backdoors.txt

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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
it.

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 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.