Wednesday, January 19, 2011

A harmless SVG + XSLT curiousity

How do you execute code in a turing complete language via the <img> tag? Why, by combining an XSL transform into an SVG image of course!

I stumbled across this old file in my archives:

http://cevans-app.appspot.com/static/expensive_xsl_svg.html

If you run it e.g. in Chrome, it'll consume a load of CPU (and subsequently memory if you let it crank). I expect it'll do the same in any WebKit browser, and Opera's error message implies it has all the pieces to follow suit if I tweaked the file a bit.

It's not a significant security issue, but it's an interesting quirk. It works because SVG and XSL are both XML formats, and XSL can use a self-referential construct to operate on itself as the input document:

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="#stylesheet"?>

If the XSL output XML is valid SVG syntax, then it will render. So you can probably pull some crazy tricks to generate a complicated SVG on the fly! My sample file doesn't get that far; it simply deliberately runs an expensive stylesheet transform with a large output.

If anyone wanted to play with this, there may be interesting issues with the unusual context the XSL is executing in. What if you used xsl:import or the document() XPath function? What origin is used for security checks?, etc.

2 comments:

  1. The loads should all fail in WebKit because SVG images don't have Frames, which means they're not connected to the loader machinery.

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  2. This curiosity may be "harmless" on modern systems, but on my old system with only 1 GB RAM, Chrome quickly ate all available memory and swap, taking down the system with it. Even if it doesn't allow arbitrary code execution, I think that a 940 byte SVG-based denial of service attack can cause quite a bit of damage.

    I guess the only real fix to resource-gobbling DoS attacks is having the browser put a resource limit on each tab and freeze it when it goes over. A popup would then let the user decide between killing the tab and letting it use more resources.

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