#22: RFC 4409 Interop problem: SMTP submission, STARTTLS, AUTH EXTERNAL
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Reporter: sm+i...@… | Owner:
Type: defect | Status: new
Priority: major | Milestone:
Component: pre-evaluation-template | Version:
Severity: - | Keywords:
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Interop issue posted by Chris Newman on April 30.
I've been dealing with client certificate authentication lately and
realized we have an interoperability problem with our standards. For SMTP
submission, when an end-user wishes to authenticate using a client
certificate, the protocol sequence to do so is unclear.
This sequence works assuming the server enables SASL EXTERNAL and the
client implements it (not true of all deployed software):
====
C: EHLO ...
S: ...
C: STARTTLS
S: 220 2.5.0 Go ahead with TLS negotiation.
<negotiate TLS with client certificate>
C: EHLO ...
S: ...
S: 250-AUTH EXTERNAL ...
S: ...
C: AUTH EXTERNAL =
S: 235 2.7.0 EXTERNAL authentication successful.
C: MAIL FROM:<[email protected]>
C: RCPT TO:<[email protected]>
<relay is allowed>
====
This sequence of commands may or may not work but RFC 4409 section 4.3 can
be interpreted to permit it assuming a client certificate via TLS counts
as "independently established authentication":
====
C: EHLO ...
S: ...
C: STARTTLS
S: 220 2.5.0 Go ahead with TLS negotiation.
<negotiate TLS with client certificate>
C: EHLO ...
S: ...
C: MAIL FROM:<[email protected]>
C: RCPT TO:<[email protected]>
<relay may or may not be allowed>
====
An SMTP submission client that implements only the latter will not
interoperate with an SMTP submission server that implements only the
former (and possibly vice versa if the client is unwilling to "just try
it" when AUTH EXTERNAL isn't there). One of these two cases must be
interpreted or declared not valid by the standards to resolve the
interoperability problem.
Option 1: relevant to YAM -- Clarify 4409 section 4.3 to state that
providing client authentication during TLS does not constitute
"independently established authentication" because there is no indication
in the TLS layer whether that authentication was deemed acceptable for
SMTP submission authentication. Perhaps note as an informational
reference that "AUTH EXTERNAL" can be used to determine the validity for
that purpose.
Pros: Doesn't alter the way SMTP relay authentication works (probably good
since relay authentication is typically manually configured trusted
peers). It's a clarification of a vague sentence. Consistent with
IMAP+STARTTLS and POP+STARTTLS. Cons: some SMTP submission clients and
servers that could be interpreted as compliant are now considered
incompliant. This change would occur in YAM and some could argue it
should derail 4409 moving to full standard.
Option 2: Incompatible change to RFC 3207 (SMTP STARTTLS) when an SMTP
client provides a client certificate the server deems valid for
authentication purposes, the server MUST enable the SASL EXTERNAL
mechanism (advertising it in EHLO and allowing it in AUTH). If the client
issues "MAIL FROM" without issuing an AUTH command in this situation, the
server MUST behave as if an implicit "AUTH EXTERNAL =" was issued by the
client.
Pros: Doesn't break any clients. Cons: Some previously complaint SMTP
STARTTLS server implementations are declared incompliant (including the
shipping product from my company ;-).
Option 3: Incompatible change to RFC 3207 (SMTP STARTTLS) to declare that
client authentication provided via STARTTLS is not considered valid
authentication at the SMTP layer until the client issues an "AUTH
EXTERNAL" command.
Pros: Consistent with IMAP+STARTTLS and POP+STARTTLS. Cons: Some
previously compliant SMTP STARTTLS client and server implementations are
declared incompliant.
--
Ticket URL: <http://trac.tools.ietf.org/wg/yam/trac/ticket/22>
yam <http://tools.ietf.org/yam/>
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