CERT® Advisory CA-1994-01 Ongoing Network Monitoring Attacks
Original issue date: February 3, 1994
Last revised: September 19, 1997
Updated copyright statement
A complete revision history is at the end of this file.
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.
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.
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.
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.
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
horse programs commonly used in conjunction with this attack,
- any suspect processes running on the system,
unauthorized use of /dev/nit,
- unexpected ASCII files in the
/dev directory, and
- 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
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
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
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:
- Become root on the system.
- Apply "method 1" as outlined in the System and
Network Administration manual, in the section, "Sun System
Administration Procedures," Chapter 9, "Reconfiguring the
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
# mv /vmunix /vmunix.old
# cp vmunix /vmunix
[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 networkmonitoring 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 accounts.
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: RECOVERING FROM A UNIX ROOT COMPROMISE
A. Immediate recovery technique
More detailed advice can be found in
- 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.
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
-- or --
Reload your system from the vendor's media.
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
- Change the password on all accounts.
Don't trust your backups for reloading any file used by
root. You do not want to re-introduce files altered by an
B. Improving the security of your system
- 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
- 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
- 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.
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:
thumper.bellcore.com /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
II Commercial Products:
Digital Pathways, Inc.
Secure Net Key (SNK)
201 Ravendale Dr.
Mountainview, Ca. 94043-5216
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.
|(complete turnkey systems)|
One Alewife Center
Cambridge, MA 02140-2312
Fax: (617) 354-8836
SecurID changing number authentication card
ACE server software
SecureID is time-synchronized using a 'proprietary' number
WatchWord and WatchWord II
480 Spring Park Place
Herndon, VA 22070
1-800-521-6261 ext 217
Watchword authentication calculator
Alpha-numeric keypad, digital signature capability
Enigma Logic, Inc.
2151 Salvio #301
Concord, CA 94520
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.
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 ftp.cert.org:pub/tools/cpm.tar.Z.
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):
% echo $status
Running cpm on a machine where no interfaces are in promiscuous
mode, under csh(1):
% 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.
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) / EDT(GMT-4)
Monday through Friday; they are on call for emergencies during other
hours, on U.S. holidays, and on weekends.
We strongly urge you to encrypt sensitive information sent by
email. Our public PGP key is available from
If you prefer to use DES, please call the CERT hotline for more
Getting security information
CERT publications and other security information are available from
our web site
* "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office.
Any material furnished by Carnegie Mellon University and the
Software Engineering Institute is furnished on an "as is"
basis. Carnegie Mellon University makes no warranties of any kind,
either expressed or implied as to any matter including, but not
limited to, warranty of fitness for a particular purpose or
merchantability, exclusivity or results obtained from use of the
material. Carnegie Mellon University does not make any warranty of any
kind with respect to freedom from patent, trademark, or copyright
Conditions for use, disclaimers, and sponsorship information
Copyright 1994, 1995, 1996, 1997 Carnegie Mellon University.
Sept. 19, 1997 Updated Copyright statement
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
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