metasploit-framework/modules/encoders/cmd/powershell_base64.rb

##
# This module requires Metasploit: http://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

require 'msf/core'

class MetasploitModule < Msf::Encoder
  Rank = ExcellentRanking

  def initialize
    super(
      'Name'             => 'Powershell Base64 Command Encoder',
      'Description'      => %q{
        This encodes the command as a base64 encoded command for powershell.
      },
      'Author'           => 'Ben Campbell',
      'Arch'             => ARCH_CMD,
      'Platform'         => 'win')
  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? '-') || (state.badchars.include? ' ')
      return buf
    end

    cmd = encode_buf(buf)

    if state.badchars.include? '='
        while cmd.include? '='
          buf << " "
          cmd = encode_buf(buf)
        end
    end

    cmd
  end

  def encode_buf(buf)
    base64 = Rex::Text.encode_base64(Rex::Text.to_unicode("cmd.exe /c start #{buf}"))
    cmd = "powershell -w hidden -nop -e #{base64}"
  end

end