A security issue has been found in Joomla 3.x. To fix that issue a new version, Joomla 3.4.5 will be released this Thursday, October 22, 2015 around 4:00 pm (Dutch time).

It is important to update your Joomla 3.x website as soon as possible because the security issue will become public. And hackers might use that knowledge to attack Joomla 3.x websites that have not been updated.

See announcement: https://www.joomla.org/announcements/release-news/5633-important-security-announcement-pre-release.html