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RE: Access Control Spec and Cookies

From: Sunava Dutta <sunavad@windows.microsoft.com>
Date: Thu, 13 Mar 2008 20:57:31 -0700
To: Marc Silbey <marcsil@windows.microsoft.com>, Arthur Barstow <art.barstow@nokia.com>, Maciej Stachowiak <mjs@apple.com>, Adele Peterson <adele@apple.com>, Doug Stamper <dstamper@exchange.microsoft.com>, Anne van Kesteren <annevk@opera.com>, "public-appformats@w3.org" <public-appformats@w3.org>, David Ross <dross@windows.microsoft.com>, Chris Wilson <Chris.Wilson@microsoft.com>, Eric Lawrence <ericlaw@exchange.microsoft.com>, Zhenbin Xu <zhenbinx@windows.microsoft.com>
Message-ID: <B3AE6274B5AA584AAADD090DD30E5A0D33AB8B196D@NA-EXMSG-W602.wingroup.windeploy.ntdev.microsoft.com>

Thanks Marc.
I do agree, this sounds like a wise plan and is a good start. Sending cookies cross domain always carries the risk of information disclosure, privacy threats and the ability for cross site request forgery attacks.
The proposal to send cookies when the user approves is much better, however, as Jonas mentions building a model that empowers the user here is challenging to say the least.
Some of the tricks around user allowing cookies include indicating to the user the information necessary know what the cookie is for?
....For example, I may trust a site for search but I may not necessarily trust the same site with my Social Network site Cookies.
.....This leads to the question, how is the Cookie's content/purpose presented to the user. Especially considering that it would need to work for all users, not power users. Should we assume that cookies can be sent if the user 'Trusts the site'?

We can work together here to see if we can build UI that gives the user enough information for a good decision, otherwise it seems defaulting to the simplest and safe solution is the best.
Maybe we need a conference call to discuss?

Dropping HTTP credentials is also recommended for the same reasons since they can give arise to CSRF attacks as well. (You can be logged into a site and cached credentials may be sent across domains now).

-----Original Message-----
From: Marc Silbey
Sent: Friday, February 29, 2008 2:30 PM
To: Arthur Barstow; Maciej Stachowiak; Adele Peterson; Doug Stamper; Anne van Kesteren; public-appformats@w3.org
Cc: Sunava Dutta; Chris Wilson
Subject: RE: Access Control Spec and Cookies

Thank you for asking Art.

We agree with stripping user data like Cookies from cross-site requests to protect user privacy. I'm adding Sunava who is our local expert on cross domain communication.

-----Original Message-----
From: Arthur Barstow [mailto:art.barstow@nokia.com]
Sent: Friday, February 29, 2008 1:25 PM
To: Maciej Stachowiak; Adele Peterson; Marc Silbey; Doug Stamper; Anne van Kesteren; public-appformats@w3.org
Subject: Access Control Spec and Cookies

Browser Vendors, Maciej, Adele, Marc, Doug, Anne,

There has been a rather lengthy discussion regarding the Access
Control for Cross-site Requests spec and Cookies, starting with this
thread from Jonas:


Due to concerns about exposing a user's cookies, Mozilla decided they
will not send cookies in this model:


We are all very interested in other browser vendors' position on this
issue. Would you please share your view on this?

Regards, Art Barstow

Begin forwarded message:

> Resent-From: public-appformats@w3.org
> From: "ext Jonas Sicking" <jonas@sicking.cc>
> Date: February 26, 2008 7:44:32 AM EST
> To: Anne van Kesteren <annevk@opera.com>
> Cc: Brad Porter <bwporter@yahoo.com>, Daniel Veditz
> <dveditz@mozilla.com>, "WAF WG (public)" <public-
> appformats@w3.org>, Window Snyder <window@mozilla.com>, Brandon
> Sterne <bsterne@mozilla.com>, Jesse Ruderman <jruderman@gmail.com>
> Subject: Re: To cookie or not to cookie
> Archived-At: <http://www.w3.org/mid/47C409B0.3040706@sicking.cc>
> Anne van Kesteren wrote:
>> I'd like to see an update on this from the Mozilla folks. I think
>> if cookies are not part of the request we should simply nuke the
>> whole idea.
> So we had another (and hopefully final :) ) security meeting at
> mozilla today. I'll post a separate mail about the other issues
> that came up as the only really big thing is the cookie issue.
> So the prevailing opinion was that sending cookies without getting
> the users consent is simply too easy to misunderstand. A server
> that sends private data based on cookie information and include a
> rule like
> allow <linkedin.com>
> has essentially just sent all the private data served on that URI
> over to linkedin, without getting the users consent. And of course
> this becomes many times worse if the rule is allow <*>. At that
> point basically any private data for anyone to read.
> While it can definitely be argued that the server should ask the
> users consent first, just as it does before selling personal data
> to other 3rd parties this seems like a much easier mistake to make.
> Sending all your users personal information to a 3rd party like
> linkedin requires an active action. Just adding a header to your
> responses in order to allow mashups requires much less thinking.
> There are three parties involved in this transaction. The user, the
> requesting site and the target site. The spec clearly enforces that
> the latter two parties are ok with this transaction. But asking the
> user is left as a responsibility to the target site.
> Unfortunately we are not convinced that all sites will get this
> right. Especially given the ease at which this spec can be deployed.
> So we have decided that we do not want to include cookies in the
> request.
> So at this point there are a few ways forward:
> 1. We can leave the spec as is and say that mozilla is intentionally
>    only implementing a subset of the spec at this point.
> I'm not at all exited about this idea. It very much increases the
> risk that server administrators will wrongly configure their
> servers such that private user data will be wrongly exposed if
> another UA implements access-control and do send cookies. This
> includes both other browsers, and later versions of firefox.
> If it comes to this we will likely simply drop support for access-
> control for firefox 3 in order to not hinder deployment by other
> vendors of the full spec.
> 2. We can change the spec to say that cookies should never be sent.
> This would support the very common usecase of the data hosted on
> the target site not being personal at all. Such as the ability to
> fetch the latest ads on craigslist.org, or fetch the directions to
> a destination from google maps.
> But I'm not exited about this idea as sending cookies and auth
> headers does have several security advantages when fetching private
> data. Such as never having to expose any credentials to the
> requesting site, and having built-in protection against distributed
> dictionary attacks. Something that won't be possible if the
> credentials have to be included in the request body.
> 3. We can change the spec to allow for requests both that include
>    cookies, and requests that don't include them. We'd further say
>    that before the UA makes a request that do include cookies it
>    should get the users permission to do so first.
> This would support the very common usecase of the data hosted on
> the target site not being personal at all. Such as the ability to
> fetch the latest ads on craigslist.org, or fetch the directions to
> a destination from google maps.
> I think this could be a very interesting option, if done right. The
> "how to ask the user for permission" part is tricky, but I think
> doable. And it's something that the spec wouldn't have to work out
> in detail, but can be left up to the UA.
> The issue of how to determine if the request should be done with or
> without cookies is something we would need to specify though. One
> solution would be to say that unless the UA has any prior knowledge
> (from for example a previous session), it should first make a
> request that does not include cookies. If that request is denied
> the UA should ask the user and then, if granted permission, do a
> request that includes cookies.
> This is very similar to how http authentication is usually done.
> Do note that I'm prepared to go with any of the above three
> options. If we really don't want to change the spec we are
> perfectly happy with holding off on this feature for a future
> firefox release.
> If we go with 3, note that for the next firefox release we would
> then act as if the user always denies the request to send cookies.
> Implementing UI to ask the user is not going to happen for this
> release. Nothing would prevent it from happening next release though.
>> One thing that might be worth considering is adopting the policy
>> Safari and Internet Explorer have for cookies. That is not
>> accepting third-party cookies, but always including cookies in the
>> request. Then again, there are already tracking methods without
>> cookies and are actively being used (Hixie pointed out paypal +
>> doubleclick on IRC) so I'm not sure whether complicated cookie
>> processing models are worth it at all.
> That wouldn't actually change anything at all. The major concern is
> sites sending replies that contain the users private data to GET
> requests that include cookies. This will happen even if the reply
> can't set additional cookies.
> The third-party-cookie blocker thing is mostly there to (poorly)
> stop sites from tracking a user across multiple sites.
> I realize a lot of people are probably going to be disappointed
> with this decision, me included. But I hope we can find a way
> forward that minimizes the disappointment, even if that includes
> removing support for any of this from the next firefox release.
> / Jonas
Received on Friday, 14 March 2008 03:58:06 UTC

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