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CA-88:01
CERT Advisory
December 1988
ftpd vulnerability
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** The sendmail portion of this advisory is superseded by CA-95:05. **
There have been several problems or attacks which have occurred in the
past few weeks. In order to help secure your systems we have gathered
the following suggestions:
1) Check that you are using version 5.59 of sendmail with the
debug option DISABLED. To verify the version try the following
commands. Use the telnet program to connect to your mail server.
Telnet to your hostname or localhost with 25 following the host.
The sendmail program will print a banner which will have the
version number in it. You need to be running version 5.59.
Version 5.61 will be released on Monday 12/12/1988. Any
version less than 5.59 is a security problem.
The following is a sample of the telnet command.
% telnet localhost 25
Trying...
Connected to localhost.SEI.CMU.EDU.
220 ed.sei.cmu.edu Sendmail 5.59 ready at Wed, 7 Dec 88 15:45:55 EST
Quit
221 ed.sei.cmu.edu closing connection
Connection closed by foreign host.
%
2) Verify with your systems support staff that the ftpd program
patches have been installed. Removing anonymous ftp is now
known to NOT plug all security holes. If you are not sure,
ftp to ucbarpa.berkeley.edu, login as anonymous password ftp
and get ftpd.shar. This file contains the sources to the
latest BSD release of the ftpd program.
3) Check your /etc/passwd file for bogus entries. Look for
unauthorized accounts with the uid field set to zero (only
the root account should have uid=0). Remove any unauthorized
entries. The following is an example of what you might find.
install::0:1::/:
To check your /etc/passwd files for spurious accounts with uid 0,
you can use the following awk program:
% awk -F: '$3 == 0 {print $0}' /etc/passwd
If you are running YP on your machine, do:
% ypcat passwd | awk [...as above]
4) Look for modified /bin/login and /usr/ucb/telnet files.
Several sites have found these programs with new "backdoors"
added. Use the strings program to search /bin/login for the
strings OURPW, knaobj, and knaboj. If in doubt, reload the
/bin/login and /usr/ucb/telnet executables from your
distribution tape.
% strings /bin/login | egrep '(OURPW|knaboj|knaobj)'
5) Educate your users to create hard to guess passwords. Account
codes, first or last names, and common words are not very
secure passwords. A few examples of common words are words
that refer to your town, location, or company and words that
are found in /usr/dict/words. Be especially careful of accounts
where the password is the account name (easy to check, easy to
guess.
6) In general, before you allow a user access to the Internet,
you must be sure you know who they are. In other words, all
users should be forced through a login/password sequence
(no unpassworded accounts and preferably someplace which logs
connections) before you let them get outside your local network.
Be especially careful with TCP/IP terminal servers.
7) check the last logs for normal logins as accounts which normally
run utility programs (sync, who, etc), watch for unreasonable
times.. watch for ftp's with funny logins (who, etc).
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Computer Emergency Response Team (CERT)
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890
Internet:
[email protected]
Telephone: 412-268-7090 24-hour hotline: CERT personnel answer
7:30a.m.-6:00p.m. EST, on call for
emergencies other hours.
Past advisories and other information are available for anonymous ftp
from cert.org (192.88.209.5).
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