Analysis of a rootkit: Tuxkit

By: spoonfork (spoonfork@hackinthebox.org)

Introduction

The following is an analysis of the Tuxkit rootkit, written by a Dutch group called Tuxtendo. This rootkit was found in one of the honeypots that we set up. The honeypot was a stock installation of Redhat 7.0, with a few services running. None of the software, such as named, sendmail and the printer daemon were patched.

There are three versions of the rootkit that are available on Tuxtendo’s website. They are tuxkit.tgz, tuxkit-1.0.tgz, and tuxkit-short.tgz. Both tuxkit.tgz and tuxkit-1.0.tgz have the same contents, while tuxkit-short.tgz contains less tools.

I’ve also tested some of tuxkit’s binaries on Redhat 7.1, and they seemed to work fine.

The following are the contents of each tuxkit. This analysis will focus on tuxkit-1.0.tgz, the one that was found on our honeypot. The rootkit was developed by Argv[], possibly modified from and based on the t0rn rootkit. The timestamp of the rootkit was December 2001. Googling for “tuxkit analysis” did not produce any hits, so I guess that this rootkit is pretty new.

NOTE: chkrootkit failed to detect tuxkit.

Packages

Listing:

.. code-block:: console

[root@angel tuxkit-1.0]# ls -l ../tuxkit (tuxkit.tgz) total 2600 -rw——- 1 sfork sfork 502884 Dec 5 07:55 bin.tgz -rw——- 1 sfork sfork 406 Dec 5 07:55 cfg.tgz -rw——- 1 sfork sfork 16213 Dec 5 07:55 lib.tgz -rw——- 1 sfork sfork 3684 Dec 5 07:55 README -rw——- 1 sfork sfork 461892 Jan 6 00:06 sshd.tgz -rw——- 1 sfork sfork 1644819 Dec 5 07:55 tools.tgz -rwx—— 1 sfork sfork 9489 Jan 6 00:53 tuxkit

[root@angel tuxkit-1.0]# ls -l ../tuxkit-1.0 (tuxkit-1.0.tgz) total 2600 -rw——- 1 sfork sfork 502884 Dec 5 07:55 bin.tgz -rw——- 1 sfork sfork 406 Dec 5 07:55 cfg.tgz -rw——- 1 sfork sfork 16213 Dec 5 07:55 lib.tgz -rw——- 1 sfork sfork 3684 Dec 5 07:55 README -rw——- 1 sfork sfork 461892 Jan 6 00:06 sshd.tgz -rw——- 1 sfork sfork 1644819 Dec 5 07:55 tools.tgz -rwx—— 1 sfork sfork 9489 Jan 6 00:53 tuxkit

[root@angel tuxkit-1.0]# ls -l ../tuxkit-short (tuxkit-1.0-short.tgz) total 1556 -rw——- 1 1001 1001 502884 Dec 5 07:55 bin.tgz -rw——- 1 1001 1001 406 Dec 5 07:55 cfg.tgz -rw——- 1 1001 1001 16213 Dec 5 07:55 lib.tgz -rw——- 1 1001 1001 3684 Dec 5 07:55 README -rw——- 1 1001 1001 461892 Jan 6 00:06 sshd.tgz -rw——- 1 1001 1001 577089 Jan 6 01:12 tools.tgz -rwx—— 1 1001 1001 9489 Jan 6 00:53 tuxkit

tuxkit-1.0.tgz

There are six files in the tuxkit which includes a README, an installation script, and four tarred/zipped files.

The following are the contents of the individual files in the tuxkit.

  • bin.tgz

    • contains precompiled trojan binaries
  • cfg.tgz

    • contains tuxkit’s configuration files
  • lib.tgz

    • contains libproc libraries, for process hiding purposes
  • sshd.tgz

    • contains precompiled sshd, complete with sshd_config
  • tools.tgz
    • contains an arsenal of tools (duh!) for the skrip kiddie who don’t know how to get their own tools. The tools are:

      [root@angel tools]# ls -la
      total 44
      drwxr-xr-x   11 root     root         4096 Mar  1 13:14 .
      drwxr-xr-x    4 root     root         4096 Mar  1 13:14 ..
      drwx------    2 root     root         4096 Nov 12 20:50 bitchx
      drwx------    2 root     root         4096 Dec 12 23:59 dos
      drwx------    2 root     root         4096 Nov 12 20:57 mirkforce
      drwx------    2 root     root         4096 Nov 12 20:57 nmapv
      drwx------    8 root     root         4096 Nov 12 23:05 psybnc
      drwx------    2 root     root         4096 Nov 13 01:00 sniffer
      drwx------    2 root     root         4096 Nov 12 20:58 ssh
      drwx------    2 root     root         4096 Nov 12 23:22 synscan
      drwx------    2 root     root         4096 Nov 12 20:58 utils
      

    The names of these tools are self-explanatory. However, they are all precompiled. utils contains only one utility - wget. This is to enable the skripkids to easily download other tools (assuming the skripkids know how to use wget).

  • tuxkit

    • an installation script
  • README

    • the obligatory README file (and greetz, of course)

The tuxkit is almost similar to the t0rn rootkit. The addition of the precompiled tools such as nmap, synscan and psybnc makes it a more handy rootkit. It is flawlessly easy to install. Tuxkit is like a pack-n-go kinda tool. The appendix shows the contents of each packages in tuxkit.

Installation

Installation of tuxkit is very straightforward. The README says:

README::

./tuxkit

Password : This will be the password you need to login onto
the compromised system.
SSD Port : This will be the port on which the SSHD will be
be listening on for incoming connections. This port will be hidden automatically in netstat.
bncport : this will be the port psyBNC will listen on.
This port will be hidden automatically in netstat.

The setup script does NOT have default settings, this forces you to provide a password, sshd and bnc ports.

The setup script also contains a variable called EMAIL, you should edit this ;)

This sets tuxkit apart from t0rn - it does not use default ports.

The default installation directory is /dev/tux. Shell script savvy skripkids may want to change this to avoid detection.

NOTE: the tuxkit installation script contains a variable EMAIL which has the default value of the author. At the end of the installation, the script will send an email which the subject “Tuxkit1.0”. The e-mail contains information about the host, the SSH backdoor port, the psyBNC port, and also the password. If you skripkid didn’t change the EMAIL (the README clearly states to change this), you have the risk of your server being owned by other people.

Trojaning process

The trojaning process is straightforward. syslogd is killed first. Then all the files that came with tuxkit-1.0.tgz are untarred and upzipped. The installation directory is created. The default installation directory is /dev/tux, and even though this is kept as the variable RDIR, the tuxkit install script hardcoded “mkdir /dev/tux”, thus changing RDIR, but forgetting to change the line above will cause your installation to skew a bit (most skripkids won’t bother to do this anyway). In fact, /dev/tux is hardcoded almost everywhere in the installation script.

The hidden files .addr, .cron, .file, .log and .proc are copied to /dev/tux/

The library files are copied to lib, and /sbin/ldconfig is executed.

This step is followed by copying files to be trojaned to /dev/tux/backup, and replacing these files with the trojaned version. A script “sz”, which is part of the bin.tgz is run against each trojaned binaries so that the size matches that of the original binaries. “sz” basically pads the trojan with zeros (from /dev/zero).

Backdooring process

The backdoored SSH is installed in /usr/bin/xsf. The trojaned sshcheck is installed in /usr/bin/xchk. Both are invoked the following way:

/usr/bin/xsf -q 1>/dev/null 2>/dev/null
/usr/bin/xchk -q 1>/dev/null 2>/dev/null

The /etc/rc.d/rc.sysinit is also edited to include the following lines:

echo "# Running Xsf ..." >> /etc/rc.d/rc.sysinit
echo "/usr/bin/xsf -q 1>/dev/null 2>/dev/null" >> /etc/rc.d/rc.sysinit
echo "# Running Xchk ..." >> /etc/rc.d/rc.sysinit
echo "/usr/bin/xchk 1>/dev/null 2>/dev/null" >> /etc/rc.d/rc.sysinit

If you string xsf, you will be able to get the passwords that the skripkid used.

The tuxkit configuration files

The tuxkit config files follows that of the original Linux rootkit. There are .addr, .cron, .file, .log and .proc. The filenames are self-explanatory. These files follow the convention of the original Linux rootkit. In forensic, what you will be interested in most is the .addr files, because it contains the IP that netstat is supposed to hide.

Detecting tuxkit

Detecting tuxkit is fairly simple.

# Look for the existence of /dev/tux # Run lsof -i +M | grep xsf

Hey, why wasn’t lsof trojaned? t0rn has a trojaned lsof :)

Detecting tuxkit - trojans

#  md5sums - if you've keep an md5sum of the virgin state of your
   installation, detecting trojans should be a walk in the park. Every
   system administrator should use file integrity checker to monitor
   critical file change.
#  Look for /usr/bin/xsf and /usr/bin/xchk
#  Look for extra lines in /etc/rc.sysinit
#  cd /etc/ssh; ls -l. The trojaned ls will return nothing, when in
   fact your ssh config files are still there.

The following are the size difference between tuxkit and Redhat 7.1 binaries. (before installation):

.. code-block:: console
crontab 29052 21280 df 27112 26812 dir 42952 45948 dmesg 3640 4252 du 25592 25788 find 55220 47516 ifconfig 36356 51164 killall 14400 12096 locate 9144 25020 (symlink to slocate) login 3980 17740 ls 42952 45948 netstat 58228 83132 ps 62748 63180 pstree 14532 12284 sshcheck 89828 - (my test machine don’t have this) sshdconfig 451260 - (my test machine don’t have this) syslogd 28324 26972 tcpd 18660 24844 top 37844 34924 updatedb 4394 25020 (symlink to slocate) vdir 42952 45948

This result is quite interesting. Trojans are not supposed to be bigger that the original binaries!

Detecting tuxkit - if you are lucky

On our honeypot, the trojaned ‘ps’ still shows xsf, even though xsf was in the list of processes to be hidden. However, ‘ls’ seems to work very well in hiding files.

Recommendations

For tuxkit developers

  • Add trojaned lsof. Borrow one from t0rn :) Also, fix ps.
  • tools.tgz is probably not needed. A skripkid who is able to crack a Linux machine (duh) should be able to download and compile his/her own tools. Furthermore, tools.tgz adds unnecessary extra bytes to the tuxkit - not really convenient for downloading.
  • Add a self-deleting script, i.e. delete the tar files and installation directory after installation. skripkids seems incapable of doing this. The config files should be kept somewhere else other than /dev/tux.

For skripkidz - vi tuxkit, type the following:

:%s//dev/tux/installation_dir_of_your_choice/g

where installation_dir_of_your_choice is, uh, the installation directory of your choice. (However, this won’t work, since /dev/tux/.{addr,proc}, etc are already hardcoded to the binaries - so hehe, just run ./tuxkit and pray that the stupid system administrators won’t notice :)

For system administrators - run file integrity checker after each fresh install.

Conclusion

The world of forensic analysis ain’t fun without rootkits.

Appendix - Contents of each packages

Contents:

.. code-block:: console

[root@angel tuxkit-1.0]# less bin.tgz -rwx—— root/root 29052 2001-12-26 21:37:57 bin/crontab -rwx—— root/root 27112 2001-12-26 21:37:57 bin/df -rwx—— root/root 42952 2001-12-26 21:37:57 bin/dir -rwx—— root/root 3640 2001-12-26 21:37:57 bin/dmesg -rwx—— root/root 25592 2001-12-26 21:37:57 bin/du -rwx—— root/root 55220 2001-12-26 21:37:57 bin/find -rwx—— root/root 36356 2001-12-26 21:37:57 bin/ifconfig -rwx—— root/root 14400 2001-12-26 21:37:57 bin/killall -rwx—— root/root 9144 2001-12-26 21:37:57 bin/locate -rwx—— root/root 3980 2001-12-26 21:37:57 bin/login -rwx—— root/root 42952 2001-12-26 21:37:57 bin/ls -rwx—— root/root 58228 2001-12-26 21:37:57 bin/netstat -rwx—— root/root 62748 2001-12-26 21:37:57 bin/ps -rwx—— root/root 14532 2001-12-26 21:37:57 bin/pstree -rwx—— root/root 89828 2001-12-26 21:37:57 bin/sshcheck -rwx—— root/root 451260 2001-12-26 21:37:57 bin/sshdconfig -rwx—— root/root 28324 2001-12-26 21:37:57 bin/syslogd -rwx—— root/root 1522 2001-12-26 21:37:57 bin/sz -rwx—— root/root 18660 2001-12-26 21:37:57 bin/tcpd -rwx—— root/root 37844 2001-12-26 21:37:57 bin/top -rwx—— root/root 4394 2001-12-26 21:37:57 bin/updatedb -rwx—— root/root 42952 2001-12-26 21:37:57 bin/vdir

[root@angel tuxkit-1.0]# less cfg.tgz -rw——- root/root 17 2001-11-11 19:12:19 cfg/.addr -rw——- root/root 69 2001-11-12 23:06:32 cfg/.cron -rw——- root/root 67 2001-12-27 20:54:23 cfg/.file -rw——- root/root 13 2001-12-27 20:54:47 cfg/.log -rw——- root/root 116 2001-12-27 20:55:29 cfg/.proc

[root@angel tuxkit-1.0]# less sshd.tgz -rw——- virus/virus 828 2001-12-13 00:22:12 ssh2/hostkey -rw——- virus/virus 697 2001-12-13 00:22:12 ssh2/hostkey.pub -rw——- virus/virus 503 2001-12-27 20:43:12 ssh2/logo -rw——- virus/virus 512 2001-12-13 23:51:33 ssh2/random_seed -rwx—— virus/virus 1040220 2002-01-06 00:05:58 ssh2/sshd -rw——- virus/virus 647 2001-12-27 22:42:20 ssh2/sshd2_config

[root@angel tuxkit-1.0]# less lib.tgz lrwxrwxrwx root/root 0 2001-11-11 02:49:02 lib/libproc.so -> libproc.so. 2.0.7