Two new Drupal releases have shown up on the Drupal website. Both the 5.x series and 6.x series have been updated.
These two new releases bring together a number of security fixes and address some critical security vulnerabilities that have arisen since the previous releases.
Full change logs are located here:
The security issues that were address can be found in the security advisory.
As expected, it's a good idea to upgrade right away to avoid exposing your site to any potential threats.