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##
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# This module requires Metasploit: https://metasploit.com/download
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# Current source: https://github.com/rapid7/metasploit-framework
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##
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class MetasploitModule < Msf::Encoder
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  # Has some issues, but overall it's pretty good
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  Rank = ManualRanking
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  def initialize
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    super(
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      'Name'             => 'Generic Shell Variable Substitution Command Encoder',
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      'Description'      => %q{
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        This encoder uses standard Bourne shell variable substitution
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      tricks to avoid commonly restricted characters.
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      },
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      'Author'           => 'hdm',
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      'Arch'             => ARCH_CMD,
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      'Platform'         => 'unix')
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  end
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  #
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  # Encodes the payload
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  #
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  def encode_block(state, buf)
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    # Skip encoding for empty badchars
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    if(state.badchars.length == 0)
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      return buf
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    end
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    if (state.badchars.include?("-"))
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      # Then neither of the others will work.  Get rid of spaces and hope
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      # for the best.  This obviously won't work if the command already
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      # has other badchars in it, in which case we're basically screwed.
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      if (state.badchars.include?(" "))
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        buf.gsub!(/\s/, '${IFS}')
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      end
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    else
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      # Without an escape character we can't escape anything, so echo
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      # won't work.  Try perl.
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      if (state.badchars.include?("\\"))
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        buf = encode_block_perl(state,buf)
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      else
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        buf = encode_block_bash_echo(state,buf)
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      end
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    end
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    return buf
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  end
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  #
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  # Uses the perl command to hex encode the command string
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  #
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  def encode_block_perl(state, buf)
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    hex = buf.unpack("H*")
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    cmd = 'perl -e '
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    qot = ',-:.=+!@#$%^&'
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    # Find a quoting character to use
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    state.badchars.unpack('C*') { |c| qot.delete(c.chr) }
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    # Throw an error if we ran out of quotes
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    raise EncodingError if qot.length == 0
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    sep = qot[0].chr
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    # Convert spaces to IFS...
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    if (state.badchars.include?(" "))
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      cmd.gsub!(/\s/, '${IFS}')
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    end
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    # Can we use single quotes to enclose the command string?
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    if (state.badchars.include?("'"))
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      if (state.badchars.match(/\(|\)/))
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        # No parenthesis...
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        raise EncodingError
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      end
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      cmd << "system\\(pack\\(qq#{sep}H\\*#{sep},qq#{sep}#{hex}#{sep}\\)\\)"
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    else
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      if (state.badchars.match(/\(|\)/))
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        if (state.badchars.include?(" "))
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          # No spaces allowed, no parenthesis, give up...
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          raise EncodingError
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        end
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        cmd << "'system pack qq#{sep}H*#{sep},qq#{sep}#{hex}#{sep}'"
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      else
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        cmd << "'system(pack(qq#{sep}H*#{sep},qq#{sep}#{hex}#{sep}))'"
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      end
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    end
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    return cmd
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  end
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  #
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  # Uses bash's echo -ne command to hex encode the command string
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  #
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  def encode_block_bash_echo(state, buf)
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    hex = ''
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    # Can we use single quotes to enclose the echo arguments?
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    if (state.badchars.include?("'"))
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      hex = buf.unpack('C*').collect { |c| "\\\\\\x%.2x" % c }.join
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    else
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      hex = "'" + buf.unpack('C*').collect { |c| "\\x%.2x" % c }.join + "'"
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    end
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    # Are pipe characters restricted?
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    if (state.badchars.include?("|"))
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      # How about backticks?
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      if (state.badchars.include?("`"))
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        # Last ditch effort, dollar paren
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        if (state.badchars.include?("$") or state.badchars.include?("("))
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          raise EncodingError
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        else
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          buf = "$(/bin/echo -ne #{hex})"
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        end
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      else
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        buf = "`/bin/echo -ne #{hex}`"
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      end
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    else
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      buf = "/bin/echo -ne #{hex}|sh"
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    end
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    # Remove spaces from the command string
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    if (state.badchars.include?(" "))
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      buf.gsub!(/\s/, '${IFS}')
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    end
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    return buf
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  end
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end
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