If you wish to stay secure while running Symantec Endpoint Protection, then you will want to download the latest update, which is definitions version 180121aw. You really need to make certain you are running this update because it offers the most secure, and up-to-date security content.
There is a dedicated page for Symantec Endpoint Protection 12.1.3 (or later), where you have a range of support documents, and also the download for the latest definitions, which you can find here.
The new definitions were created on January 21, but was not released until today, Jan. 22. the extended version is 1/21/2016 rev. 49, the sequence number 172303, and also the number of signatures 41,070,705.
If you have been experiencing issues with 12.1.3, then you might want to visit the Endpoint Protection Technical Support page, as it gives you an array of information and support.
You will be able to Troubleshoot and Manage Threats, read and study Recommended Articles, look at the latest Alerts, and also take time reading through Documentation and Training.
One final thing, you do not need to worry about the lack of support for this security software, as support will be offered up until July 5, 2018.