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 a single direc
tory, by default /etc/smrsh, allowing the system adminis
trator to choose the set of acceptable commands, and to
the shell builtin commands ``exec'', ``exit'', and
``echo''. It also rejects any commands with the charac
ters ``', `<', `>', `;', `$', `(', `)', `\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/procmail -f-
/etc/procmailrcs/user || exit 75"''
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 ``/etc/smrsh/vacation''.
System administrators should be conservative about popu
lating the /etc/smrsh directory. 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 /etc/smrsh direc
tory. Note that this does not restrict the use of shell
or perl scripts in the /etc/smrsh directory (using the
``#!'' syntax); it simply disallows execution of arbitrary
programs.
FILES
/etc/smrsh - directory for restricted programs
SEE ALSO
sendmail(8)
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