metasploit-framework/modules/encoders/cmd/generic_sh.rb

##
# This file is part of the Metasploit Framework and may be subject to
# redistribution and commercial restrictions. Please see the Metasploit
# web site for more information on licensing and terms of use.
#   http://metasploit.com/
##


require 'msf/core'


class Metasploit3 < Msf::Encoder

	# Has some issues, but overall it's pretty good
	Rank = GoodRanking

	def initialize
		super(
			'Name'             => 'Generic Shell Variable Substitution Command Encoder',
			'Description'      => %q{
				This encoder uses standard Bourne shell variable substitution
			tricks to avoid commonly restricted characters.
			},
			'Author'           => 'hdm',
			'Arch'             => ARCH_CMD)
	end


	#
	# Encodes the payload
	#
	def encode_block(state, buf)

		# Skip encoding for empty badchars
		if(state.badchars.length == 0)
			return buf
		end

		if (state.badchars.include?("-"))
			# Then neither of the others will work.  Get rid of spaces and hope
			# for the best.  This obviously won't work if the command already
			# has other badchars in it, in which case we're basically screwed.
			if (state.badchars.include?(" "))
				buf.gsub!(/\s/, '${IFS}')
			end
		else
			# Without an escape character we can't escape anything, so echo
			# won't work.  Try perl.
			if (state.badchars.include?("\\"))
				buf = encode_block_perl(state,buf)
			else
				buf = encode_block_bash_echo(state,buf)
			end
		end

		return buf
	end

	#
	# Uses the perl command to hex encode the command string
	#
	def encode_block_perl(state, buf)

		hex = buf.unpack("H*")
		cmd = 'perl -e '
		qot = ',-:.=+!@#$%^&'

		# Find a quoting character to use
		state.badchars.unpack('C*') { |c| qot.delete(c.chr) }

		# Throw an error if we ran out of quotes
		raise RuntimeError if qot.length == 0

		sep = qot[0].chr

		# Convert spaces to IFS...
		if (state.badchars.include?(" "))
			cmd.gsub!(/\s/, '${IFS}')
		end

		# Can we use single quotes to enclose the command string?
		if (state.badchars.include?("'"))

			if (state.badchars.match(/\(|\)/))

				# No paranthesis...
				raise RuntimeError
			end

			cmd << "system\\(pack\\(qq#{sep}H\\*#{sep},qq#{sep}#{hex}#{sep}\\)\\)"

		else
			if (state.badchars.match(/\(|\)/))
				if (state.badchars.include?(" "))
					# No spaces allowed, no paranthesis, give up...
					raise RuntimeError
				end

				cmd << "'system pack qq#{sep}H*#{sep},qq#{sep}#{hex}#{sep}'"
			else
				cmd << "'system(pack(qq#{sep}H*#{sep},qq#{sep}#{hex}#{sep}))'"
			end
		end

		return cmd
	end

	#
	# Uses bash's echo -ne command to hex encode the command string
	#
	def encode_block_bash_echo(state, buf)

		hex = ''

		# Can we use single quotes to enclose the echo arguments?
		if (state.badchars.include?("'"))
			hex = buf.unpack('C*').collect { |c| "\\\\\\x%.2x" % c }.join
		else
			hex = "'" + buf.unpack('C*').collect { |c| "\\x%.2x" % c }.join + "'"
		end

		# Are pipe characters restricted?
		if (state.badchars.include?("|"))
			# How about backticks?
			if (state.badchars.include?("`"))
				# Last ditch effort, dollar paren
				if (state.badchars.include?("$") or state.badchars.include?("("))
					raise RuntimeError
				else
					buf = "$(/bin/echo -ne #{hex})"
				end
			else
				buf = "`/bin/echo -ne #{hex}`"
			end
		else
			buf = "/bin/echo -ne #{hex}|sh"
		end

		# Remove spaces from the command string
		if (state.badchars.include?(" "))
			buf.gsub!(/\s/, '${IFS}')
		end

		return buf
	end

end