Sakki's Client Essentials are plugins for Minecraft Servers and Client's. It eliminates the problems with hacked client's and gives server administrators control by restricting clients to use whitelisted plugins and mods and ends unfairness.
SCE is currently in Development phase for Forge / Sponge.
This mod / plugin is used as a way to protect a players client, while giving the Server Admin the ability to approve of who can, or cannot connect. This is not hack proof, however, it is a means of preventing (or allowing) players from modifying their clients. There are also some other client functions that have been implemented,