The good folks at WordPress just released Security Update 4.0.1 that fixes a host of security vulnerabilities. Go get it now.

https://wordpress.org/news/2014/11/wordpress-4-0-1/

4.0.1 fixes the following in 4.0, and fixes a LOT more from previous versions.

• Three cross-site scripting issues that a contributor or author could use to compromise a site. Discovered by Jon Cave, Robert Chapin, and John Blackbournof the WordPress security team.
• A cross-site request forgery that could be used to trick a user into changing their password.
• An issue that could lead to a denial of service when passwords are checked. Reported by Javier Nieto Arevalo and Andres Rojas Guerrero.
• Additional protections for server-side request forgery attacks when WordPress makes HTTP requests. Reported by Ben Bidner (vortfu).
• An extremely unlikely hash collision could allow a user’s account to be compromised, that also required that they haven’t logged in since 2008 (I wish I were kidding). Reported by David Anderson.
• WordPress now invalidates the links in a password reset email if the user remembers their password, logs in, and changes their email address. Reported separately by Momen Bassel, Tanoy Bose, and Bojan Slavković of ManageWP.

ALWAYS keep your WordPress up-t0-date. WordPress.org doesn't release Security Updates for nothing.