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Home : Advisories : Internet Intruder Warning

Title: Internet Intruder Warning
Released by: CERT
Date: 19th March 1990
Printable version: Click here

Hash: SHA1


Last revised: September 17,1997

                Attached copyright statement

                            CERT Advisory

                            March 19, 1990

                      Internet Intruder Warning

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

There have been a number of media reports stemming from a March 19 New York

Times article entitled 'Computer System Intruder Plucks Passwords and

Avoids Detection.'  The article referred to a program that attempts to

get into computers around the Internet.

At this point, the Computer Emergency Response Team Coordination

Center (CERT/CC) does not have hard evidence that there is such a

program.  What we have seen are several persistent attempts on systems

using known security vulnerabilities.  All of these vulnerabilities

have been previously reported.  Some national news agencies have

referred to a 'virus' on the Internet; the information we have now

indicates that this is NOT true.  What we have seen and can confirm is

an intruder making persistent attempts to get into Internet systems.

It is possible that a program may be discovered.  However, all the

techniques used in these attempts have also been used, in the past, by

intruders probing systems manually.

As of the morning of March 19, we know of several systems that have

been broken into and several dozen more attempts made on Thursday and

Friday, March 15 and 16.

Systems administrators should be aware that many systems around the

Internet may have these vulnerabilities, and intruders know how to

exploit them.  To avoid security breaches in the future, we recommend

that all system administrators check for the kinds of problems noted

in this message.

The rest of this advisory describes problems with system

configurations that we have seen intruders using.  In particular, the

intruders attempted to exploit problems in Berkeley BSD derived UNIX

systems and have attacked DEC VMS systems.  In the advisory below,

points 1 through 12 deal with Unix, points 13 and 14 deal with the VMS


If you have questions about a particular problem, please get

in touch with your vendor.

The CERT makes copies of past advisories available via anonymous FTP

(see the end of this message).  Administrators may wish to review

these as well.

We've had reports of intruders attempting to exploit the following


1) Use TFTP (Trivial File Transfer Protocol) to steal password files.  

   To test your system for this vulnerability, connect to your system

using TFTP and try 'get /etc/motd'.  If you can do this, anyone else

can get your password file as well.  To avoid this problem, disable


   In conjunction with this, encourage your users to choose passwords

that are difficult to guess (e.g. words that are not contained in any

dictionary of words of any language; no proper nouns, including names

of "famous" real or imaginary characters; no acronyms that are common

to computer professionals; no simple variations of first or last

names, etc.)  Furthermore, inform your users not to leave any clear

text username/password information in files on any system.

   If an intruder can get a password file, he/she will usually take it

to another machine and run password guessing programs on it. These

programs involve large dictionary searches and run quickly even on slow

machines.  The experience of many sites is that most systems that do

not put any controls on the types of passwords used probably have at

least one password that can be guessed.

2) Exploit accounts without passwords or known passwords (accounts

with vendor supplied default passwords are favorites).  Also uses

finger to get account names and then tries simple passwords.  

   Scan your password file for extra UID 0 accounts, accounts with no

password, or new entries in the password file.  Always change vendor

supplied default passwords when you install new system software.

3) Exploit holes in sendmail.

   Make sure you are running the latest sendmail from your vendor.

BSD 5.61 fixes all known holes that the intruder is using.  

4) Exploit bugs in old versions of FTP; exploit mis-configured

   anonymous FTP

   Make sure you are running the most recent version of FTP which is

the Berkeley version 4.163 of Nov.  8 1988.  Check with your vendor

for information on configuration upgrades.  Also check

your anonymous FTP configuration.  It is important to follow the

instructions provided with the operating system to properly configure

the files available through anonymous ftp (e.g., file permissions,

ownership, group, etc.).  Note especially that you should not use your

system's standard password file as the password file for FTP.

5) Exploit the fingerd hole used by the Morris Internet worm.

   Make sure you're running a recent version of finger.  Numerous

Berkeley BSD derived versions of UNIX were vulnerable.

Some other things to check for:

6) Check user's .rhosts files and the /etc/hosts.equiv files for systems

outside your domain.  Make sure all hosts in these files are

authorized and that the files are not world-writable.

7) Examine all the files that are run by cron and at.  We've seen

intruders leave back doors in files run from cron or submitted to at.

These techniques can let the intruder back on the system even after

you've kicked him/her off.  Also, verify that all files/programs

referenced (directly or indirectly) by the cron and at jobs, and the

job files themselves, are not world-writable.

8) If your machine supports uucp, check the L.cmds file to see if

they've added extra commands and that it is owned by root (not by uucp!)

and world-readable.  Also, the L.sys file should not be world-readable

or world-writable.

9) Examine the /usr/lib/aliases (mail alias) file for unauthorized

entries.  Some alias files include an alias named 'uudecode'; if this

alias exists on your system, and you are not explicitly using it, then

it should be removed.

10) Look for hidden files (files that start with a period and are

normally not shown by ls) with odd names and/or setuid capabilities,

as these can be used to "hide" information or privileged (setuid root)

programs, including /bin/sh.  Names such as '..  ' (dot dot space

space), '...', and .xx have been used, as have ordinary looking names

such as '.mail'.  Places to look include especially /tmp, /usr/tmp,

and hidden directories (frequently within users' home directories).

11) Check the integrity of critical system programs such as su, login,

and telnet.  Use a known, good copy of the program, such as the

original distribution media and compare it with the program you are


12) Older versions of systems often have security vulnerabilities that

are well known to intruders.  One of the best defenses against

problems is to upgrade to the latest version of your vendor's system.


13) The intruder exploits system default passwords that have not been

changed since installation.  Make sure to change all default passwords

when the software is installed.  The intruder also guesses simple user

passwords.  See point 1 above for suggestions on choosing good


14) If the intruder gets into a system, often the programs

loginout.exe and show.exe are modified.  Check these programs against

the files found in your distribution media.

If you believe that your system has been compromised, contact CERT via

telephone or e-mail.

Computer Emergency Response Team (CERT)

Software Engineering Institute

Carnegie Mellon University

Pittsburgh, PA 15213-3890

Internet E-mail: cert@cert.org

Telephone: 412-268-7090 24-hour hotline: CERT personnel answer 

           7:30a.m.-6:00p.m. EST, on call for emergencies

            other hours.

Past advisories and other information are available for anonymous ftp

from cert.org (

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

Copyright 1989 Carnegie Mellon University. Conditions for use, disclaimers,

and sponsorship information can be found in

http://www.cert.org/legal_stuff.html and http://ftp.cert.org/pub/legal_stuff .

If you do not have FTP or web access, send mail to cert@cert.org with

"copyright" in the subject line.

CERT is registered in the U.S. Patent and Trademark Office.


Revision History

September 17, 1997  Attached Copyright Statement


Version: PGP for Personal Privacy 5.0

Charset: noconv





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