Apple released an update for Safari on 12/13/16. This update, which brings Safari up to 10.0.2, patches a number of bugs that may allow malicious websites to run code that corrupts your computer’s memory. Once that happens, it may be possible to plant malware on your computer. Once that happens, all kinds of bad things may result. See the long list of specific bugs fixed, and who gets credit for finding them, at https://support.apple.com/en-us/HT207421. Get the update itself at the App Store.
There was an earlier Safari update, released on 11/25, which had kicked the release up to 10.0.1. So if you are running behind on your patching, this will take care of a 2 for 1.