This is a custom firmware written for the Proxmark3 device. It extends the currently available firmware. This release is nicknamed Frostbit.
You can download it from the following link: https://packetstormsecurity.com/files/download/165531/proxmark3-4.14831.tar.gz
Source: https://packetstormsecurity.com/files/165531/Proxmark3-4.14831.html