Is the restricted shell really secure?

[P McFerrin]

The restricted shell CAN be secure if the administrator follows certain
guidelines, some of which are included:

1-	Do not have PATH setup to search the standard directories.  Have
	it search only a directory that contains ONLY the commands you
	are allowing the restricted user. (called the restricted bin directory)

2-	Do not permit the restricted user access to the bin directory
	established in step 1.

3-	Use the .profile to setup any important variables you want and
	set them to read-only if necessary.  Do a cd(1) to the appropiate
	directory.  Include SHELL=/bin/rsh in the .profile.

4-	The commands you put in the restricted bin directory should
	check the arguments to insure that the restrictions are not
	being bypassed.  (e.g. arguments beginning with '/' or '../'.

5-	If you want to include some standard Unix commands in the
	restricted bin directory, use the following example:

	cd $RESTRICTED_BIN
	ed .cmd
	a
	PATH="/bin:/usr/bin:..........$RESTRICTED_BIN"
	#                   ^^^^^^  your normal bin directories here
	for arg in $*
	do
		case $arg in
		/*|../)	:
			echo "$0: \"$arg\" not allowed, restricted." 1>&2
			exit 255
			;;
		*)	: ok
			;;
		esac
	done
	exec $0 $*
	.
	w
	q
	chmod 775 .cmd
	ln .cmd ls
	ln .cmd cat
	ln .cmd diff
	# The last 3 commands will make the ls, cat, & diff commands available
	# to the restricted user.

Remember, shell scripts will be executed by a NON-RESTRICTED shell, thus
giving you, the administrator, the full power in controlling what you are
allowing the restricted user.

			Paul McFerrin
			AT&T Consumer Products