WordPress 2.6.5 is immediately available and fixes one security problem and three bugs. We recommend everyone upgrade to this release, in order to make your blog as secure as possible.
The security issue is an XSS exploit discovered by Jeremias Reith that fortunately only affects IP-based virtual servers running on Apache 2.x. If you are interested only in the security fix, copy wp-includes/feed.php and wp-includes/version.php from the 2.6.5 release package.
2.6.5 contains three other small fixes in addition to the XSS fix. The first prevents accidentally saving post meta information to a revision. The second prevents XML-RPC from fetching incorrect post types. The third adds some user ID sanitization during bulk delete requests.
Note that WordPress are skipping version 2.6.4 and jumping from 2.6.3 to 2.6.5 to avoid confusion with a fake 2.6.4 release that made the rounds. There is not and never will be a version 2.6.4.