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

UDF object files cannot be placed in arbitrary directories. They must be located in some system directory that the dynamic linker is configured to search. To enforce this restriction and prevent attempts at specifying path names outside of directories searched by the dynamic linker, MySQL checks the shared object file name specified in CREATE FUNCTION statements for path name delimiter characters. As of MySQL 5.0.3, MySQL also checks for path name delimiters in file names stored in the mysql.func table when it loads functions. This prevents attempts at specifying illegitimate path names through direct manipulation of the mysql.func table. For information about UDFs and the runtime linker, see Section 21.2.2.5, “Compiling and Installing User-Defined Functions”.

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. As of MySQL 5.0.3, mysqld 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 object 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.