Altered System Binaries Incident

Original issue date: June 22, 1992
Last revised: September 19, 1997
Attached copyright statement

A complete revision history is at the end of this file.

The Computer Emergency Response Team/Coordination Center (CERT/CC) has received information regarding a series of significant intrusion incidents on the Internet.  Systems administrators should be aware that many systems on the Internet have been compromised due to this activity.  To identify whether your systems have been affected by the activity we recommend that all system administrators check for the signs of intrusion detailed in this advisory.

This advisory describes the activities that have been identified as part of this particular incident.  This does not address the possibility that systems may have been compromised due to other, unrelated intrusion activity.

I. Description

The intruders gain initial access to a host by discovering a password for a user account on the system, exploiting a "+" in the "/etc/hosts.equiv" file, or any ".rhosts" files on the system.  The intruder then connects to the system using rsh and attempts to become root on the compromised system.  An alias of "decode" may used to gain root privileges.

II. Impact

Having gained root access on a system, the intruders may make unauthorized changes to system binaries that can capture account information for both local and remote systems.  In addition, the intruder adds "+ +" to any ".rhosts" files to which the intruder has access.

III. Solution

A. Check your systems for signs of intrusion due to this incident.

  1. Check the login, telnet, and uucpd binaries (for example, "/bin/login", "/usr/ucb/telnet", and "/usr/etc/in.uucpd" on Sun systems) against copies from distribution media.  Note that   a check for creation or modification times and sizes is not sufficient to assure that the files have not been modified.   The CERT/CC suggests that you compare the output of the "sum(1)" or "cmp(1)" command on both the distribution and installed versions of the binaries.
  2. If the check from (A.1) indicates that your binaries have been   modified, check for the presence of a password log file.  Since the name of the logfile is often changed,   the name of the file should be obtained using the "strings(1)" command on the Trojan login, uucpd, or telnet binary.  Examples of filenames used on other systems are:

    "/usr/spool/. " (dot space)

    Verify that the contents of files found using the "strings(1)" command do not contain valid username/password combinations. 

  3. Check for the presence of "+" in the "/etc/hosts.equiv" file. 

    NOTE that Sun Microsystems installs the SunOS operating system with a default "+" in the /etc/hosts.equiv   file for easy network access.  This should be removed unless required in your operating environment and protected by a firewall network configuration.  Leaving the "+"   intact will allow any non-root user on the Internet to login to the system without a password.

  4. Check the home directory for each entry in the "/etc/passwd" file for the presence of a ".rhosts" file containing "+ +" (plus space plus).

  5. Assure that your "/etc/fstab", "/etc/inetd.conf", and "/etc/exports" files have not been modified.

B. Take the following steps to secure your systems.

  1. Save copies of the identified files to removable media and remove any password log files as found in (A.2) above.
  2. Replace any modified binaries with copies from distribution media.
  3. Remove the "+" entry from the "/etc/hosts.equiv" file and the "+ +" (plus space plus) entry from any ".rhosts" files. 

  4. Change ownership of the "/etc" directory to userid "root" if it is owned by "bin" (as distributed by Sun).

  5. Change every password on the system and assure that the new passwords are robust using a package such as Crack or Cops (available via anonymous ftp from

  6. Inspect and restore any changes made to your "/etc/fstab", "/etc/exports", or "/etc/inetd.conf" files.  If any modifications are found in these files, you will need to   unmount file systems and restart daemons once the files   have been restored.  Alternatively the system could be rebooted.

  7. Remove the "decode" alias from your global mail aliases file ("/etc/aliases" on Sun systems, "/usr/lib/aliases" on other UNIX systems).

This document is available from:

CERT/CC Contact Information

Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
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.

Using encryption

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 information.

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 infringement.

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Copyright 1992 Carnegie Mellon University.

Revision History
September 19,1997  Attached copyright statement