[whatwg] font security on measureText

Ian Hickson ian at hixie.ch
Fri Jun 7 16:08:30 PDT 2013

On Thu, 2 May 2013, Rik Cabanier wrote:
> The canvas 2d spec currently states that a font has to be local in order 
> for measureText to work: [1][2]
> If doing these measurements requires using a font that has an origin 
> that is not the same as that of the Document object that owns the canvas 
> element (even if "using a font" means just checking if that font has a 
> particular glyph in it before falling back to another font), then the 
> method must throw a SecurityError exception.
> Does anyone know what this is?

If you import a font from another origin (without CORS), then this 
prevents you from reading its metrics.

> It seems to us, that if the font is available to CSS (depending on if 
> the browser supports CORS for fonts), it should be fine to call 
> measureText.

Only if CSS prevents cross-origin font loads.

On Thu, 2 May 2013, Rik Cabanier wrote:
> So, if my css specifies a font:
> @font-face {
> font-family: "Bitstream Vera Serif Bold";
> src: url("http://developer.mozilla.org/@api/deki/files/2934/=VeraSeBd.ttf");
> }
> it seems that the origin of the font is not the same as the document so 
> measureText won't work. (I'm not that familiar with the verbiage so I 
> might be mistaken)

Right, that's the idea.

On Fri, 3 May 2013, Boris Zbarsky wrote:
> The text at 
> http://dev.w3.org/csswg/css-fonts/#default-same-origin-restriction and 
> http://dev.w3.org/csswg/css-fonts/#allowing-cross-origin-font-loading 
> predates your introduction of the mode values, but clearly corresponds 
> to the "CORS" mode, no?
> And while browsers are not aligned yet, they did plan to align last I 
> heard, in that their representatives in the WG had agreed to the above 
> text.
> Of course it's possible some of the browsers involved are just planning 
> to ignore the spec altogether without bothering to argue to get it 
> changed to what they think is the right thing.

If browsers align on the above text the HTML spec indeed would no longer 
need to worry about this, since there'd no longer be any cross-origin 
fonts. Has this occurred?

(Personally I don't really see why we'd limit this to same-origin and 
CORS-cross-origin only. It makes loading fonts from other origins a pain.)

On Mon, 6 May 2013, Rik Cabanier wrote:
> I logged https://www.w3.org/Bugs/Public/show_bug.cgi?id=21943

In the future, please note that there's no need to file a bug for topics 
raised on the WHATWG list -- just one or the other is fine.

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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