[whatwg] Canvas and drawWindow

[Jonas Sicking]

And this is why it's a bad idea to separate "right to embed" from
"right to read" :(

http://weblogs.mozillazine.org/roc/archives/2011/02/distinguishing.html

/ Jonas

On Mon, Mar 14, 2011 at 5:52 PM, Gregg Tavares (wrk) <gman at google.com> wrote:
> Someone pointed out that once you have HTML5->Canvas->WebGL, even though you
> can't call readPixels or toDataURL or getImageData because of cross origin
> issues you can write a shader that takes longer depending on the color and
> then just time draw calls to figure out what's in the texture.
>
> In other words, if you want to prevent security issues you could only do
> this on same origin content.
>
> But then you open another can of worms. Once you can put content in a
> texture you want to be able to let the user interact with it (like they can
> with 3d css) but then you run into the issue that you don't know what the
> user's shaders are doing so you have to let JavaScript translate mouse
> coordinates which is probably another security issue on top of being a PITA
> to implement.
>
>
> On Fri, Mar 11, 2011 at 8:35 AM, Erik Möller <emoller at opera.com> wrote:
>
>> I bet this has been discussed before, but I'm curious as to what people
>> think about breathing some life into a more general version of Mozillas
>> canvas.drawWindow() that draws a snapshot of a DOM window into the canvas?
>> https://developer.mozilla.org/en/drawing_graphics_with_canvas#section_9
>>
>> I know there are some security considerations (for example listed in the
>> source of drawWindow):
>>
>>  // We can't allow web apps to call this until we fix at least the
>>  // following potential security issues:
>>  // -- rendering cross-domain IFRAMEs and then extracting the results
>>  // -- rendering the user's theme and then extracting the results
>>  // -- rendering native anonymous content (e.g., file input paths;
>>  // scrollbars should be allowed)
>>
>> I'm no security expert, but it seems to me there's an easy way to at least
>> cater for some of the use-cases by always setting origin-clean to false when
>> you use drawWindow(). Sure it's a bit overkill to always mark it dirty, but
>> it's simple and would block you from reading any of the pixels back which
>> would address most (all?) of the security concerns.
>>
>> I'm doing a WebGL demo, so the use-case I have for this would be to render
>> a same-origin page to a canvas and smack that on a monitor in the 3d-world.
>> Intercept mouse clicks, transform them into 2d and passing them on would of
>> course be neat as well and probably opens up the use-cases you could dream
>> up.
>>
>> So, I'm well aware its a tad unconventional, but perhaps someone has a
>> better idea of how something like this could be accomplished... i.e. via SVG
>> and foreignObject or punching a hole in the canvas and applying a transform
>> etc. I'd like to hear your thoughts.
>>
>> --
>> Erik Möller
>> Core Developer
>> Opera Software
>>
>