Yeah, sounds like a malicious plugin. They usually hide as a legitimate plugin, but contain a script that inserts a script into your game whenever the plugin is loaded (i.e. when you open a game). The script then requires a module from elsewhere and runs code from that module, usually some code that allows exploits to be run on the server.
Some best practices to avoid these plugins;
- Consider getting an antivirus plugin, I use this one by Christbru01: V2.1 - Plugin: Hidden Backdoor/Infection Script Detector (Detects/Removes infections from malicious plugins)
- Avoid leaked plugins - usually, the leaker will include malicious scripts in these
- Always check the author of a plugin before installing it - if you don’t recognise the author, do some research! Plugins can be pretty dangerous since they have a lot more access than regular scripts, and run in every place you visit.
- Disable or uninstall plugins that you don’t use - that way, you’re minimising the number of plugins active at any one time, reducing the number of possible attack vectors.
Hope that helps 