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Re: [cors] unaddressed security concerns

From: Maciej Stachowiak <mjs@apple.com>
Date: Mon, 12 Oct 2009 08:19:27 -0700
Cc: "Mark S. Miller" <erights@google.com>, Anne van Kesteren <annevk@opera.com>, "Henry S. Thompson" <ht@inf.ed.ac.uk>, Jonas Sicking <jonas@sicking.cc>, Arthur Barstow <Art.Barstow@nokia.com>, public-webapps <public-webapps@w3.org>
Message-id: <5D7511B2-DA9D-40AF-A536-D799FB6EEFD8@apple.com>
To: Maciej Stachowiak <mjs@apple.com>

On Oct 12, 2009, at 7:04 AM, Maciej Stachowiak wrote:

> On Oct 9, 2009, at 4:36 PM, Mark S. Miller wrote:
>> The last of the links above should make the application to CORS
>> concrete. See also the dismissive replies which followed in that
>> thread. If you find these dismissals plausible, please imagine back  
>> to
>> the world in which CSRF was first diagnosed (second bullet above) as
>> ask if CSRFs would have also seemed merely theoretical back then? In
>> both cases, the answer "well don't do that" seems to make sense on
>> first analysis for the same reasons.
> Tyler claimed the vulnerability in his example could be avoided by  
> not identifying the origin of a cross-site request, but he did not  
> explain how. For example, if photo.example.com instead used a shared  
> secret to identify itself to strage.example.com, the exact same  
> scenario could occur.
> I would argue that the vulnerability here is created by using a URI  
> to identify an access-controlled resource on a third-party site.  
> Some ways to safely perform the transaction include:
> 1) printer.example.net asks storage.example.org for a one-time write  
> access token for its per-user job spool (perhaps indicating that  
> photo.example.com is the only site authorised to use it), and hands  
> it to photo.example.com, which then uses it to write. It can't tell  
> photo.example.com to overwrite a different photo, because it cannot  
> acquire a write access token for that photo. CORS can help implement  
> this approach - it knows which part of a user's space is writable to  
> printer.example.net, and can use the Origin header to decide that  
> printer.example.net
> 2) storage.example.org can offer a "conditional write" service that  
> copies a file only if all parties listed in the request, plus those  
> listed in Origin, have write access. Then when photo.example.com  
> uploads a photo for spooling on behalf of printer.example.net, it  
> can list printer.example.net as one of the domains that must have  
> write access.
> Either of these approaches works fine with CORS. Thus, I do not see  
> how the scenario argues against providing the Origin header.
> I am sure there are solutions that do not use the Origin header at  
> all, solely using one-time token schemes. However, popular one-time  
> token schemes for cross-origin networking typically require a shared  
> secret, and thus cannot be done client-to-server. Thus, no form of  
> cross-origin client-side networking is helpful for implementing them.
> CORS is helpful in these kinds of multi-party scenarios because it  
> allows you to bootstrap a secret token scheme from the client side,  
> by using the combination of Origin and a user credential such as a  
> cookie to grant the initial token, based on the user's pre-existing  
> access grants, and without the need for a shared secret between the  
> two sites that are party to the transaction.

As a side note, I should add that Tyler's scenario would be much  
simpler overall if printer.example.net used a grant of read access to  
the photo it wants to print on storage.example.org, instead of  
granting write access to a new location. Or it can just ask  
photo.example.com to read the photo from storage.example.org and send  
the actual data. Either of these much less abusable than writing, and  
would be a safer approach, whether or not the overall design depends  
on secret tokens. The grant of read access can be one-time or time- 
limited if necessary. Thus, I think the scenario is somewhat  
contrived. Likely cross-site interactions would not involve such a  
complex process to transfer data. The root of the vulnerability in  
Tyler's scenario is writing in a shared namespace to transfer data,  
instead of simply granting read access to the existing copy, or  
transferring the actual data bits.

Received on Monday, 12 October 2009 15:20:03 UTC

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