Trusted Hosts Configuration Vulnerability

Original issue date: August 22, 1991
Last revised: September 18, 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 concerning a vulnerability in the configuration of several system files.  This advisory discusses a workaround since there are no permanent patches available at this time.

This vulnerability is present in a very large number of UNIX-based operating systems. Therefore, we recommend that ALL sites take the corrective actions listed below.

I. Description

The presence of a '-' as the first character in /etc/hosts.equiv, /etc/hosts.lpd and .rhosts files may allow unauthorized access to the system.

II. Impact

Remote users can gain unauthorized root access to the system.

III. Solution

Rearrange the order of entries in the hosts.equiv, hosts.lpd, and .rhosts files so that the first line does not contain a leading '-' character.

Remove hosts.equiv, hosts.lpd, and .rhosts files containing only entries beginning with a '-' character.

.rhosts files in ALL accounts, including root, bin, sys, news, etc., should be examined and modified as required.  .rhosts files that are not needed should be removed.   

Please note that the CERT/CC strongly cautions sites about the use of hosts.equiv and .rhosts files.  We suggest that they NOT be used unless absolutely necessary. 

The CERT/CC wishes to thank Alan Marcum, NeXT Computer, for bringing this security vulnerability to our attention.  We would also like to thank CIAC for their assistance in testing this vulnerability.

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.

Conditions for use, disclaimers, and sponsorship information

Copyright 1991 Carnegie Mellon University.

Revision History
September 18,1997  Attached Copyright Statement