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TUCoPS :: Unix :: General :: cert0073.txt

CERT Advisory CA-94:01 network monitoring attacks


CERT* Advisory CA-94:01
Original issue date:  February 3, 1994
Last revised: April 3, 1997
                 Appendix B - corrected "Public Domain" to read "Publicly

                    A complete revision history is at the end of this file.

Topic: Ongoing Network Monitoring Attacks
- -----------------------------------------------------------------------------

In the week before we originally issued this advisory, the CERT/CC staff
observed a dramatic increase in reports of intruders monitoring network
traffic.  Systems of some service providers have been compromised, and all
systems that offer remote access through rlogin, telnet, and FTP are at risk.
Intruders have already captured access information for tens of thousands of
systems across the Internet.

The current attacks involve a network monitoring tool that uses the
promiscuous mode of a specific network interface, /dev/nit, to capture
host and user authentication information on all newly opened FTP,
telnet, and rlogin sessions.

In the short-term, we recommend that all users on sites that offer
remote access change passwords on any network-accessed account. In
addition, all sites having systems that support the /dev/nit interface
should disable this feature if it is not used and attempt to prevent
unauthorized access if the feature is necessary. A procedure for
accomplishing this is described in Section III.B.2 below.  Systems
known to support the interface are SunOS 4.x (Sun3 and Sun4
architectures) and Solbourne systems; there may be others. Sun Solaris
systems do not support the /dev/nit interface. If you have a system
other than Sun or Solbourne, contact your vendor to find if this
interface is supported.

While the attack is specific to /dev/nit, the short-term workaround does not
constitute a solution. The best long-term solution currently available for
this attack is to reduce or eliminate the transmission of reusable passwords
in clear-text over the network.

- -----------------------------------------------------------------------------

I.   Description

     Root-compromised systems that support a promiscuous network
     interface are being used by intruders to collect host and user
     authentication information visible on the network.

     The intruders first penetrate a system and gain root access
     through an unpatched vulnerability. Solutions and workarounds for
     these vulnerabilities have been described in previous CERT
     advisories, which are available from

     The intruders then run a network monitoring tool that captures up
     to the first 128 keystrokes of all newly opened FTP, telnet, and
     rlogin sessions visible within the compromised system's domain.
     These keystrokes usually contain host, account, and password
     information for user accounts on other systems; the intruders log
     these for later retrieval.  The intruders typically install
     Trojan horse programs to support subsequent access to the
     compromised system and to hide their network monitoring process.

II.  Impact

     All connected network sites that use the network to access remote
     systems are at risk from this attack.

     All user account and password information derived from FTP,
     telnet, and rlogin sessions and passing through the same network
     as the compromised host could be disclosed.

III. Approach

     There are three steps in our recommended approach to the

     - Detect if the network monitoring tool is running on any of your
       hosts that support a promiscuous network interface.

     - Protect against this attack either by disabling the network
       interface for those systems that do not use this feature or by
       attempting to prevent unauthorized use of the feature on systems
       where this interface is necessary.

     - Scope the extent of the attack and recover in the event that
       the network monitoring tool is discovered.

     A.  Detection

         The network monitoring tool can be run under a variety of
         process names and log to a variety of filenames.  Thus, the
         best method for detecting the tool is to look for 1) Trojan
         horse programs commonly used in conjunction with this attack,
         2) any suspect processes running on the system, 3) the
         unauthorized use of /dev/nit, 4) unexpected ASCII files in the
         /dev directory, and 5) modifications to /etc/rc* files and

         1) Trojan horse programs:

         The intruders have been found to replace one or more of the
         following programs with a Trojan horse version in conjunction
         with this attack:

           and /bin/login -  Used to provide back-door access for the
                             intruders to retrieve information
           /bin/ps  - Used to disguise the network monitoring process
           binaries referred in /etc/inetd.conf

         Because the intruders install Trojan horse variations of
         standard UNIX commands, we recommend not using other
         commands such as the standard UNIX sum(1) or cmp(1) commands
         to locate the Trojan horse programs on the system until these
         programs can be restored from distribution media, run from
         read-only media (such as a mounted CD-ROM), or verified using
         cryptographic checksum information.

         In addition to the possibility of having the checksum
         programs replaced by the intruders, the Trojan horse programs
         mentioned above may have been engineered to produce the same
         standard checksum and timestamp as the legitimate version.
         Because of this, the standard UNIX sum(1) command and the
         timestamps associated with the programs are not sufficient to
         determine whether the programs have been replaced.

         We recommend that you use both the /usr/5bin/sum and
         /bin/sum commands to compare against the distribution media
         and assure that the programs have not been replaced.  The use
         of cmp(1), MD5, Tripwire (only if the baseline checksums were
         created on a distribution system), and other cryptographic
         checksum tools are also sufficient to detect these Trojan
         horse programs, provided these programs were not available
         for modification by the intruder.  If the distribution is
         available on CD-ROM or other read-only device, it may be
         possible to compare against these volumes or run programs off
         these media.

         2) Suspect processes:

         Although the name of the network monitoring tool can vary from
         attack to attack, it is possible to detect a suspect process
         running as root using ps(1) or other process-listing commands.
         Until the ps(1) command has been verified against distribution
         media, it should not be relied upon--a Trojan horse version
         is being used by the intruders to hide the monitoring process.
         Some process names that have been observed are sendmail, es,
         and in.netd.  The arguments to the process also provide an
         indication of where the log file is located.  If the "-F" flag
         is set on the process, the filename following indicates the
         location of the log file used for the collection of
         authentication information for later retrieval by the intruders.

         3) Unauthorized use of /dev/nit:

         If the network monitoring tool is currently running on your
         system, it is possible to detect this by checking for
         unauthorized use of the /dev/nit interface. We have created
         a minimal tool, cpm, for this purpose.

         We urge you to use the cpm tool on every machine at your site (where
         applicable). Some sites run this as a cron job at regular intervals,
         such as every 15 minutes, to report any result that indicates a
         possible compromise.

         cpm (version 1.2) can be obtained from


         Below are the MD5 checksums for the tarfiles and the contents of the
         cpm.1.2 directory, when created.

                MD5 (cpm.1.2.tar) = 5f0489e868fbf213c026343cca7ec6ff
                MD5 (cpm.1.2.tar.Z) = b76285923ad17d8dbfffd9dd0082ce5b
                MD5 (cpm.1.2.tar.gz) = e689ca1c663e4c643887245f41f13a84

                MD5 (cpm.1.2/MANIFEST) = ed6ec1ca374113c074547eb0580d9240
                MD5 (cpm.1.2/README) = 34713d2be42b434a117165a5002f0a27
                MD5 (cpm.1.2/cpm.1) = 84df06d9c6687314599754f3515c461b
                MD5 (cpm.1.2/cpm.c) = 3da08fe657b96a75697a41e2700d456e
                MD5 (cpm.1.2/cpm.txt) = 5860bfb9c383f519e494a38c682c22fb

         This archive contains a readme file, also included as
         Appendix C of this advisory, containing instructions on
         installing and using this detection tool.

         Note that some sites have reported intruders gaining root access then
         reinstalling a kernel with /dev/nit functionality.

         4) Unexpected ASCII files in /dev

         Look for unexpected ASCII files in the /dev directory.
         Some of the Trojan binaries listed above rely on configuration files,
         which are often found in /dev.

         5) Modifications to /etc/rc* files and /etc/shutdown

         Check for modifications to /etc/rc* files and /etc/shutdown.
         Some intruders have modified /etc/rc files to ensure that
         the sniffer restarts after a shutdown or reboot. Others
         have modified the shutdown sequence to remove all traces of

     B.  Prevention

         There are two actions that are effective in preventing this
         attack.  A long-term solution requires eliminating
         transmission of clear-text passwords on the network.  For
         this specific attack, however, a short-term workaround
         exists.  Both of these are described below.

         1) Long-term prevention:

         We recognize that the only effective long-term solution to
         prevent these attacks is by not transmitting reusable
         clear-text passwords on the network. We have collected some
         information on relevant technologies.  This information is
         included as Appendix B in this advisory.  Note: These
         solutions will not protect against transient or remote access
         transmission of clear-text passwords through the network.

         Until everyone connected to your network is using the above
         technologies, your policy should allow only authorized users
         and programs access to promiscuous network interfaces.  The
         tool described in Section III.A.3 above may be helpful in
         verifying this restricted access.

         2) Short-term workaround:

         Regardless of whether the network monitoring software is
         detected on your system, we recommend that ALL SITES take
         action to prevent unauthorized network monitoring on their
         systems. You can do this either by removing the interface, if
         it is not used on the system or by attempting to prevent the
         misuse of this interface.

         For systems other than Sun and Solbourne, contact your vendor
         to find out if promiscuous mode network access is supported
         and, if so, what is the recommended method to disable or
         monitor this feature.

         For SunOS 4.x and Solbourne systems, the promiscuous
         interface to the network can be eliminated by removing the
         /dev/nit capability from the kernel.  The procedure for doing
         so is outlined below (see your system manuals for more
         details).  Once the procedure is complete, you may remove the
         device file /dev/nit since it is no longer functional.

         Procedure for removing /dev/nit from the kernel:

         1. Become root on the system.

         2. Apply "method 1" as outlined in the System and Network
         Administration manual, in the section, "Sun System
         Administration Procedures," Chapter 9, "Reconfiguring the
         System Kernel."  Excerpts from the method are reproduced

         # cd /usr/kvm/sys/sun[3,3x,4,4c]/conf
         # cp CONFIG_FILE SYS_NAME

         [Note that at this step, you should replace the CONFIG_FILE
         with your system specific configuration file if one exists.]

         # chmod +w SYS_NAME
         # vi SYS_NAME

            # The following are for streams NIT support.  NIT is used by
            # etherfind, traffic, rarpd, and ndbootd.  As a rule of thumb,
            # NIT is almost always needed on a server and almost never
            # needed on a diskless client.
            pseudo-device   snit            # streams NIT
            pseudo-device   pf              # packet filter
            pseudo-device   nbuf            # NIT buffering module

         [Comment out the preceding three lines; save and exit the
         editor before proceeding.]

         # config SYS_NAME
         # cd ../SYS_NAME
         # make

         # mv /vmunix /vmunix.old
         # cp vmunix /vmunix

         # /etc/halt
         > b

         [This step will reboot the system with the new kernel.]

         [NOTE that even after the new kernel is installed, you need
         to take care to ensure that the previous vmunix.old , or
         other kernel, is not used to reboot the system.]

     C.  Scope and recovery

         If you detect the network monitoring software at your site,
         we recommend following three steps to successfully
         determine the scope of the problem and to recover from this

         1. Restore the system that was subjected to the network
         monitoring software.

         The systems on which the network monitoring and/or Trojan
         horse programs are found have been compromised at the root
         level; your system configuration may have been altered.  See
         Appendix A of this advisory for help with recovery.

         2. Consider changing router, server, and privileged account
         passwords due to the wide-spread nature of these attacks.

         Since this threat involves monitoring remote connections,
         take care to change these passwords using some mechanism
         other than remote telnet, rlogin, or FTP access.

         3. Urge users to change passwords on local and remote

         Users who access accounts using telnet, rlogin, or FTP either
         to or from systems within the compromised domain should
         change their passwords after the intruder's network monitor
         has been disabled.

         4. Notify remote sites connected from or through the local
         domain of the network compromise.

         Encourage the remote sites to check their systems for
         unauthorized activity.  Be aware that if your site routes
         network traffic between external domains, both of these
         domains may have been compromised by the network monitoring


Appendix A:

A.   Immediate recovery technique

        1) Disconnect from the network or operate the system in
           single- user mode during the recovery.  This will keep users
           and intruders from accessing the system.

        2) Verify system binaries and configuration files against the
           vendor's media (do not rely on timestamp information to
           provide an indication of modification).  Do not trust any
           verification tool such as cmp(1) located on the compromised
           system as it, too, may have been modified by the intruder.
           In addition, do not trust the results of the standard UNIX
           sum(1) program as we have seen intruders modify system
           files in such a way that the checksums remain the same.
           Replace any modified files from the vendor's media, not
           from backups.

                                -- or --

           Reload your system from the vendor's media.

        3) Search the system for new or modified setuid root files.

                find / -user root -perm -4000 -print

           If you are using NFS or AFS file systems, use ncheck to
           search the local file systems.

                ncheck -s /dev/sd0a

        4) Change the password on all accounts.

        5) Don't trust your backups for reloading any file used by
           root.  You do not want to re-introduce files altered by an

        More detailed advice can be found in

B.   Improving the security of your system

        1) CERT Security Technical Tips
           The CERT/CC staff has developed technical tips and checklists based
           on information gained from computer security incidents reported to
           us. These tips are available from

        2) Security Tools
           Use security tools such as COPS and Tripwire to check for
           security configuration weaknesses and for modifications
           made by intruders.  We suggest storing these security
           tools, their configuration files, and databases offline or
           encrypted.  TCP daemon wrapper programs provide additional
           logging and access control.  These tools are available


        3) CERT Advisories
           Review past CERT advisories (both vendor-specific and
           generic) and install all appropriate patches or workarounds
           as described in the advisories.  CERT advisories and other
           security-related information are available from


           To join the CERT Advisory mailing list, send a request to:


           Please include contact information, including a telephone number.

CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890

Copyright (c) Carnegie Mellon University 1994


Appendix B:
                         ONE-TIME PASSWORDS

Given today's networked environments, CERT recommends that sites
concerned about the security and integrity of their systems and
networks consider moving away from standard, reusable passwords. CERT
has seen many incidents involving Trojan network programs (e.g.,
telnet and rlogin) and network packet sniffing programs.  These
programs capture clear-text hostname, account name, password triplets.
Intruders can use the captured information for subsequent access to
those hosts and accounts.  This is possible because 1) the password is
used over and over (hence the term "reusable"), and 2) the password
passes across the network in clear text.

Several authentication techniques have been developed that address
this problem. Among these techniques are challenge-response
technologies that provide passwords that are only used once (commonly
called one-time passwords). This document provides a list of sources
for products that provide this capability. The decision to use a
product is the responsibility of each organization, and each
organization should perform its own evaluation and selection.

I.  Publicly Available Packages

        The S/KEY package is publicly available (no fee) via
        anonymous FTP from:

                  /pub/nmh directory

        There are three subdirectories:

                skey            UNIX code and documents on S/KEY.
                                Includes the change needed to login,
                                and stand-alone commands (such as "key"),
                                that computes the one-time password for
                                the user, given the secret password and
                                the S/KEY command.

                dos             DOS or DOS/WINDOWS S/KEY programs.  Includes
                                DOS version of "key" and "termkey" which is
                                a TSR program.

                mac             One-time password calculation utility for
                                the Mac.

II.  Commercial Products

Secure Net Key (SNK)                            (Do-it-yourself project)
        Digital Pathways, Inc.
        201 Ravendale Dr.
        Mountainview, Ca. 94043-5216
        Phone: 415-964-0707
        Fax: (415) 961-7487

                        handheld authentication calculators  (SNK004)
                        serial line auth interruptors (guardian)

        Note: Secure Net Key (SNK) is des-based, and therefore restricted
        from US export.

Secure ID                                       (complete turnkey systems)
        Security Dynamics
        One Alewife Center
        Cambridge, MA   02140-2312
        Phone: 617-547-7820
        Fax: (617) 354-8836

                        SecurID changing number authentication card
                        ACE server software

        SecureID is time-synchronized using a 'proprietary' number
        generation algorithm

WatchWord and WatchWord II
        480 Spring Park Place
        Herndon, VA 22070
        1-800-521-6261 ext 217

                        Watchword authentication calculator
                        Encrypting modems

        Alpha-numeric keypad, digital signature capability

        Enigma Logic, Inc.
        2151 Salvio #301
        Concord, CA 94520
        Fax: (510)827-2593

                        DES Silver card authentication calculator
                        SafeWord Multisync card authentication calculator

        Available for UNIX, VMS, MVS, MS-DOS, Tandum, Stratus, as well as
        other OS versions.  Supports one-time passwords and super
        smartcards from several vendors.


Appendix C:
                         cpm 1.0 README FILE

       cpm -  check for network interfaces in promiscuous mode.

Copyright (c) Carnegie Mellon University 1994
Thursday Feb 3 1994

CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890

   This program is free software; you can distribute it and/or modify
   it as long as you retain the Carnegie Mellon copyright statement.

   It can be obtained via anonymous FTP from

   This program is distributed WITHOUT ANY WARRANTY; without the IMPLIED
   WARRANTY of merchantability or fitness for a particular purpose.

   This package contains:

   To create cpm under SunOS, type:
   % cc -Bstatic -o cpm cpm.c

   On machines that support dynamic loading, such as Sun's, CERT recommends
   that programs be statically linked so that this feature is disabled.

   CERT recommends that after you install cpm in your favorite directory,
   you take measures to ensure the integrity of the program by noting
   the size and checksums of the source code and resulting binary.

   The following is an example of the output of cpm and its exit status.

   Running cpm on a machine where both the le0 and le2 interfaces are
   in promiscuous mode, under csh(1):

   % cpm
   % echo $status

   Running cpm on a machine where no interfaces are in promiscuous
   mode, under csh(1):

   % cpm
   % echo $status

- ---------------------------------------------------------------------------
The CERT Coordination Center thanks the members of the FIRST community
as well as the many technical experts around the Internet who
participated in creating this advisory.  Special thanks to Eugene
Spafford of Purdue University for his contributions.
- ---------------------------------------------------------------------------

 If you believe that your system has been compromised, contact the CERT
 Coordination Center or your representative in Forum of Incident
 Response and Security Teams (FIRST).

 Internet E-mail:
 Telephone: 412-268-7090 (24-hour hotline)
            CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4),
            and are on call for emergencies during other hours.

 CERT Coordination Center
 Software Engineering Institute
 Carnegie Mellon University
 Pittsburgh, PA 15213-3890

 Past advisories, information about FIRST representatives, and other
 information related to computer security are available for anonymous
 FTP from

Copyright 1994, 1995, 1996 Carnegie Mellon University
This material may be reproduced and distributed without permission provided
it is used for noncommercial purposes and the copyright statement is

* Registered U.S. Patent and Trademark Office


      * We have seen sniffers for other platforms, i.e., Solaris.

      * Sites have reported intruders using sniffers to capture
        authentication to routers. Using that data, they compromise
        the routers and modify the configuration file.

Revision history

Apr. 03, 1997  Appendix B - corrected "Public Domain" to read "Publicly
Oct. 09, 1996  Sentence 1 - Clarified the time of the increase in the reports.
               Appendix A - Added the URL for our tech tip on root compromises.
Aug. 30, 1996  Information previously in the README was inserted
                into the advisory. Updated URLs.
July 31, 1996  Appendix B - referred to new tech tips, which replace the single
                            security checklist
Mar. 20, 1996  Sec.III.A.3 - additional information concerning cpm (v. 1.2)
Sept. 21, 1995 Sec. III.A.3 - suggestions regarding cpm
Feb. 02, 1995  Sec. III - additional information on Trojan binaries (III.A),
                          use of the /dev directory (III.A.3), and two more
                          activities (III.A.4 & III.A.5)
Feb. 02, 1995  Updates section - additional information about sniffer activity

Version: 2.6.2


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