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Re: Mutt: failure to check server certificate in SMTP TLS connection
From: Joachim Schipper <joachim () joachimschipper nl>
Date: Tue, 8 Mar 2011 09:00:05 +0100

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 () smtp gmail com:587/"
to be
set smtp_url = "smtp://MYUSERNAME () mail 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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