SMRSH(8) SMRSH(8)

NAME
smrsh - restricted shell for sendmail

SYNOPSIS
smrsh -c command

DESCRIPTION
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 lim-
its 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 ‘‘’, ‘<’, ‘>’, ‘|’, ‘;’,’ (carriage return), or ‘0 (new-
‘&’, ‘$’, ‘(’, ‘)’, ‘
line) 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 popu-
lating /usr/adm/sm.bin. Reasonable additions are vaca-
tion(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
Compilation should be trivial on most systems. You may
need to use -DPATH=
path (defaults to ‘‘/bin:/usr/bin:/usr/ucb’’) and/or
-DCMDBIN=
(defaults to ‘‘/usr/adm/sm.bin’’).

FILES
/usr/adm/sm.bin - directory for restricted programs