render one yourself. You can just literally copy the markup above, paste into notepad and save with a .svg extension, then open in your favorite browser.

Alternativly, you can download the .svg in a .zip file at the following address: https://gist.github.com/AdamRakaska/6acc80eaf9c5ddae9c881a929bb33fc0/archive/8d4cd28b5f0dd6490216a07151d38e3f12d9209d.zip