CERT Summary CS-95:01

CERT Advisory cert-advisory at cert.org
Wed Jul 26 20:35:12 UTC 1995

CERT Summary CS-95:01
July 26, 1995

As part of our ongoing efforts to disseminate timely information about
Internet security issues, the CERT Coordination Center is pleased to announce
the CERT Summary.  CERT Summary will be distributed periodically to call
attention to the types of attacks currently being reported to the CERT
Coordination Center.  The summary will include pointers to sources of
information for dealing with the problems.


Recent Activity

The majority of incidents reported to the CERT Coordination Center in 
recent weeks fall into three categories:

1.  IP Spoofing:  We have seen a surge in IP spoofing.  In recent weeks, we
    have received more than 170 reports of IP spoofing attacks or probes, many
    of them resulting in a successful break in.  Several sites believed
    incorrectly that they were blocking such packets.  Others planned to block
    them but hadn't yet done so. 

    We urge you to take the time to review CERT Advisory CA-95:01, "IP
    Spoofing Attacks and Hijacked Terminal Connections," which has
    details on this type of attack and how to prevent it. Vulnerability 
    to IP spoofing attacks is NOT limited to any specific router or OS vendor.
    This advisory is available from


2.  Packet Sniffers:  We receive new reports daily that describe sniffers
    installed on compromised hosts.  These sniffers, which are used to collect
    account names and passwords, are frequently installed using a kit.
    Further information on packet sniffers is available from


    Once root is compromised on a system, the sniffer kit can be activated
    to collect account names and passwords.  Note that even if sniffing
    capabilities are disabled by recompiling and rebooting the kernel, we
    have received reports of intruders re-enabling these capabilities by
    recompiling and rebooting systems.  Pay particular attention to every
    system reboot.

    In the attacks that we have seen, intruders frequently install (as root)
    Trojan horse system software that is available with the sniffer kit.
    Further information on the Trojan horses that we have seen is available


3.  NFS Attacks:  We have seen a large increase in the number of attacks
    and probes against weaknesses within NFS. Again, many of these are
    successful.  Programs to automate such attacks have become widespread.

    Please review CERT Advisory CA-94:15, "NFS Vulnerabilities," and its
    associated README file. They are available from


    A successful attack usually results in the intruders gaining root access.
    Intruders have been targeting machines with vendor-licensed source code.
    They appear to be in search of the code for system software with the view
    to creating and installing copies containing Trojan horses on compromised
    systems.  If you manage systems that contain vendor-licensed source code,
    pay particular attention to the "Security Measures" section of the

New Trojan Horse Programs
    Once root has been compromised on a system, we are finding new Trojan
    horses installed in the inetd and in.rexecd daemons.  These Trojan horses
    allow an intruder to gain access at a later time, bypassing most firewall
    and TCP wrapper configurations.  Do not rely on checksums determined by
    the sum(1) program because intruders are creating files whose
    checksums match those of the vendor-distributed versions.  Do not rely
    on time stamps of these files either because intruders are setting
    these to previous values as well.

    We recommend that you use a known clean version of cmp(1) to make a direct
    comparison of the binaries and the appropriate distribution media.

    Alternatively, you can check the MD5 results on suspect binaries against
    a list of MD5 checksums from known good binaries.  Ask your vendor to make
    MD5 checksums available for their distribution binaries.  You can also
    consult the following for some additional information on MD5:


   In addition, tools such as Tripwire can archive MD5 checksums of known good
   binaries when used immediately after a system installation.  If you use
   Tripwire, you should regularly maintain the checksums on removable or
   read-only media.  For more details on Tripwire and MD5, see the CERT
   security checklist:


    We are also seeing Trojan horses introduced into shared object libraries.
    Examples are /usr/lib/libc.so.* and /usr/kvm/libkvm.so.* on
    SunOS-based machines.  Although we have only received reports of
    SunOS-based machines being altered, the techniques used by intruders are
    applicable to other systems that use shared object libraries.  These
    libraries are being modified so that the presence of certain directories
    and processes cannot be detected with vendor-provided programs or public
    domain programs built to use shared object libraries.  This means that ANY
    program using these shared libraries will act in the manner described by
    the intruder without the intruder necessarily having to modify the program

    The Trojan horse daemons previously described typically become "invisible"
    to programs such as ps once the kvm shared object library has been
    modified.  Similarly, the directories used by intruders for building
    these daemons are "invisible" once the libc shared object library
    has been modified.  Again, do not rely on checksums using sum(1) or time
    stamps to detect altered files.  Use a known clean version of cmp(1) or a
    strong checksum technique such as MD5 to verify your files against the
    appropriate distribution.


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

If you wish to send sensitive incident or vulnerability information to
CERT staff by electronic mail, we strongly advise that the e-mail be
encrypted.  The CERT Coordination Center can support a shared DES key, PGP
(public key available via anonymous FTP on info.cert.org), or PEM (contact
CERT staff for details). 

Internet e-mail: cert at cert.org
Telephone: +1 412-268-7090 (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.
Fax: +1 412-268-6989

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

CERT advisories and bulletins are posted on the USENET news group
comp.security.announce. If you would like to have future advisories and
bulletins mailed to you or to a mail exploder at your site, please send
mail to cert-advisory-request at cert.org. 

Past CERT publications, information about FIRST representatives,
and other information related to computer security are available by
anonymous FTP from info.cert.org. 


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

CERT is a service mark of Carnegie Mellon University.

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