Re: [secdir] secdir review of draft-saintandre-tls-server-id-check-09

Peter Saint-Andre <stpeter@stpeter.im> Wed, 22 September 2010 18:18 UTC

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Date: Wed, 22 Sep 2010 12:18:53 -0600
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Cc: IETF cert-based identity <certid@ietf.org>, secdir@ietf.org, Barry Leiba <barryleiba.mailing.lists@gmail.com>, IETF discussion list <ietf@ietf.org>, tls@ietf.org
Subject: Re: [secdir] secdir review of draft-saintandre-tls-server-id-check-09
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On 9/22/10 12:14 PM, Jeffrey Hutzelman wrote:
> --On Wednesday, September 22, 2010 12:34:50 PM -0400 Barry Leiba
> <barryleiba.mailing.lists@gmail.com>; wrote:
> 
>> There's a distinction, here, between a protocol and a user interface
>> for configuration.  My mother doesn't know whom to trust, except that
>> she knows that she (at least kinda-sorta) trusts the mail program
>> she's decided to use, and an entity she calls "gmail" (not
>> "google.com", not "gmail.com", but just "gmail").  She's relying to
>> the mail program's "easy configuration feature" to sort this out.
>>
>> The text I reviewed appeared to be saying normative things about what
>> client software MUST and MUST NOT do with regard to this sort of
>> configuration situation, which goes well beyond what the client
>> software is doing on the wire.  Unless I'm mis-reading it, it's
>> explicitly saying that my client software is not allowed to do
>> something like this, for example:
>> 1. Ask the user, "What email service do you use?"
>> 2. Receive the answer "gmail" from the user.
>> 3. Auto-configure itself for the known gmail servers based only on
>> that user input.
> 
> I think that's reasonable behavior _if_ the mail client knows that
> "gmail" is "mail.google.com".  What's _not_ reasonable is for it to
> arbitrarily transform "gmail" into a domain by adding ".com", then look
> up "gmail.com" and see that it is an alias for "mail.google.com" and not
> only follow the (insecure) alias to mail.google.com but also use it to
> decide that "mail.google.com" is an appropriate name to find in a
> certificate.
> 
> If your mother's mail client does that, then all I have to do to steal
> her password is convince said client that "gmail.com" is actually an
> alias for "stealgmailpassword.attacker.org".

In my experience, some user agents have interface elements such as a
drop-down box that lists popular service providers, and the account
configuration wizard behaves differently (e.g., asks for different
information) depending on which popular service provider the user chooses.

Peter

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