Re: [secdir] [xmpp] SecDir review of draft-ietf-xmpp-3920bis-17

Peter Saint-Andre <stpeter@stpeter.im> Wed, 03 November 2010 21:53 UTC

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Cc: Security Area Directorate <secdir@ietf.org>, "draft-ietf-xmpp-3920bis.all@tools.ietf.org" <draft-ietf-xmpp-3920bis.all@tools.ietf.org>, XMPP Working Group <xmpp@ietf.org>, The IESG <iesg@ietf.org>, Matthew Wild <mwild1@gmail.com>
Subject: Re: [secdir] [xmpp] SecDir review of draft-ietf-xmpp-3920bis-17
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On 11/3/10 3:03 PM, Kurt Zeilenga wrote:
> 
> On Nov 3, 2010, at 11:10 AM, Matthew Wild wrote:
> 
>> On 3 November 2010 17:31, Peter Saint-Andre <stpeter@stpeter.im>;
>> wrote:
>>> 
>> 
>>> as described under Section 13.7.2.1 (however, the lack of
>>> channel binding in the PLAIN mechanism implies that even
>>> authenticated TLS cannot fully protect the SASL negotiation and
>>> subsequent communications when PLAIN is used).
>>> 
>> 
>> I'm no expert on this, but is this text technically true? I
>> thought channel binding would only be beneficial where there is not
>> full TLS authentication.
> 
> Channel binding is beneficial regardless of whether there the client
> performed TLS server cert/subject checks.  Note that the server has
> no clue what checks, if any, the client (or its user) performed.
> With channel bindings, it becomes irrelevant to the server whether
> the client performed these checks or not.  That is, (amongst other
> things) channel bindings provide a means for the server itself to
> protect against MITM attacks.
> 
> I do agree, however, that he wording of the however comment is
> confusing and seems should either be removed or replaced.

Removed in my working copy.

> My main concern is that Unauthenticated TLS is subject to downgrade
> to PLAIN by two vectors, spoofing mechanisms and spoofing transition
> required.
> 
> Removing <transition-needed/> (in favor of a yet to be specified
> extension) addresses one vector.
> 
> There seems no good way to address the other vector at this time.  I
> do suggest however that the 13.9.4 text: To help prevent this attack,
> the parties SHOULD protect the channel using TLS before attempting
> SASL negotiation. be replaced with: To mitigate this attack, the
> partied SHOULD protect the channel using TLS before attempting SASL
> negotiation and either perform full certificate validation as
> described in Section 13.7.2.1 or utilize a mechanism which provides
> channel bindings, such as SCRAM-SHA-1-PLUS.

Text incorporated, with minor tweaks.

Peter

-- 
Peter Saint-Andre
https://stpeter.im/