
CA-89:03
                                 CERT Advisory
                                August 16, 1989
                            Telnet Breakin Warning

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Many computers connected to the Internet have recently experienced
unauthorized system activity.  Investigation shows that the activity
has occurred for several months and is spreading.  Several UNIX
computers have had their "telnet" programs illicitly replaced with
versions of "telnet" which log outgoing login sessions (including
usernames and passwords to remote systems).  It appears that access
has been gained to many of the machines which have appeared in some of
these session logs.  (As a first step, frequent telnet users should
change their passwords immediately.)  While there is no cause for
panic, there are a number of things that system administrators can do
to detect whether the security on their machines has been compromised
using this approach and to tighten security on their systems where
necessary.  At a minimum, all UNIX site administrators should do the
following:

o Test telnet for unauthorized changes by using the UNIX "strings"
  command to search for path/filenames of possible log files.  Affected
  sites have noticed that their telnet programs were logging information
  in user accounts under directory names such as "..." and ".mail".

In general, we suggest that site administrators be attentive to
configuration management issues.  These include the following:


o Test authenticity of critical programs - Any program with access to
  the network (e.g., the TCP/IP suite) or with access to usernames and
  passwords should be periodically tested for unauthorized changes.
  Such a test can be done by comparing checksums of on-line copies of
  these programs to checksums of original copies.  (Checksums can be
  calculated with the UNIX "sum" command.)  Alternatively, these
  programs can be periodically reloaded from original tapes.

o Privileged programs - Programs that grant privileges to users (e.g.,
  setuid root programs/shells in UNIX) can be exploited to gain
  unrestricted access to systems.  System administrators should watch
  for such programs being placed in places such as /tmp and /usr/tmp (on
  UNIX systems).  A common malicious practice is to place a setuid shell
  (sh or csh) in the /tmp directory, thus creating a "back door" whereby
  any user can gain privileged system access.

o Monitor system logs - System access logs should be periodically
  scanned (e.g., via UNIX "last" command) for suspicious or unlikely
  system activity.

o Terminal servers - Terminal servers with unrestricted network access
  (that is, terminal servers which allow users to connect to and from
  any system on the Internet) are frequently used to camouflage network
  connections, making it difficult to track unauthorized activity.
  Most popular terminal servers can be configured to restrict network
  access to and from local hosts.

o Passwords - Guest accounts and accounts with trivial passwords
  (e.g., username=password, password=none) are common targets.  System
  administrators should make sure that all accounts are password
  protected and encourage users to use acceptable passwords as well as
  to change their passwords periodically, as a general practice.  For
  more information on passwords, see Federal Information Processing
  Standard Publication (FIPS PUB) 112, available from the National
  Technical Information Service, U.S. Department of Commerce,
  Springfield, VA 22161.

o Anonymous file transfer - Unrestricted file transfer access to a
  system can be exploited to obtain sensitive files such as the UNIX
  /etc/passwd file.  If used, TFTP (Trivial File Transfer Protocol -
  which requires no username/password authentication) should always be
  configured to run as a non-privileged user and "chroot" to a file
  structure where the remote user cannot transfer the system /etc/passwd
  file.  Anonymous FTP, too, should not allow the remote user to access
  this file, or any other critical system file.  Configuring these
  facilities to "chroot" limits file access to a localized directory
  structure.

o Apply fixes - Many of the old "holes" in UNIX have been closed.
  Check with your vendor and install all of the latest fixes.


If system administrators do discover any unauthorized system activity,
they are urged to contact the Computer Emergency Response Team (CERT).

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

Internet: 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.

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