Rootkit detection techniques used by the OSSEC HIDS
by Daniel B. Cid, daniel.cid@gmail.com


Starting on version 0.4, the OSSEC HIDS will perform
rootkit detection on every system where the agent is
installed. The rootcheck (rootkit detection engine) will
be executed every X minutes (user specified --by default
every 2 hours) to detect any possible rootkit installed.
Used witht the log analysis and the integrity checking
engine, it will become a very powerful monitoring solution
(the OSSEC HIDS performs log analysis and integrity
checking since version 0.1).

Other feature included on version 0.4 is that the analysis
server will automatically forward the rootkit detection
signatures to the agents, reducing the administration
overhead for the system admin. The agents and server will
keep contact every 10 minutes and if the server is
updated with a new signature file, it will forward them
to all configured agents. Take a look at the management
documentation for more information.

The rootcheck will perform the following steps on the
system trying to find rootkits:


1- Read the rootkit_files.txt which contains a big database
   of rootkits and files used by them. It will try to stats,
   fopen and opendir each specified file. We use all these
   system calls, because some kernel-level rootkits, hide
   files from some system calls. The more system calls we
   try, the better the detection. This method is more like
   an anti-virus rule that needs to be updated constantly.
   The chances of false-positives are small, but false
   negatives can be produced by modifying the rootkits.

2- Read the rootkit_trojans.txt which contains a database
   of signatures of files trojaned by rootkits. This
   technique of modifying binaries with trojaned versions
   was commonly used by most of the popular rootkits 
   available. This detection method will not find any
   kernel level rootkit or any unknown rootkit.   
   
3- Scan the /dev directory looking for anomalies. The /dev
   should only have device files and the Makedev script.
   A lot of rootkits use the /dev to hide files. This 
   technique can detect even non-public rootkits.

4- Scan the whole filesystem looking for unusual files and
   permission problems. Files owned by root, with written
   permission to others are very dangerous and the rootkit
   detection will look for them. Suid files, hidden directories
   and files will also be inspected.

5- Look for the presence of hidden processes. We use getsid()
   and kill() to check if any pid is being used or not. If
   the pid is being used, but "ps" can't see it, it is the
   indication of kernel-level rootkit or a trojaned version
   of "ps". We also verify the output of kill and getsid that
   should be the same.

6- Look for the presence of hidden ports. We use bind() to
   check every tcp and udp port on the system. If we can't
   bind to the port (it's being used), but netstat does not
   show it, we probably have a rootkit installed.

7- Scan all interfaces on the system and look for the ones
   with "promisc" mode enabled. If the interface is in promiscuous
   mode, the output of "ifconfig" should show that. If not,
   we probably have a rootkit installed.


EOF
