Someone posted a client (no download available) on YouTube about making you an OP on any server. It's probably fake but I don't know. I hope this isn't possible. If it is, how would I prevent it on my server?
Mod Edit :- I removed the video link as it was linking to a hacked client, which is against forum rules
There was a way to force op on somebodies server, but I believe that it was patched recently. If it somehow still exists, here is how they would do it, and how you can protect yourself.
Note: This method only works if the server is hosted from your computer.
First, they would connect to your server. Then, they would ask you to check out their server. They would use some data that you leave behind to hijack your session and gain op on your server. Although I think this was patched, you should be able to protect yourself by avoiding servers that your players invite you to. (Especially if they aren't a trusted player.)