WordPress 2.8.6 has just been released. This new version fixes two security problems that can be exploited by registered and logged in users who have posting privileges. If you have new or untrusted authors on your blog, upgrading to 2.8.6 is very recommended.
In the WordPress Blog, they describe the first problem as an XSS vulnerability in Press This discovered by Benjamin Flesch. The second problem, discovered by Dawid Golunski, is an issue with sanitizing uploaded file names that can be exploited in certain Apache configurations. Thanks to Benjamin and Dawid for finding and reporting these two issues.