Re: [xmppwg] Last Call: 'XMPP Core' to Proposed Standard

["Kurt D. Zeilenga" <Kurt@OpenLDAP.org>]

At 01:28 PM 10/8/2003, Eric Rescorla wrote:
>"Kurt D. Zeilenga" <Kurt@OpenLDAP.org> writes:
>> I concur with Alexey's recent comments regarding XMPP's SASL
>> Profile.  I have a few addition concerns regarding this profile.
>> The scope of my review was, due to time constraints, was limited
>> to the profile.
>> 
>> I not sure why the documents says that SASL and TLS security
>> layers SHOULD NOT be enabled simultaneously (Section 6.2, rule 2),
>> but this recommendation is, I believe, flawed.  There are numerous use
>> cases where implementations SHOULD establish additional layers.
>> For example, establishing additional SASL layers may prevent
>> certain kinds of tunneling man-in-the-middle attacks.
>Could you describe some of them?

XMPP's TLS profile allows establishment of TLS even though the
implementation's certificate verification has failed:
  If the above methods fail, the certificate SHOULD be presented
  to a human (e.g., an end user or server administrator) for
  approval; if presented, the receiver MUST deliver the entire
  certificate chain to the human, who SHOULD be given the option
  to store the Root CA certificate (not the service or End Entity
  certificate) and to not be queried again regarding acceptance of
  the certificate for some reasonable period of time.

Given the likelihood that the end user will inappropriately
accept the certificate, attackers can easily insert a
TLS-protected machine in the middle.  Given this, it is quite
reasonable for implementors to offer (and deployers to demand)
additional safeguards.

Kurt