Here's a paper on backdoors I wrote. Feedback welcomed.
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 out.
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
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
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
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
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