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Re: Cross-Origin Resource Embedding Restrictions

From: Adam Barth <w3c@adambarth.com>
Date: Tue, 1 Mar 2011 00:26:01 -0800
Message-ID: <AANLkTikV63cpQ3J-fMpmop33DVN4GrjuL=6YChwYqafe@mail.gmail.com>
To: Maciej Stachowiak <mjs@apple.com>
Cc: Anne van Kesteren <annevk@opera.com>, WebApps WG <public-webapps@w3.org>
On Mon, Feb 28, 2011 at 11:57 PM, Maciej Stachowiak <mjs@apple.com> wrote:
> For what it's worth, I think this is a useful draft and a useful technology. Hotlinking prevention is of considerable interest to Web developers, and doing it via server-side Referer checks is inconvenient and error-prone. I hope we can fit it into Web Apps WG, or if not, find another goo home for it at the W3C.
> One thing I am not totally clear on is how this would fit into CSP. A big focus for CSP is to enable site X to have a policy that prevents it from accidentally including scripts from site Y, and things of that nature. In other words, voluntarily limit the embedding capabilities of site X itself But the desired feature is kind of the opposite of that. I think it would be confusing to stretch CSP to this use case, much as it would have been confusing to reuse CORS for this purpose.

There's been a bunch of discussion on the public-web-security mailing
list about the scope of CSP.  Some folks think that CSP should be a
narrow feature targeted at mitigating cross-site scripting.  Other
folks (e.g., as articulated in
<http://w2spconf.com/2010/papers/p11.pdf>) would like to see CSP be
more of a one-stop shop for configuring security-relevant policy for a
web site.

From-Origin is closely related to one of the proposed CSP features,
namely frame-ancestors, which also controls how the given resource can
be embedded in other documents:


Aside from the aesthetic questions, I'd imagine folks will want to
include a list of permissible origins in the From-Origin header (or
else they'd have to give up caching their resources).  CSP already has
syntax, semantics, and processing models for lists of origins,
including wildcards.  At a minimum, we wouldn't want to create a
gratuitously different syntax for the same thing.


> On Feb 28, 2011, at 11:35 PM, Anne van Kesteren wrote:
>> The WebFonts WG is looking for a way to prevent cross-origin embedding of fonts as certain font vendors want to license their fonts with such a restriction. Some people think CORS is appropriate for this, some don't. Here is some background material:
>> http://weblogs.mozillazine.org/roc/archives/2011/02/distinguishing.html
>> http://annevankesteren.nl/2011/02/web-platform-consistency
>> http://lists.w3.org/Archives/Public/public-webfonts-wg/2011Feb/0066.html
>> More generally, having a way to prevent cross-origin embedding of resources can be useful. In addition to license enforcement it can help with:
>> * Bandwidth "theft"
>> * Clickjacking
>> * Privacy leakage
>> To that effect I wrote up a draft that complements CORS. Rather than enabling sharing of resources, it allows for denying the sharing of resources:
>> http://dvcs.w3.org/hg/from-origin/raw-file/tip/Overview.html
>> And although it might end up being part of the Content Security Policy work I think it would be useful if publish a Working Draft of this work to gather more input, committing us nothing.
>> What do you think?
>> Kind regards,
>> --
>> Anne van Kesteren
>> http://annevankesteren.nl/
Received on Tuesday, 1 March 2011 08:27:06 UTC

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