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

Charles Pritchard chuck at jumis.com
Mon Dec 5 15:59:14 PST 2011

On 12/5/11 3:34 PM, Chris Marrin wrote:
> 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.

Can this proposal be moved forward on CORS + HTMLMediaElement, 
HTMLImageElement and HTMLCanvasElement?

The proposal would really benefit users and authors on those media 
types, even if it falls short of applying to general HTML elements and 
CSS urls in the first draft.

I realize that it falls short of the lofty goals of the presentation, 
but it would make a good impact and set the stage for further work. It 
seems entirely do-able to disable a:visited on elements that have custom 
filters applied, but, like the timing issues, there needs to be some 
empirical data on risks before moving forward on them.

> 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.  :-)
> -----
> ~Chris
> cmarrin at apple.com

I still love my iPhone. ;-)


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