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CA-91:18                        CERT Advisory
                              September 27, 1991
                         Active Internet tftp Attacks

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The Computer Emergency Response Team/Coordination Center (CERT/CC) would
like to alert you to automated tftp probes that have been occurring over
the last few days.  These probes have attacked Internet sites throughout 
the world and in most cases the file retrieved was /etc/passwd.  However, 
other files such as /etc/rc may have been retrieved. 

The CERT/CC is working with the site(s) that were used by intruders
to launch the attacks.  We are actively contacting those sites where we
believe the retrievals were successful.  We are urging all sites to 
carefully check their system configurations concerning tftp usage.

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I.   Description

     Unrestricted tftp access allows remote sites to retrieve
     a copy of any world-readable file.

II.  Impact

     Anyone on the Internet can use tftp to retrieve copies of a
     site's sensitive files.  For example, the recent incident
     involved retrieving /etc/passwd.  The intruder can later
     crack the password file and use the information to login 
     to the accounts.  This method may provide access to the 
     root account.

III. Solution
        
     A.  Sites that do not need tftp should disable it immediately by
     editing the system configuration file to comment out, or remove,
     the line for tftpd.  This file may be /etc/inetd.conf, /etc/servers,
     or another file depending on your operating system.  To cause 
     the change to be effective, it will be necessary to restart
     inetd or force inetd to read the updated configuration file.

     B.  Sites that must use tftp (for example, for booting diskless
     clients) should configure it such that the home directory is changed.  
     Example lines from /etc/inetd.conf might look like:

     ULTRIX 4.0
     tftp   dgram  udp  nowait  /etc/tftpd  tftpd -r /tftpboot

     SunOS 4.1
     tftp   dgram  udp  wait  root  /usr/etc/in.tftpd in.tftpd -s /tftpboot

     As in item A. above, inetd must be restarted or forced to read 
     the updated configuration file to make the change effective.

     C.  If your system has had tftp configured as unrestricted, the CERT/CC
     urges you to consider taking one of the steps outlined above and
     change all the passwords on your system.

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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/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.
