[webkit-dev] Timing attacks on CSS Shaders (was Re: Security problems with CSS shaders)

Chris Marrin cmarrin at apple.com
Mon Dec 5 15:34:25 PST 2011

On Dec 5, 2011, at 11:32 AM, Adam Barth wrote:

> On Mon, Dec 5, 2011 at 10:53 AM, Chris Marrin <cmarrin at apple.com> wrote:
>> To be clear, it's not the difference between white and black pixels, it's
>> the difference between pixels with transparency and those without.
> Can you explain why the attack is limited to distinguishing between
> black and transparent pixels?  My understanding is that these attacks
> are capable of distinguishing arbitrary pixel values.

This is my misunderstanding. I was referring to the attacks using WebGL, which measure the difference between rendering alpha and non-alpha pixels. But I think there is another, more dangerous attack vector specific to CSS shaders. Shaders have the source image (the image of that part of the page) available. So it is an easy thing to make a certain color pixel take a lot longer to render (your "1000x slower" case). So you can easily and quickly detect, for instance, the color of a link. 

So I take back my statement that CSS Shaders are less dangerous than WebGL. They are more!!! As I've said many times (with many more expletives), I hate the Internet.

I think the solution is clear. We should create a whole new internet where we only let in people we trust.  :-)

cmarrin at apple.com

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