Copyright (c) 1998-2004 Sendmail, Inc. and its suppliers.
All rights reserved.
Copyright (c) 1993 Eric P. Allman. All rights reserved.
Copyright (c) 1993
The Regents of the University of California. All rights reserved.

By using this file, you agree to the terms and conditions set
forth in the LICENSE file which can be found at the top level of
the sendmail distribution.


$Id: smrsh.8,v 8.22 2004/08/06 03:55:35 gshapiro Exp $

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 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 `\`', `<', `>', `;', `$', `(', `)', `\er' (carriage return), or `\en' (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=\e"path\e" to adjust the default search path (defaults to ``/bin:/usr/bin'') and/or -DSMRSH_CMDDIR=\e"dir\e" 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

sendmail(8) $FreeBSD$