Security Plugin
An open source predictive anticheat for Minecraft servers that simulates player movement to detect cheating clients.
Grim Anticheat is a Minecraft security plugin for Paper, Spigot, Purpur, Folia, Bukkit and Fabric, listed under Server Security Plugins. It lists support for Minecraft 26.2. It is free.
From the source listing
Grim is a multithreaded anticheat that predicts the movement the game's rules allow and flags clients that do not follow it. It runs on Paper, Spigot, Purpur and Folia and has a Fabric version, and it is published under the GPL with a wiki. Any anticheat can flag honest players, so test it on your own server first.
From the source listing
Same category
Most recently approved first. This is not a recommendation or a ranking.
Server Security Plugins · Security Plugin · by AuthMe
An authentication plugin that adds registration and login with passwords for servers that cannot use Mojang's online mode.
Paper · Spigot · Velocity · +3MC 26.3 · 26.2
Folia support · Open source · +1
Guides
Twenty checks that close the most common ways servers are taken over: accounts, ports, proxies, panels, plugins and backups.