full-disclosure - Re: Mutt: failure to check server certificate in SMTP TLS connection

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Date: Tue, 8 Mar 2011 21:09:49 +1100
From: dave b <db.pub.mail@...il.com>
To: full-disclosure@...ts.grok.org.uk
Cc: Joachim Schipper <joachim@...chimschipper.nl>
Subject: Re: Mutt: failure to check server certificate in
 SMTP TLS connection

On 8 March 2011 19:00, Joachim Schipper <joachim@...chimschipper.nl> wrote:
> On Tue, Mar 08, 2011 at 12:36:01PM +1100, dave b wrote:
>> Hi all. It seems that mutt fails to check the validity of a SMTP
>> servers certificate during a TLS connection. In my mutt configuration
>> I have
>>
>> set ssl_starttls = yes
>> set ssl_force_tls = yes
>>
>> However, after performing the steps below I found that mutt did not
>> properly validate the remote servers SMTP tls certificate. This means
>> that an attacker could potentially MITM a mutt user connecting to
>> their SMTP server even when the user has forced a TLS connection.
>>
>> Steps to test this:
>> 1. I set in my hosts file the ip for smtp.gmail.com to be bound to
>> mail.lolok.com
>>
>> in /etc/hosts
>> 74.125.127.109 mail.LOLOK.com
>>
>> 2.Then I changed my
>>
>> set smtp_url = "smtp://MYUSERNAME@...p.gmail.com:587/"
>> to be
>> set smtp_url = "smtp://MYUSERNAME@...l.lolok.com:587/"
>>
>> 3. I opened up mutt and emailed my self. I note that I saw mutt say
>> "connecting to mail.lolok.com".
>>
>> I feel that this is an issue because mutt _does_ actually perform IMAP
>> server certificate validation (at least it did when I last tested it
>> :P).
>
> I'm on the train and not able to test, but the muttrc(5) man page has
>
>       smtp_url
>              Type: string
>              Default: ""
>
>              Defines  the  SMTP  smarthost where sent messages should relayed
>              for delivery. This should take the form of an SMTP URL, e.g.:
>
>
>
>              smtp[s]://[user[:pass]@]host[:port]
>
>
>
>              where "[...]" denotes an optional part.  Setting  this  variable
>              overrides the value of the $sendmail variable.
>
> Note the "[s]". But yes, you should arguably file a documentation-bug
> with the Mutt maintainers, since ssl_starttls does suggest that it works
> for SMTP too.

Oh really?
I'll test it out now!

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