The wordpress development team reports:
Block unsafe PHP unserialization that could occur in limited situations and setups, which can lead to remote code execution. Prevent a user with an Author role, using a specially crafted request, from being able to create a post "written by" another user. Fix insufficient input validation that could result in redirecting or leading a user to another website.
Additionally, we've adjusted security restrictions around file uploads to mitigate the potential for cross-site scripting.