Here's a paper I wrote on backdoors. Feedback welcome.
By Christopher Klaus 8/4/97
Since the early days of intruders breaking into computers, they have tried
to develop techniques or backdoors that allow them to get back into the
system. In this paper, it will be focused on many of the common backdoors
and possible ways to check for them. Most of focus will be on Unix
backdoors with some discussion on future Windows NT backdoors. This will
describe the complexity of the issues in trying to determine the methods
that intruders use and the basis for administrators understanding on how
they might be able to stop the intruders from getting back in. When an
administrator understands how difficult it would be to stop intruder once
they are in, the appreciation of being proactive to block the intruder from
ever getting in becomes better understood. This is intended to cover many
of the popular commonly used backdoors by beginner and advanced intruders.
This is not intended to cover every possible way to create a backdoor as
the possibilities are limitless.
The backdoor for most intruders provide two or three main functions:
Be able to get back into a machine even if the administrator tries to
secure it, e.g., changing all the passwords.
Be able to get back into the machine with the least amount of visibility.
Most backdoors provide a way to avoid being logged and many times the
machine can appear to have no one online even while an intruder is using
Be able to get back into the machine with the least amount of time. Most
intruders want to easily get back into the machine without having to do all
the work of exploiting a hole to gain access.
In some cases, if the intruder may think the administrator may detect any
installed backdoor, they will resort to using the vulnerability repeatedly
to get on a machine as the only backdoor. Thus not touching anything that
may tip off the administrator. Therefore in some cases, the
vulnerabilities on a machine remain the only unnoticed backdoor.
Password Cracking Backdoor
One of the first and oldest methods of intruders used to gain not only
access to a Unix machine but backdoors was to run a password cracker. This
uncovers weak passworded accounts. All these new accounts are now possible
backdoors into a machine even if the system administrator locks out the
intruder's current account. Many times, the intruder will look for unused
accounts with easy passwords and change the password to something
difficult. When the administrator looked for all the weak passworded
accounts, the accounts with modified passwords will not appear. Thus the
administrator will not be able to easily determine which accounts to lock
Rhosts + + Backdoor
On networked Unix machines, services like Rsh and Rlogin used a simple
authentication method based on hostnames that appear in rhosts. A user
could easily configure which machines not to require a password to log
into. An intruder that gained access to someone's rhosts file could put a
"+ +" in the file and that would allow anyone from anywhere to log into
that account without a password. Many intruders use this method especially
when NFS is exporting home directories to the world. These accounts
become backdoors for intruders to get back into the system. Many intruders
prefer using Rsh over Rlogin because it is many times lacking any logging
capability. Many administrators check for "+ +" therefore an intruder may
actually put in a hostname and username from another compromised account on
the network, making it less obvious to spot.
Checksum and Timestamp Backdoors
Early on, many intruders replaced binaries with their own trojan versions.
Many system administrators relied on time-stamping and the system checksum
programs, e.g., Unix's sum program, to try to determine when a binary file
has been modified. Intruders have developed technology that will recreate
the same time-stamp for the trojan file as the original file. This is
accomplished by setting the system clock time back to the original file's
time and then adjusting the trojan file's time to the system clock. Once
the binary trojan file has the exact same time as the original, the system
clock is reset to the current time. The sum program relies on a CRC
checksum and is easily spoofed. Intruders have developed programs that
would modify the trojan binary to have the necessary original checksum,
thus fooling the administrators. MD5 checksums is the recommended choice
to use today by most vendors. MD5 is based on an algorithm that no one has
yet to date proven can be spoofed.
On Unix, the login program is the software that usually does the password
authentication when someone telnets to the machine. Intruders grabbed the
source code to login.c and modified it that when login compared the user's
password with the stored password, it would first check for a backdoor
password. If the user typed in the backdoor password, it would allow you to
log in regardless of what the administrator sets the passwords to. Thus
this allowed the intruder to log into any account, even root. The
password backdoor would spawn access before the user actually logged in and
appeared in utmp and wtmp. Therefore an intruder could be logged in and
have shell access without it appearing anyone is on that machine as that
account. Administrators started noticing these backdoors especially if
they did a "strings" command to find what text was in the login program.
Many times the backdoor password would show up. The intruders then
encrypted or hid the backdoor password better so it would not appear by
just doing strings. Many of the administrators can detect these backdoors
with MD5 checksums.
When a user telnets to the machine, inetd service listens on the port and
receive the connection and then passes it to in.telnetd, that then runs
login. Some intruders knew the administrator was checking the login
program for tampering, so they modified in.telnetd. Within in.telnetd, it
does several checks from the user for things like what kind of terminal the
user was using. Typically, the terminal setting might be Xterm or VT100.
An intruder could backdoor it so that when the terminal was set to
"letmein", it would spawn a shell without requiring any authentication.
Intruders have backdoored some services so that any connection from a
specific source port can spawn a shell.
Almost every network service has at one time been backdoored by an
intruder. Backdoored versions of finger, rsh, rexec, rlogin, ftp, even
inetd, etc., have been floating around forever. There are programs that
are nothing more than a shell connected to a TCP port with maybe a backdoor
password to gain access. These programs sometimes replace a service like
uucp that never gets used or they get added to the inetd.conf file as a new
service. Administrators should be very wary of what services are running
and analyze the original services by MD5 checksums.
Cronjob on Unix schedules when certain programs should be run. An intruder
could add a backdoor shell program to run between 1 AM and 2 AM. So for 1
hour every night, the intruder could gain access. Intruders have also
looked at legitimate programs that typically run in cronjob and built
backdoors into those programs as well.
Almost every UNIX system uses shared libraries. The shared libraries are
intended to reuse many of the same routines thus cutting down on the size
of programs. Some intruders have backdoored some of the routines like
crypt.c and _crypt.c. Programs like login.c would use the crypt() routine
and if a backdoor password was used it would spawn a shell. Therefore,
even if the administrator was checking the MD5 of the login program, it was
still spawning a backdoor routine and many administrators were not checking
the libraries as a possible source of backdoors.
One problem for many intruders was that some administrators started MD5
checksums of almost everything. One method intruders used to get around
that is to backdoor the open() and file access routines. The backdoor
routines were configured to read the original files, but execute the trojan
backdoors. Therefore, when the MD5 checksum program was reading these
files, the checksums always looked good. But when the system ran the
program, it executed the trojan version. Even the trojan library itself,
could be hidden from the MD5 checksums. One way to an administrator could
get around this backdoor was to statically link the MD5 checksum checker
and run on the system. The statically linked program does not use the
trojan shared libraries.
The kernel on Unix is the core of how Unix works. The same method used for
libraries for bypassing MD5 checksum could be used at the kernel level,
except even a statically linked program could not tell the difference. A
good backdoored kernel is probably one of the hardest to find by
administrators, fortunately kernel backdoor scripts have not yet been
widely made available and no one knows how wide spread they really are.
File system backdoors
An intruder may want to store their loot or data on a server somewhere
without the administrator finding the files. The intruder's files can
typically contain their toolbox of exploit scripts, backdoors, sniffer
logs, copied data like email messages, source code, etc. To hide these
sometimes large files from an administrator, an intruder may patch the
files system commands like "ls", "du", and "fsck" to hide the existence of
certain directories or files. At a very low level, one intruder's backdoor
created a section on the hard drive to have a proprietary format that was
designated as "bad" sectors on the hard drive. Thus an intruder could
access those hidden files with only special tools, but to the regular
administrator, it is very difficult to determine that the marked "bad"
sectors were indeed storage area for the hidden file system.
In the PC world, many viruses have hid themselves within the bootblock
section and most antivirus software will check to see if the bootblock has
been altered. On Unix, most administrators do not have any software that
checks the bootblock, therefore some intruders have hidden some backdoors
in the bootblock area.
Process hiding backdoors
An intruder many times wants to hide the programs they are running. The
programs they want to hide are commonly a password cracker or a sniffer.
There are quite a few methods and here are some of the more common:
An intruder may write the program to modify its own argv to make it look
like another process name.
An intruder could rename the sniffer program to a legitimate service like
in.syslog and run it. Thus when an administrator does a "ps" or looks at
what is running, the standard service names appear.
An intruder could modify the library routines so that "ps" does not show
all the processes.
An intruder could patch a backdoor or program into an interrupt driven
routine so it does not appear in the process table. An example backdoor
using this technique is amod.tar.gz available on
An intruder could modify the kernel to hide certain processes as well.
One of the most popular packages to install backdoors is rootkit. It can
easily be located using Web search engines. From the Rootkit README, here
are the typical files that get installed:
z2 - removes entries from utmp, wtmp, and lastlog.
Es - rokstar's ethernet sniffer for sun4 based kernels.
Fix - try to fake checksums, install with same dates/perms/u/g.
Sl - become root via a magic password sent to login.
Ic - modified ifconfig to remove PROMISC flag from output.
ps: - hides the processes.
Ns - modified netstat to hide connections to certain machines.
Ls - hides certain directories and files from being listed.
du5 - hides how much space is being used on your hard drive.
ls5 - hides certain files and directories from being listed.
Network traffic backdoors
Not only do intruders want to hide their tracks on the machine, but also
they want to hide their network traffic as much as possible. These network
traffic backdoors sometimes allow an intruder to gain access through a
firewall. There are many network backdoor programs that allow an intruder
to set up on a certain port number on a machine that will allow access
without ever going through the normal services. Because the traffic is
going to a non-standard network port, the administrator can overlook the
intruder's traffic. These network traffic backdoors are typically using
TCP, UDP, and ICMP, but it could be many other kinds of packets.
TCP Shell Backdoors
The intruder can set up these TCP Shell backdoors on some high port number
possibly where the firewall is not blocking that TCP port. Many times,
they will be protected with a password just so that an administrator that
connects to it, will not immediately see shell access. An administrator
can look for these connections with netstat to see what ports are listening
and where current connections are going to and from. Many times, these
backdoors allow an intruder to get past TCP Wrapper technology. These
backdoors could be run on the SMTP port, which many firewalls allow traffic
to pass for e-mail.
UDP Shell Backdoors
Administrator many times can spot a TCP connection and notice the odd
behavior, while UDP shell backdoors lack any connection so netstat would
not show an intruder accessing the Unix machine. Many firewalls have been
configured to allow UDP packets for services like DNS through. Many times,
intruders will place the UDP Shell backdoor on that port and it will be
allowed to by-pass the firewall.
ICMP Shell Backdoors
Ping is one of the most common ways to find out if a machine is alive by
sending and receiving ICMP packets. Many firewalls allow outsiders to ping
internal machines. An intruder can put data in the Ping ICMP packets and
tunnel a shell between the pinging machines. An administrator may notice a
flurry of Ping packets, but unless the administrator looks at the data in
the packets, an intruder can be unnoticed.
An administrator can set up a sniffer trying to see data appears as someone
accessing a shell, but an intruder can add encryption to the Network
traffic backdoors and it becomes almost impossible to determine what is
actually being transmitted between two machines.
Because Windows NT does not easily allow multiple users on a single machine
and remote access similar as Unix, it becomes harder for the intruder to
break into Windows NT, install a backdoor, and launch an attack from it.
Thus you will find more frequently network attacks that are spring boarded
from a Unix box than Windows NT. As Windows NT advances in multi-user
technologies, this may give a higher frequency of intruders who use Windows
NT to their advantage. And if this does happen, many of the concepts from
Unix backdoors can be ported to Windows NT and administrators can be ready
for the intruder. Today, there are already telnet daemons available for
Windows NT. With Network Traffic backdoors, they are very feasible for
intruders to install on Windows NT.
As backdoor technology advances, it becomes even harder for administrators
to determine if an intruder has gotten in or if they have been successfully
One of the first steps in being proactive is to assess how vulnerable your
network is, thus being able to figure out what holes exist that should be
fixed. Many commercial tools exist to help scan and audit the network and
systems for vulnerabilities. Many companies could dramatically improve
their security if they only installed the security patches made freely
available by their vendors.
One necessary component of a system scanner is MD5 checksum baselines.
This MD5 baseline should be built up before a hacker attack with clean
systems. Once a hacker is in and has installed backdoors, trying to create
a baseline after the fact could incorporate the backdoors into the
baseline. Several companies had been hacked and had backdoors installed on
their systems for many months. Overtime, all the backups of the systems
contained the backdoors. When some of these companies found out they had
a hacker, they restored a backup in hopes of removing any backdoors. The
effort was futile since they were restoring all the files, even the
backdoored ones. The binary baseline comparison needs to be done before an
Intrusion detection is becoming more important as organizations are hooking
up and allowing connections to some of their machines. Most of the older
intrusion detection technology was log-based events. The latest intrusion
detection system (IDS) technology is based on real-time sniffing and
network traffic security analysis. Many of the network traffic backdoors
can now easily be detected. The latest IDS technology can take a look at
the DNS UDP packets and determine if it matches the DNS protocol requests.
If the data on the DNS port does not match the DNS protocol, an alert flag
can be signaled and the data captured for further analysis. The same
principle can be applied to the data in an ICMP packet to see if it is the
normal ping data or if it is carrying encrypted shell session.
Boot from CD-ROM.
Some administrators may want to consider booting from CD-ROM thus
eliminating the possibility of an intruder installing a backdoor on the
CD-ROM. The problem with this method is the cost and time of implementing
this solution enterprise wide.
Because the security field is changing so fast, with new vulnerabilities
being announced daily and intruders are constantly designing new attack and
backdoor techniques, no security technology is effective without vigilance.
Be aware that no defense is foolproof, and that there is no substitute for