PRISM is an user space reverse shell backdoor. It offers ICMP mode where it awaits a packet containing a security key and host ip / port destination information. It also offers static mode where it can connect to a hardcoded ip / port.
You can download it from the following link: https://packetstormsecurity.com/files/download/85340/prism.c
Source: https://packetstormsecurity.com/files/85340/PRISM-ICMP-Reverse-Shell-0.5.html