Plague is an odd proof of concept backdoor tool based on the premise of using existing system files and commands to keep and maintain a backdoor on Linux systems. Uses awk and sed to make malicious commands seem innocuous.
You can download it from the following link: https://packetstormsecurity.com/files/download/51369/plague.txt
Source: https://packetstormsecurity.com/files/51369/plague.txt.html