Rootkit Sleuth

The chkrootkit package examines key system files to determine whether they have been tampered with or infected by malware known as rootkits. An infected system can leak valuable data such as user credentials, can allow hackers to steal banking information, can provide remote control capability, and can disrupt security software. The fact that rootkits can remain on a system for long periods of time undetected makes them particularly dangerous.

Changing Attitudes on Linux Malware

Contrary to popular belief, Linux does suffer from malware infections and attacks. Rootkits are one type of malware designed to infect, gain elevated access, and remain hidden from detection for long periods of time. Rootkits are especially malicious, because they not only remain hidden but also can disable or disrupt security software making them hard to remove. Once infected, often the only recourse is to re-image (wipe and reinstall) the system.Symantec has identified five types of rootkits:

• Hardware/firmware – installed in the computer’s BIOS, firmware, or other hardware components
• Bootloader – the system’s bootloader is active before the operating system loads
• Memory – RAM-based rootkits only last while the computer is powered on and generally do not persist after a reboot unless the file is set to reload on boot
• Application – application rootkits alter standard programs that appear to work normally while infected
• Kernel mode – core operating system alterations access and control the computer’s lowest operational levels

Rootkits are generally downloaded and installed by the root user, which means that the malware author now has full control of the infected computer. Malicious file attachments, social engineering, and malicious web payloads account for most of the known rootkit infections.Great care must be taken by manufacturers, suppliers, and system administrators to guard against infections of this kind. Regular updates and best practice security measures can decrease the likelihood of malware infections. But those who download, execute, and install software as the root user must ensure that this material is first tested in a non-production environment.

Installing chkrootkit

On Debian-based systems, use:

$ sudo apt install chkrootkit

On Red Hat Enterprise Linux, the procedure is more complex, because there is no chkrootkit RPM in the standard repositories. You’ll have to install from source and there are some prerequisites:

$ sudo yum –y update

I installed the minimal CentOS 7.x option, so I don’t have any development tools on my system:

$ sudo yum –y install wget gcc-c++ glibc-static

Download the latest tarball for chkrootkit:

$ sudo wget -c ftp://ftp.pangeia.com.br/pub/seg/pac/chkrootkit.tar.gz

Unzip the tarball:

$ tar xvf chkrootkit.tar.gz $ mkdir /usr/local/chkrootkit$ sudo cp chkrootkit-0.53/* /usr/local/chkrootkit

At editorial deadline, the current version is chkrootkit v0.53, but your version and directory name might vary:

$ cd /usr/local/chkrootkit

Compile the code:

$ sudo make sense

Checking Out chkrootkit

Now you have a small collection of chkrootkit related tools.

• chkrootkit – the main script that checks system binaries for rootkit infection and modification
• ifpromisc – checks network interfaces for promiscuous mode
• chklastlog – checks lastlog for deletions
• chkwtmp – checks wtmp for deletions
• chkproc and chkdirs – check for evidence of loadable kernel module (LKM) Trojans
• strings – script for strings replacements
• chkutmp – checks for utmp deletions

Although you can run any of the scripts, the chkrootkit script runs all checks for you.To list all available options, use the code in Listing 1.

$ sudo /usr/local/chkrootkit/chkrootkit –h

Usage: /usr/local/chkrootkit/chkrootkit [options] [test ...]
Options:
  -h                  show this help and exit
  -V                  show version information and exit
  -l                  show available tests and exit
  -d                  debug
  -q                  quiet mode
  -x                  expert mode
  -r dir              use dir as the root directory
  -p dir1:dir2:dirN   path for the external commands used by chkrootkit
  -n                  skip NFS mounted dirs

Searching for Rootkit InfectionsUnless you have a specific need, you can simply run chkrootkit with no options:

$ sudo /usr/local/chkrootkit/chkrootkit

Because the output is very long, you can check to see if there are any obvious infections, by filtering for the term INFECTED:

$ sudo /usr/local/chkrootkit/chkrootkit | grep INFECTED

If there is a positive (false or not), you’ll see a message similar to the following:

Searching for Linux.Xor.DDoS ... INFECTED: Possible Malicious Linux.Xor.DDoS installed

This infection notice is a false positive on my new CentOS 7 system. You should investigate each positive notification to be sure that they are either false or real. Further investigation will determine the extent of the damage and possibly how long your system has been infected and compromised. You should update and run chkrootkit on a regular basis to halt any infections before they do extensive damage to your systems or to your users.

Going the Extra Mile

Chkrootkit uses a handful of native commands (awk, cut, echo, egrep, etc.). If you believe that your system might be compromised, then the best option is to mount its disk onto a clean system for inspection. Do not trust executables on a compromised system. Rootkit writers know about chkrootkit and will attempt to infect those commands it uses.You can also run chkrootkit in “expert” mode, which provides you with a verbose output of its checks so that you can inspect what it finds for yourself. Chkrootkit runs from cron so that you can schedule a daily scan of your system and mail the output to your user account or to a shared account.

Summary

Chkrootkit has been around for more than 20 years and is still actively developed. It, like other security solutions, is not a panacea. It is simply a part of a layered approach to your overall security strategy. Exercise caution before relying too heavily on a single approach in your own networks. Getting a clean bill of health from a chkrootkit scan doesn’t mean that your system is clean or uninfected by malware. There are many different types of malware, and chkrootkit checks for a single type.