by Toby Miller
The purpose of this paper is to inform the IDS community of signatures related to the t0rn rootkit. This paper will not serve as a how-to guide to the t0rn rootkit; rather, it is designed to identify binaries and ports that t0rn uses. This paper will also provide md5sums of binaries and analysis on how to detect t0rn.
The t0rn rootkit is designed for speed. By that I mean that it was designed to install quickly on Linux machines. T0rn can do this because it takes very little skill to install and run. All of the binaries that the attacker would need come pre-compiled and the installation process is as simple as ./t0rn. T0rn comes standard with a log cleaner called t0rnsb, a sniffer named t0rns and a log parser called t0rnp.
Red Hat 6.1 Details
T0rn has many details that need to be discussed and analyzed in order to detect it in the wild. The computer that was used in this analysis is a RH 6.1 box with no applied patches, the inetd.conf file had been secured, the password had 6 characters and was connected to an internal network. In order to analyze t0rn, I had to complete some pre-installation t0rn data collection that included documenting the sizes and creation dates of both the RH binaries and the pre-compiled t0rn binaries.
First, we want to take a look at the Red Hat 6.1 binaries (before t0rn is installed) their date, size and timestamps. Figure 1 is a list of RH 6.1 binaries and their characteristics.
Figure 1. RH 6.1 Binaries and properties
Why is this information important? As you will see in a minute, the file size is a key indicator in detecting t0rn. Another piece of data that I collected was the md5sums of the RH 6.1 binaries. I thought that the creator of this rootkit might be able to mask the file size and creation timestamp(s) that are included in this rootkit with the good ones in the operating systems. But there would be little chance he/she could recreate the md5sums. Figure 2 illustrates the md5sums of the RH 6.1 binaries that would eventually be replaced(by t0rn).
Figure 2. RH 6.1 md5sums (before t0rn)
I also copied most of the RH 6.1 binaries to a different directory so that I could use them after the Trojan binaries were loaded.
Now that we have analyzed the Red Hat binaries, lets look at the t0rn binaries before installation. After unpacking t0rn I made a list (Figure 3) of the t0rn binaries and their properties (did not get the year but that's not important).
Figure 3. t0rn binaries and properties (before installation)
After I documented the t0rn file sizes and properties I also ran a md5sum on the t0rn binaries and came up with figure 4. These checksums can be critical in determining if t0rn has been installed on your Linux box.
Figure 4. t0rn md5sums
When t0rn is installed a couple of things happen. First, it creates its own directory /usr/src/.puta. There you will find all the files (sniffer, log cleaner, etc.) needed to run t0rn.
Default t0rn is not really hard to detect. The first command I ran after installing the rootkit was ps -ef. The output of ps -ef was totally different the output from running the binary /bin/ps. The next step I took was to check the file(s) size and timestamp. T0rn is tricky in this department, the trojaned binaries keep the same exact timestamp as the good binaries. What stands out like an eye-sore is the file size. An example of this is /bin/ps. Normally, if you were to run ls-la on /bin/ps (Red Hat 6.1) you would have the following output:
-r-xr-xr-x 1 root root 61244 Sept 26 1999
If t0rn is installed the user would see the following:
-r-xr-xr-x 1 root root 31336 Sept 26 1999
Notice the difference in the file size. Also, just as a side note that the file size is one byte off from being eleet. This holds true for all t0rn binaries. The one binary that I found the most interesting was netstat. Why? Well, t0rn's version of netstate causes a segmentation fault.
T0rn can be detected by using lsof. (Yes, the guys who wrote this rootkit forgot to change an important tool.) Running lsof | grep LISTEN will show port 47017(highlighted in red) is the listening state (Figure 5).
Figure 5. lsof | grep LISTEN output
This port is the default port used by t0rn. By using lsof | grep t0rn a person can look at anything being ran as t0rn. Figure 6 shows us the results of lsof -grep t0rn
Figure 6. Output of lsof
Here we see a few key items. First, we see the file /usr/src/.puta/t0rns (sniffer) running (highlighted in red). We also see /usr/srec/.puta, again this is the hidden directory for t0rn. These two files can be a key indicator for identifying t0rn.
Finally, I also found t0rn by running nmap and scanning for destination ports 45k -48k. The nmap output would look like this:
Starting nmap V. 2.54BETA7 ( www.insecure.org/nmap/ ) Interesting ports on (192.168.1.3): (The 4000 ports scanned but not shown below are in state: closed) Port State Service 47017/tcp open unknown TCP Sequence Prediction: Class=random positive increments Difficulty=3980866 (Good luck!) Remote operating system guess: Linux 2.1.122 - 2.2.16 Nmap run completed -- 1 IP address (1 host up) scanned in 5 seconds
Detecting t0rn or any other rootkit requires planning when installing the operating systems. The best way to prevent these kinds of attacks is by using programs like Tripwire, maintaining good backups and keeping up with the latest patches. One other suggestion is to run md5sum on binaries such as /bin/ps, /bin/ls and many others and save the results to a floppy that will be stored in a secure place.
T0rn is a very sneaky toolkit and can be hard to detect if an administrator does not know what to look for. If a person follows the recommendations stated above he or she could save themselves a lot of heartache and time trying to look for programs like this.
Toby Miller currently works at SYTEX Inc. based out of Pennsylvania. Toby holds a B.Sc. in Computer Information Systems . Toby is a GIAC Certified Intrusion Analyst and a Microsoft Certified Professional. In his seven years in the computer field he has worked in many area such as Firewalls, Unix administration, NT Administration and some mainframe work.
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