The smrsh program is intended as a replacement for sh for use in the prog mailer in sendmail(8) configuration files. It sharply limits the commands that can be run using the |program syntax of sendmail in order to improve the over all security of your system. Briefly, even if a bad guy can get sendmail to run a program without going through an alias or forward file, smrsh limits the set of programs that he or she can execute.
Briefly, smrsh limits programs to be in a single directory, by default /usr/libexec/sm.bin, allowing the system administrator to choose the set of acceptable commands, and to the shell builtin commands exec, exit, and echo. It also rejects any commands with the characters `, <, >, ;, $, (, ), \r (carriage return), or \n (newline) on the command line to prevent end run attacks. It allows || and && to enable commands like: "|exec /usr/local/bin/filter || exit 75"
Initial pathnames on programs are stripped, so forwarding to /usr/bin/vacation, /home/server/mydir/bin/vacation, and vacation all actually forward to /usr/libexec/sm.bin/vacation.
System administrators should be conservative about populating the sm.bin directory. For example, a reasonable additions is vacation(1), and the like. No matter how brow-beaten you may be, never include any shell or shell-like program (such as perl(1)) in the sm.bin directory. Note that this does not restrict the use of shell or perl scripts in the sm.bin directory (using the #! syntax); it simply disallows execution of arbitrary programs. Also, including mail filtering programs such as procmail(1) is a very bad idea. procmail(1) allows users to run arbitrary programs in their procmailrc(5).
Compilation should be trivial on most systems. You may need to use -DSMRSH_PATH=\"path\" to adjust the default search path (defaults to /bin:/usr/bin) and/or -DSMRSH_CMDDIR=\"dir\" to change the default program directory (defaults to /usr/libexec/sm.bin).
/usr/adm/sm.bin - default directory for restricted programs on most OSs
/var/adm/sm.bin - directory for restricted programs on HP UX and Solaris
/usr/libexec/sm.bin - directory for restricted programs on FreeBSD (>= 3.3) and DragonFly BSD