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 built-in 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 /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 is a very bad idea. procmail allows users to run arbitrary programs in their procmailrc.

FILES
/usr/libexec/sm.bin
    directory for restricted programs

SEE ALSO
sendmail(8)