SMRSH(8) OpenBSD System Manager's Manual SMRSH(8) NAME smrsh - restricted shell for sendmail SYNOPSIS smrsh -c command DESCRIPTION The smrsh program is intended as a replacement for /bin/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(8) in order to improve the overall 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 the shell builtin commands ``exec'', ``exit'', and ``echo''. It also rejects any commands with the characters `\', `<', `>', `;', `$', `(', `)', `\r' (carriage return), or `\n' (new- line) 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 /usr/libexec/sm.bin/vacation. System administrators should be conservative about populating the sm.bin 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 sm.bin directory. Note that this does not restrict the use of shell or perl scripts in the sm.bin di- rectory (using the ``#!'' syntax); it simply disallows execution of arbi- trary programs. FILES /usr/libexec/sm.bin directory for restricted programs SEE ALSO sendmail(8) OpenBSD 3.1 January 24, 2001 1