I've recently read about the vulnerabilities within ntp, here which can be closed by installing ntp version 4.2.8p1.
However, I've used
apt-get to install the latest version of ntp, on an Ubuntu 14.04.2 server, but this is only version 4.2.6p5. I can't find a newer version by browsing the repositories, although I'm fairly new to doing this, so may have missed it. Even Vivid Vervet is showing 4.2.6p5. I'm assuming there's a review process for version changes which is still pending.