Thursday, November 5, 2009

smrsh - restricted shell for sendmail

SkyHi @ Thursday, November 05, 2009
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 /etc/smrsh, 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 ''', '<', '>', ';', '$', '(', ')', '\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/filter || 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 populating the /etc/smrsh 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 /etc/smrsh 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).
/etc/smrsh - directory for restricted programs
See Also