After Recon?

I did my recon (as suggested by most users around here, I just knew why it is really needed now) on a remote server, it is running a linux server with open ports (ssh, 80, etc), there are also ports that are opened by a node.js app, the node.js code looks like this: pastebin.com/QWikgNk7

I have done a lot of research (plus my programming experience) about possible vulnerabilities this type of server has but I failed. Maybe anyone knows something about this.

All I want is to install a backdoor on the server.