Drupal 6.18 and 5.23, maintenance releases which fix security vulnerabilities are now available for download.
Drupal 6.19 also fixes other small issues reported through the bug tracking system.
Upgrading your existing Drupal 5 and 6 sites is strongly recommended. There are no new features in these releases. For more information about the Drupal 6.x release series, consult the Drupal 6.0 release announcement, more information on the 5.x releases can be found in the Drupal 5.0 release announcement. Drupal 5 will no longer be maintained when Drupal 7 is released. Upgrading to Drupal 6 is recommended.
For further details check out below link
Read more about this news