Subject: sasl-rfc2222bis: multiple authentication attack

From: "Kurt D. Zeilenga" <[email-masked]>
Date: Wed, 25 Feb 2004 15:08:30 -0800

In review of the latest draft, I've come across a number of
technical and editorial concerns which I intend to start a number
of separate threads on.  This is the first.

I am particularity perplexed by the long discussion of the
"Multiple Authentication attack", an attack which only
impacts certain kinds of distributed server implementations,
which seems to overshadow higher risk, more generally
applicable attacks.  I am quite concerned about the
addition of:
>   The protocol profiles that allow for
>   reauthentication SHOULD recommend that another security layer is
>   negotiated once a security layer was installed.

as, if the implementation were subject to this kind of attack,
simply recommending, or even requiring, negotiation of another
security layer does not fully mitigate the risk.  A server which
is subject to such attack should simply refuse re-authentication
(or if the implementor was only concerned about post re-authentication
attacks, it can refuse re-authentications which don't result in
installation of new layers).  It seems odd to address what
is a protocol-neutral security consideration in this fashion (a recommendation upon protocol profiles).

Anyways, as previously discussed (and, I believe, agreed), this issue
is best addressed through statement of security considerations.
Hence, I suggest that all mention of this attack be removed from
section 6.3, including all of section 6.3.1, and that the Security
Consideration mention of 6.3.1 be replaced with (and merged with
the preceding paragraph about distributed server implementations):
    It may be appropriate for a server to disallow all     
    re-authentication, or re-authentications which do not result
    in new security layers being established, involving non-
    (or insufficiently) trusted parties.  In some cases, it may
    be appropriate for the server to require separate connections
    to access information in different protected information spaces.

Kurt