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. You can download the actual raw data used for this paper here.
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. For
more information on how this information was captured, check out To Build a
Honeypot". Throughout this paper our intruder is refered to as he, however
we have no idea what the true gender of the intruder is.
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: connect from 18.104.22.168
Apr 27 00:12:27 bach imapd: connect from 22.214.171.124
Apr 27 00:12:30 vivaldi imapd: connect from 126.96.36.199
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: Unauthorized access by NFS
Apr 27 16:47:28 mozart syslogd: Cannot glue message parts together
Apr 27 16:47:28 mozart mountd: Blocked attempt of 188.8.131.52 to mount
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: 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: (login) session opened for user crak0
Apr 27 16:50:38 mozart login: LOGIN ON ttyp0 BY crak0 FROM 1Cust102.tnt1.long-branch.nj.da.uu.net
Apr 27 16:50:47 mozart PAM_pwdb: (su) session opened for user rewt by
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.
For more detailed information on rootkits, see the README that comes
with lrk4. This will give you a better idea how rootkits work in general.
I also recommend you check out hide-and-seek,
a black-hat paper on covering your tracks.
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.
mkdir ". "
cd ". "
tar -zxvf lrk4.unshad.tar.gz
mv lrk4 proc
mv proc ". "
cd ". "
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 "*.*"
/dev/. /. /procps-1.01/proc/.depend
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
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.
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]
cant open log
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
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.
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 Linux or 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".