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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.

Veja a relação completa dos artigos de Marcos Aguinaldo Forquesato