MySQL takes several measures to prevent misuse of user-defined functions.

UDF library files cannot be placed in arbitrary directories. They must be located in the server's plugin directory. This directory is given by the value of the plugin_dir system variable.

To use CREATE FUNCTION or DROP FUNCTION, you must have the INSERT or DELETE privilege, respectively, for the mysql database. This is necessary because those statements add and delete rows from the mysql.func table.

UDFs should have at least one symbol defined in addition to the xxx symbol that corresponds to the main xxx() function. These auxiliary symbols correspond to the xxx_init(), xxx_deinit(), xxx_reset(), xxx_clear(), and xxx_add() functions. mysqld also supports an --allow-suspicious-udfs option that controls whether UDFs that have only an xxx symbol can be loaded. By default, the option is off, to prevent attempts at loading functions from shared library files other than those containing legitimate UDFs. If you have older UDFs that contain only the xxx symbol and that cannot be recompiled to include an auxiliary symbol, it may be necessary to specify the --allow-suspicious-udfs option. Otherwise, you should avoid enabling this capability.