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Re: [cors] Allow-Credentials vs Allow-Origin: * on image elements?

From: Charlie Reis <creis@chromium.org>
Date: Wed, 7 Jul 2010 16:46:42 -0700
Message-ID: <AANLkTikWiGVENyw0waIsLsL6oGGEcwQO0MmwNASCyANk@mail.gmail.com>
To: Devdatta Akhawe <dev.akhawe@gmail.com>
Cc: "Mark S. Miller" <erights@google.com>, Anne van Kesteren <annevk@opera.com>, public-webapps@w3.org
On Wed, Jul 7, 2010 at 4:14 PM, Devdatta Akhawe <dev.akhawe@gmail.com>wrote:

> > Because it's undesirable to prevent the browser from sending cookies on
> an
> > <img> request,
>
> Why ? I can understand why you can't do it today - but why is this
> undesirable even for new applications? Ad tracking ?
>
> ~devdatta
>
>
I meant "undesirable" in that it will require much deeper changes to
browsers.

I wouldn't mind making it possible to request an image or other subresource
without cookies, but I don't think there's currently a mechanism for that,
is there?  And if there's consensus that user agents shouldn't send cookies
at all on third party subresources, I'm ok with that, but I imagine there
would be pushback on that sort of proposal-- it would likely affect
compatibility with existing web sites.  I haven't gathered any data on it,
though.

The benefit to allowing * with credentials is that it lets CORS work with
the existing browser request logic for images and other subresources, where
cookies are currently sent with the request.

Charlie



> On 7 July 2010 16:11, Charlie Reis <creis@chromium.org> wrote:
> >
> >
> > On Wed, Jul 7, 2010 at 4:04 PM, Mark S. Miller <erights@google.com>
> wrote:
> >>
> >> On Wed, Jul 7, 2010 at 1:09 PM, Charlie Reis <creis@chromium.org>
> wrote:
> >> [...]
> >>>
> >>> That's unfortunate-- at least for now, that prevents servers from
> echoing
> >>> the origin in the Access-Control-Allow-Origin header, so servers cannot
> host
> >>> "public" images that don't taint canvases.  The same problem likely
> exists
> >>> for other types of requests that might adopt CORS, like fonts, etc.
> >>
> >> Why would public images or fonts need credentials?
> >
> > Because it's undesirable to prevent the browser from sending cookies on
> an
> > <img> request, and the user might have cookies for the image's site.
>  It's
> > typical for the browser to send cookies on such requests, and those are
> > considered a type of credentials by CORS.
> > Charlie
> >
> >>
> >>
> >>>>
> >>>> I believe the plan is to change HTML5 once CORS is somewhat more
> stable
> >>>> and use it for various pieces of infrastructure there. At that point
> we can
> >>>> change <img> to transmit an Origin header with an origin. We could
> also
> >>>> decide to change CORS and allow the combination of * and the
> credentials
> >>>> flag being true. I think * is not too different from echoing back the
> value
> >>>> of a header.
> >>>>
> >>>
> >>> I would second the proposal to allow * with credentials.  It seems
> >>> roughly equivalent to echoing back the Origin header, and it would
> allow
> >>> CORS to work on images and other types of requests without changes to
> HTML5.
> >>> Thanks,
> >>> Charlie
> >>
> >>
> >>
> >> --
> >>     Cheers,
> >>     --MarkM
> >
> >
>
Received on Wednesday, 7 July 2010 23:49:06 GMT

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