Improving the Security of Mail Services

The following precautions should be taken to ensure secure operation of sendmail [GS91]:

1. Verify that the version of sendmail used is recent. Older versions of sendmail have several bugs that allow security violations.

2. Remove the ``uudecode'' and ``decode'' alias from the aliases file. This file is usually /etc/aliases or /usr/lib/aliases.

3. For aliases that allow messages to be sent to programs, make sure that there is no way to obtain a shell or send commands to a shell from these programs.

4. Verify that the ``wizard'' password is disable in the configuration file sendmail.cf.

5. Verify that sendmail does not support the ``debug'' command. This can be done with the following commands:
   % telnet localhost 25
   Connected to localhost
   Escape character is ``^]''.
   220 hostname sendmail 5.61 ready at Fri, 18 Sep 92 15:10:48 EDT
   debug
   500 Command unrecognized
   quit
   %
   If sendmail responds to the ``debug'' command with the message ``200 Debug set'', then sendmail is vulnerable to attack and should be replaced with a newer version.