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Colaboração: Marcos Aguinaldo Forquesato

Data de Publicação: 24 de Agosto de 1997

Estou enviando um texto hoje sobre as chamadas backdoors. Este texto foi escrito por Christopher Klaus <<cklaus (a) ISS NET>>. Backdoors são as alterações que um hacker cria em um sistema que tenha invadido para que possa voltar.

Este texto faz uma explicação abrangente destas técnicas. Boa leitura!

   Date: 	Sat, 16 Aug 1997 19:07:58 -0400
   Sender: Bugtraq List <BUGTRAQ@NETSPACE.ORG>
   From: Christopher Klaus <cklaus@ISS.NET>
   Subject:      Backdoor Paper
   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.
   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.

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