Imagine a space shooter with a scrolling level. What methods are there for preventing a malicious player from modifying the game to their benefit? Things he could do that are hard to limit server-side is auto-aiming, peeking outside the visible area, speed hacking and other things.
What ways are there of preventing this? Assume that the server is any language and that the clients are connected via WebSocket.
Always assume that the code is 100% hackable. Think of ways to prevent a client completely rewritten (for the purposes of cheating) from cheating. These can be things such as methods for writing a secure game protocol, server-side detection, etc.