Drupal 6.15 and 5.21, maintenance releases which fix issues reported through the bug tracking system, as well as non critical security vulnerabilities, are now available for download. Both releases fix other smaller issues as well.
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 Drupal 5.0 release announcement.
The full list of changes between the 6.14 and 6.15 releases can be found by reading the 6.15 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 5.20 and 5.21 releases can be found by reading the 5.21 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 5.21 and 6.15 were released in response to the discovery of security vulnerabilities. Details can be found in the official security advisory:
To fix the security problem, you can either (1) upgrade Drupal or (2) patch Drupal.
We strongly recommend you do a full upgrade (which is also detailed in the security announcement) as the patches do not contain the additional bug fixes that went into the releases. Applying the patches will leave your site in an unversioned state and confuse the update status module, which will keep reminding you to upgrade to 6.15 or 5.21. Please read the announcement for details on the patch.
If you still prefer to patch Drupal, apply the http://drupal.org/files/sa-core-2009-009/SA-CORE-2009-009-6.14.patch file to your Drupal 6.14 codebase or http://drupal.org/files/sa-core-2009-009/SA-CORE-2009-009-5.20.patch to your Drupal 5.20 codebase.
Important update notes
These releases did not change the (default.)settings.php and robots.txt files, so you can keep your existing files intact, if you have modifications in them.
The .htaccess file was changed in Drupal 6.15, adding settings to let all PHP scripts set their own caching headers without those being overridden by the server. See http://drupal.org/node/550488 for more information.