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CA-91:12                        CERT Advisory
                               August 22, 1991
                    Trusted Hosts Configuration Vulnerability

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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.

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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.  

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

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

Internet E-mail: cert@cert.org
Telephone: 412-268-7090 24-hour hotline:
           CERT/CC personnel answer 7:30a.m.-6:00p.m. EST,
           on call for emergencies during other hours.

Past advisories and other computer security related information are available
for anonymous ftp from the cert.org (192.88.209.5) system.

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