The guys behind WordPress have released a new version 2.6.5 that fixes a security issue and three bugs as can be read on the official WordPress Blog, so go update!
If you are already running version 2.6.3:
As the changeset between 2.6.3 and 2.6.5 is pretty small, you don’t really need to undergo the whole “I have to delete all the old and then re-upload all the new files” update procedure – just overwrite the old files mentioned in the changeset with the new ones from 2.6.5.
The authors of WordPress point out that there is not and never will be a version 2.6.4! This is mentioned due to a fake WordPress version 2.6.4 that made the rounds. You can read more about that fake version on Craig Murphy’s blog.