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