Tiny SHell is an open-source UNIX backdoor that compiles on all variants, has full pty support, and uses strong crypto for communication.
You can download it from the following link: https://packetstormsecurity.com/files/download/31650/tsh-0.6.tgz
Source: https://packetstormsecurity.com/files/31650/tsh-0.6.tgz.html