smrsh - restricted shell for sendmail
smrsh -c command
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 the directory /usr/adm/sm.bin, allowing the system administrator to choose the set of acceptable commands. It also rejects any commands with the characters ’`’, ’<’, ’>’, ’|’, ’;’, ’&’, ’$’, ’(’, ’)’, ’\r’ (carriage return), or ’\n’ (newline) on the command line to prevent ’’end run’’ attacks.
Initial pathnames on programs are stripped, so forwarding to ’’/usr/ucb/vacation’’, ’’/usr/bin/vacation’’, ’’/home/server/mydir/bin/vacation’’, and ’’vacation’’ all actually forward to ’’/usr/adm/sm.bin/vacation’’.
System administrators should be conservative about populating /usr/adm/sm.bin. Reasonable additions are vacation(1), procmail(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.
Compilation should be trivial on most systems. You may need to use -DPATH=\"path\" to adjust the default search path (defaults to ’’/bin:/usr/bin:/usr/ucb’’) and/or -DCMDBIN=\"dir\" to change the default program directory (defaults to ’’/usr/adm/sm.bin’’).
/usr/adm/sm.bin - directory for restricted programs
sendmail(8)