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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.
Drupal 6.18 only includes fixes for security issues. Drupal 6.19 also includes bugfixes. The full list of changes between the 6.17 and 6.19 releases can be found by reading the 6.19 release notes. A complete list of all bug fixes in the stable DRUPAL-6 branch can be found at http://drupal.org/project/cvs/3060/?branch=DRUPAL-6.
The full list of changes between the 5.22 and 5.23 releases can be found by reading the 5.23 release notes. A complete list of all bug fixes in the stable DRUPAL-5 branch can be found at http://drupal.org/project/cvs/3060/?branch=DRUPAL-5.
Drupal 5.23, 6.18 and 6.19 were released in response to the discovery of security vulnerabilities. Details can be found in the official security advisory:
To fix the security problem, please upgrade Drupal. We made two versions of Drupal 6 available, so you can choose to only include security fixes (Drupal 6.18) or security fixes and bugfixes (Drupal 6.19). You can choose your preferred version. We are trying to make it easier and quicker to roll out security updates by making security-only releases available as well as ones with bugfixes included. We hope this helps you roll out the fixes as soon as possible.
These releases did not change the .htaccess, (default.)settings.php and robots.txt files, so you can keep your existing files intact, if you have modifications in them.