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Re: WebID discussion in Debian

From: Olivier Berger <olivier.berger@it-sudparis.eu>
Date: Sat, 18 May 2013 17:04:35 +0200
To: public-webid <public-webid@w3.org>
Cc: Jonas Smedegaard <dr@jones.dk>, Russ Allbery <rra@debian.org>
Message-ID: <8761yggwho.fsf@inf-8657.int-evry.fr>

Olivier Berger <olivier.berger@it-sudparis.eu> writes:

> Hi.
> Jonas Smedegaard <dr@jones.dk> writes:
>> Debian already has PGP-based WoT.  So question remains: how is WebID 
>> relevant for *Debian*?
> For the records, I've provided some feedback about WebID in the
> debian-devel@ thread (and have as well forwarded a mail by Andrei), that
> I hope will have provided more useful details to felow Debian project
> members.

In the following posts on the Debian list, Russ Allbery has challenged
the security of WebID + TLS for authentication. 

Here's below a forward of his post [0], with permission.

I'd be interested in hearing your thoughts on the security issue he
highlights (item 4 below). Maybe this is FAQ ? 

My answer to him [1] was that some signature of the WebID/FOAF document
may be necessary to ensure some trust in the process.

I may have overlooked some recent developments of WebID, so some other
opinions would be interesting on the matter.

Thanks in advance.

[0] http://lists.debian.org/debian-devel/2013/05/msg01067.html
[1] http://lists.debian.org/debian-devel/2013/05/msg01070.html

---- Forwarded ----

obergix@debian.org writes:
> Russ Allbery <rra@debian.org> writes:

>> I'd never heard of WebID before this thread, but looking briefly at the
>> spec, I share Daniel's concerns.  I don't see how this eliminates
>> reliance on the normal CAs.  You still have to do certificate
>> validation to be able to trust the link between URL and keypair, and
>> the WebID protocol provides no way to do that certificate validation
>> other than the normal CA process (and doesn't provide any alternative
>> CA).

> I'm not sure I understand all aspects of the recent evolutions of the
> WebID auth protocols nor the big picture, but my understanding is that
> to auth to a server using a WebID (i.e. a URI pointing to a RDF document
> which declares a SSL cert public key), all that is required is that the
> connecting client owns the corresponding private key.

Here's the security problem in a nutshell (since I'm not sure anyone has
said it outright in this thread): suppose that I am known to a particular
server as <https://www.eyrie.org/~eagle/personal/id#me>.  Suppose an
attacker wishes to authenticate as me.  The attacker would do the

1. Generate a new public/private key pair with that URI in the appropriate
   field so that it looks like a WebID certificate for that URI.

2. Set up a web server that serves the appropriate WebID metadata
   including their new certificate at that URI.

3. Visit the server they wish to attack to trigger the metadata request to
   my identity URI.

4. Hijack that metadata identity request so that it goes to their server
   instead of mine.  This can be done in any number of ways (DNS cache
   poisoning, compromise of www.eyrie.org, compromise of my account on
   www.eyrie.org, TCP active MITM, etc.) depending on the situation.

The server then retrieves the attacker's WebID document, verifies that the
certificates match, and allows the attacker into the system as me.

The only way to prevent this attack in WebID that I see is to either do
leap-of-faith permanent caching (in other words, any server that I
authenticate to caches all my cert information and never lets me change it
subsequently), which is probably an unacceptable loss in functionality, or
to secure the connection to my identity URI.  If that endpoint is
compromised, WebID loses in general (and probably can't be expected to
defend against that).  However, other major authentication systems are at
least robust against DNS poisoning and TCP MITM.

The obvious way to authenticate the connection to www.eyrie.org to
retrieve my metadata is to validate the www.eyrie.org certificate against
a CA, which is where the CA cartel is reintroduced into the picture.

Please note that 4 is not as difficult as it looks, particularly if one of
the goals is to allow more ad hoc servers that are possibly mobile, using
untrusted wireless networks, or the like, rather than hosted in data
centers with locked-down networks and physical security.

Put more succinctly, as I understand the protocol, WebID is only as secure
as the connection from the authenticating server to the metadata URI.

Russ Allbery (rra@debian.org)               <http://www.eyrie.org/~eagle/>

---- /Forwarded ----

Olivier BERGER 
http://www-public.telecom-sudparis.eu/~berger_o/ - OpenPGP-Id: 2048R/5819D7E8
Ingenieur Recherche - Dept INF
Institut Mines-Telecom, Telecom SudParis, Evry (France)
Received on Saturday, 18 May 2013 15:05:03 UTC

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