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CA-91:15                        CERT Advisory
                               September 10, 1991
			Mac/PC NCSA Telnet Vulnerability

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The Computer Emergency Response Team/Coordination Center (CERT/CC) has
received information concerning a vulnerability in the default
configurations of National Center for Supercomputing Applications
(NCSA) Telnet for both the Macintosh and the PC.  The vulnerability
also affects the version of NCSA Telnet with IBM 3270 terminal
emulation distributed by Clarkson University.  Two workarounds are
available that correct this problem.

NCSA has committed to changing the default configurations in future
releases.  Maintenance updates for both the Macintosh and the PC are
planned to be released in about 2 months.

NCSA provides two e-mail addresses for Telnet questions, comments,
and bug reports:

     PC Telnet		pctelnet@ncsa.uiuc.edu
     Mac Telnet		mactelnet@ncsa.uiuc.edu

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I.   Description

     The default configuration of NCSA Telnet for both the Macintosh
     and the PC has a serious vulnerability in its implementation of
     an ftp server.

     The default configuration file enables ftp via the "ftp=yes"
     line.  However, sites should be aware that ftp is also enabled
     in the absence of any ftp statement in the configuration file.

II.  Impact

     Any Internet user can connect via ftp to a PC or Macintosh
     running the default configuration of NCSA Telnet and gain
     unauthorized read and write access to any of its files, including
     system files.

III. Solution
        
     Either disable ftp server functionality or provide password
     protection as described below.

     To disable the ftp server, add an "ftp=no" line in the
     configuration file.

     If the ftp server option is enabled (via either an "ftp=yes" line 
     in the configuration file or the absence of an ftp statement in the 
     configuration file), then the Telpass program (included with both 
     Mac and PC versions) can be used to provide password protection.  
     Telpass is used to enter usernames and encrypted passwords into a 
     password file.  The configuration file specifies the name and 
     location of the password file in the "passfile=" statement.  The 
     usage of Telpass is documented in Chapter 5 of version 2.4 of the 
     Macintosh version documentation and Chapter 7 of version 2.3 of the 
     PC version.  Note that the documentation (as well as the package 
     itself) is available by anonymous ftp from ftp.ncsa.uiuc.edu 
     (141.142.20.50).

     The instructions for enabling password protection differ between
     the Macintosh and PC versions, but in both cases they involve
     enabling the "passfile" option in the configuration file, and
     creating usernames and encrypted passwords with the Telpass
     program.  

     CERT/CC strongly urges all sites running NCSA Telnet to implement
     one of these two workarounds.

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The CERT/CC would like to thank NCSA and Clarkson University for their
assistance.
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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. EDT,
           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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