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TUCoPS :: Unix :: General :: 9124.txt

CERT/CC Generic Security Information

Security Bulletin 9124                  DISA Defense Communications System
5 December 1991             Published by: DDN Security Coordination Center
                                      (SCC@NIC.DDN.MIL)   1-(800) 365-3642

                        DEFENSE  DATA  NETWORK
                          SECURITY  BULLETIN

The DDN  SECURITY BULLETIN is distributed  by the  DDN SCC  (Security
Coordination Center) under DISA contract as  a means of  communicating
information on network and host security exposures, fixes, &  concerns
to security & management personnel at DDN facilities.  Back issues may
be  obtained  via  FTP  (or  Kermit)  from  NIC.DDN.MIL  []
using login="anonymous" and password="guest".  The bulletin pathname is
SCC:DDN-SECURITY-yynn (where "yy" is the year the bulletin is issued
and "nn" is a bulletin number, e.g. SCC:DDN-SECURITY-9124).

+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
!                                                                       !
!     The following important information was issued by the Computer    !
!     Emergency Response Team (CERT)  and is being relayed unedited     !
!     via the Defense Information Systems Agency's Security             !
!     Coordination Center  distribution  system  as a  means  of        !
!     providing  DDN subscribers with useful security information.      !
!                                                                       !
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +


                    CERT/CC Generic Security Information

The following information is provided to sites that have, or may have,
experienced a break-in.  Section A lists several ways to determine if a system
has been compromised.  Sections B and C contain lists of vulnerabilities that
have been exploited by intruders on UNIX and VMS systems respectively.
Section D gives pointers to some tools that may be used to assist in securing
your system.

The information in this document can be used to prevent several types of
break-ins.  We encourage system administrators to review all sections of this
document and modify their systems accordingly to close these potential


A.  How To Determine If Your System Has Been Compromised

    1.  Examine log files such as your 'last' log, process accounting, syslog,
        and C2 security logs for logins from unusual locations or other unusual
        activity.  Note that this is not foolproof; many intruders edit
        accounting files in an attempt to hide their activity.

    2.  Look everywhere on the system for unusual or hidden files (files that
        start with a period and are normally not shown by ls) as these can be
        used to hide information such as password cracking programs and
        password files from other systems.  A favorite trick on UNIX systems
        is to put a hidden directory in a user's account with an unusual name;
        something like '...' or '..  ' (dot dot space space) or '..^G' (dot
        dot control-G).  Also, files with names such as '.xx' and '.mail' have
        been used.

    3.  Look for set-uid files everywhere on your system.  Intruders often
        leave set-uid copies of /bin/sh around to allow them root access at a
        later time.  The UNIX find program can be used to hunt for setuid root
        files.  The following example will find setuid root files on the '/'
        (root) partition (Note:  The find command will not follow symbolic

                  find / -user root -perm -4000 -print

    4.  Check your system binaries to make sure that they haven't been changed.
        We've seen intruders change programs on UNIX systems such as login,
        su, telnet, and other critical network and system programs.  On VMS
        systems, we've seen intruders change programs such as loginout.exe and
        show.exe.  Compare the versions on your systems with known good copies
        such as those from your initial installation tapes.  Be careful of
        trusting backups; your backups could also contain Trojan horses.

    5.  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 an intruder back on the system even after
        you've kicked him or 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.

    6.  Inspect /etc/inetd.conf for unauthorized additions or changes.  In
        particular, hunt for entries that execute a shell program
        (for example, /bin/sh or /bin/csh) and check all programs that are
        specified in /etc/inetd.conf to verify that they are correct and
        haven't been replaced by Trojan horses.

    7.  Check your system and network configuration files for unauthorized
        entries.  In particular, look for '+' (plus sign) entries and
        inappropriate non-local host names in /etc/hosts.equiv, /etc/hosts.lpd,
        and in all ~/.rhost files (especially ~root, ~uucp, ~ftp, and other
        system accounts) on the system.  These files should not be
        world-writable.  Furthermore, ensure that these files existed prior to
        any intrusion and have not been created by the intruder.  

    8.  Examine all machines on the local network when searching for signs of
        intrusion.  In particular, check those hosts that share NIS (yellow
        pages) or NFS mounted partitions, or that are referenced in
        /etc/hosts.equiv files.  Also, check any hosts with which your users
        share .rhost access.

    9.  Examine the /etc/passwd file on the system and check for any
        additional or modified accounts.  In particular, look for the
        unauthorized creation of new accounts, accounts with no passwords, or
        UID changes to existing accounts.

B.  UNIX System Configuration Problems That Have Been Exploited

    1.  Weak passwords

        Intruders often use finger or ruser to discover account names and then
        try simple passwords.  Encourage your users to choose passwords that
        are difficult to guess (for example, words that are not contained in
        any dictionary of words of any language; no proper nouns, including
        names of "famous" real or fictitious characters; no acronyms that are
        common to computer professionals; no simple variations of first or last
        names.  Furthermore, inform your users not to leave any clear text
        username/password information in files on any system. 

        A good heuristic for choosing a password is to choose an
        easy-to-remember phrase, such as "By The Dawn's Early Light", and take
        the first letters to form a password.  Insert in some punctuation or
        mixed case letters as well.  For the phrase above, one example password
        might be: bt}DeL{.  (DO NOT use this sample phrase for your password.)

        If intruders can get a password file, they 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 easily guessed.

        If you believe that your password file may have been taken, change all
        the passwords on the system.  At the very least, you should always
        change all system passwords because an intruder may concentrate on
        those and may be able to guess even a reasonably 'good' password.

        Section D details proactive steps that can be taken to ensure that
        users set 'good' passwords and that encrypted passwords are not
        visible to system users.

    2.  Use of TFTP (Trivial File Transfer Protocol) to steal password 

        To test your system for this vulnerability, connect to your system
        using tftp and try 'get /etc/passwd'.  If you can do this, anyone else
        on the network can probably get your password file.  To avoid this
        problem, either disable tftpd if you don't require it or ensure that
        it is configured with restricted access. 

        If you believe your password file may have been taken, the safest
        course is to change all passwords in the system.

    3.  Accounts without passwords or known passwords (accounts
        with vendor-supplied default passwords are favorites) 

        Intruders often exploit system default passwords that have not been
        changed since installation.  Be sure to change all default passwords
        when the software is installed.  Also, be aware that product upgrades
        can quietly change account passwords to a new default.  It is best to
        change the passwords of default accounts after updates are applied.

        Scan your password file for extra UID 0 accounts, accounts with no
        password, or new entries in the password file.  Do not allow any
        accounts without passwords.  Remove entries for unused accounts from
        the password file.  To disable an account, change the password field in
        the /etc/passwd file to an asterisk '*', and change the login shell to
        /bin/false to ensure that an intruder cannot login to the account from
        a trusted system on the network.

    4.  Holes in sendmail

        Make sure that you are running the latest sendmail from your vendor.
        BSD 5.65 fixes all known holes.  To establish which version of
        sendmail that you are running, use telnet to connect to the SMTP
        port (25) on your system:

                    telnet <your hostname> 25

    5.  Old versions of FTP;  misconfigured anonymous FTP

        Make sure that you are running the most recent version of ftpd, which
        is the Berkeley version 5.60 of July 22, 1990.  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 and
        directories available through anonymous FTP (for example, file and
        directory permissions, ownership and group).  Note that you should
        not use your system's standard password file or group file as the
        password file or group file for FTP.  The anonymous FTP root directory
        and its two subdirectories, etc and bin, should not be owned by ftp.

    6.  Fingerd hole used by the Morris Internet worm

        Make sure that you're running a version of finger that is more recent
        than November 1988.  Numerous Berkeley-derived versions of UNIX were

    7.  Inappropriate network configuration file entries

        Several vendors supply /etc/hosts.equiv files with a '+' (plus sign)
        entry.  The '+' entry should be removed from this file because it means
        that your system will trust all other systems.  Other files that
        should not contain a '+' entry include /etc/hosts.lpd and all ~/.rhost
        files on the system.  These files should not be world-writable.  We
        recommend that you do not support the following services in your
        /etc/inetd.conf file unless you specifically require them: 

                   port 11 - systat
                   port 69 - tftp
                   port 87 - link

    8.  Misconfiguration of uucp

        If your machine supports uucp, check the L.cmds (Permissions) file and
        ensure that only the commands you require are included. This file
        should be owned by root (not by uucp!) and world-readable.  The L.sys
        (Systems) file should be owned by uucp and protected (600) so that
        only programs running setuid uucp can access it.

    9.  Inappropriate 'secure' settings in /etc/ttys and /etc/ttytab

        Check the file /etc/ttys or /etc/ttytab depending on the release of
        UNIX being used.  The default setting should be that no terminal lines,
        pseudo terminals or network terminals are set secure except for the

   10.  Inappropriate entries in /usr/lib/aliases

        Examine the /usr/lib/aliases (mail alias) file for inappropriate
        entries.  Some alias files include an alias named 'uudecode' or just
        'decode'.  If this alias exists on your system, and you are not
        explicitly using it, then it should be removed.

   11.  Inappropriate file and directory protections

        Check your system documentation to establish the correct file and
        directory protections and ownership for system files and directories.
        In particular, check the '/' (root) and '/etc' directories, and all
        system and network configuration files.  Examine file and directory
        protections before and after installing software or running
        verification utilities.  Such procedures can cause file and directory
        protections to change.

   12.  Old versions of system software

        Older versions of operating systems often have security
        vulnerabilities that are well known to intruders.  To minimize your
        vulnerability to attacks, keep the version of your operating system
        up-to-date and apply security patches appropriate to your system(s) as
        soon as they become available.

C.  VMS System Vulnerabilities

    1.  Accounts with known default passwords

        Intruders often exploit system default passwords that have not been
        changed since installation.  Make sure to change all default passwords
        when the software is installed.  Be aware that product upgrades can
        quietly change account passwords to a new default.  It is best to
        change the passwords of default accounts after updates are applied.
        Accounts no longer in use should be removed from the authorization
        file and rights database.  Dormant accounts should be set to DISUSER.

        Intruders also try guessing simple user passwords.  See the discussion
        on weak passwords in Section A for suggestions on choosing good

    2.  Unauthorized versions of system files

        If an intruder gets into a system, the programs patch.exe,
        loginout.exe, and show.exe are often modified.  Compare these programs
        with those found in your distribution media.

D.  Software Tools To Assist In Securing Your System

 *  The CERT/CC will not formally review, evaluate, or endorse the tools     *
 *  and techniques described.  The decision to use the tools and             *
 *  techniques described is the responsibility of each user or               *
 *  organization, and we encourage each organization to thoroughly evaluate  *
 *  new tools and techniques before installation or use.                     *

    1.  Shadow passwords

        If your UNIX system has a shadow password capability, you should
        consider using it.  Under a shadow password system the /etc/passwd
        file does not have encrypted passwords in the password field.  Instead
        the encrypted passwords are held in a shadow file that is not world
        readable.  Consult your system manuals to determine whether a shadow
        password capability is available on your system, and to get details of
        how to set up and manage such a facility.

    2.  COPS (The Computer Oracle and Password System)

        COPS is a publicly available collection of programs that attempt to
        identify security problems in a UNIX system.  COPS does not attempt to
        correct any discrepancies found; it simply produces a report of its
        findings.  COPS was written by Dan Farmer and is available via
        anonymous FTP from the system ( in the
        /pub/cops directory and via uucp from

    3.  passwd+

        passwd+ is a replacement program suite that allows a system
        administrator to enforce policies for selecting passwords.  The suite
        also provides a logging capability.  passwd+ was written by Matt
        Bishop and can be obtained by anonymous FTP from the
        system ( in the file /pub/passwd+.tar.Z.  Please read the
        README.IMPORTANT file which accompanies this software distribution.

    4.  TCP/IP Wrapper Program

        This program provides additional network logging information and gives
        a system administrator the ability to deny or allow access from certain
        systems or domains to the host on which the program is installed.
        Installation of this software does not require any modification to
        existing network software or network configuration files.
        The program was written by Wietse Venema from Eindhoven University of
        Technology in the Netherlands and is available via anonymous FTP from
        the system ( in the file


If you believe that your system has been compromised, contact CERT/CC via
telephone or e-mail.

Internet E-mail:
Telephone: 412-268-7090 24-hour hotline:

      CERT/CC personnel answer 7:30a.m. to 6:00p.m. EST(GMT-5)/EDT(GMT-4),
      and are on call for emergencies during other hours.

Computer Emergency Response Team/Coordination Center (CERT/CC)
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890


*                                                                        *
*  Additionally, if you are a DDN user and you believe that your system  *
*  has been compromised, you need to contact the DDN Security            *
*  Coordination Center (SCC) via telephone or e-mail.                    *
*                                                                        *
*     E-mail address: SCC@NIC.DDN.MIL                                    *
*     Telephone: 1-(800) 365-3642                                        *
*                                                                        *
*  NIC help desk personnel answer 7:00 a.m. to 7:00 p.m. EST(GMT-5)      *
*  Monday through Friday except holidays.                                *
*                                                                        *

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