The MITRE CVE dictionary describes this issue as:
MySQL 5.1.x before 5.1.41 allows local users to bypass certain privilege checks by calling CREATE TABLE on a MyISAM table with modified (1) DATA DIRECTORY or (2) INDEX DIRECTORY arguments that are originally associated with pathnames without symlinks, and that can point to tables created at a future time at which a pathname is modified to contain a symlink to a subdirectory of the MySQL data home directory, related to incorrect calculation of the mysql_unpacked_real_data_home value. NOTE: this vulnerability exists because of an incomplete fix for CVE-2008-4098 and CVE-2008-2079.
Find out more about CVE-2009-4030 from the
MITRE CVE dictionary dictionary and