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

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Date: Tue, 8 Mar 2011 09:00:05 +0100
From: Joachim Schipper <joachim@...chimschipper.nl>
To: full-disclosure@...ts.grok.org.uk
Subject: Re: Mutt: failure to check server certificate in
 SMTP TLS connection

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.

		Joachim

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