There is no sandbox. A plugin can read every file your server can read, open network connections, and run any command as the console. That is what makes plugins powerful and it is why where you got one matters as much as what it does.
The five-minute check
- Download it from the project's own page
The official listing, the developer's repository, or the marketplace the developer publishes on. Not a re-upload, not a forum attachment, not a link someone pasted.
- Look at when it was last updated
A plugin abandoned two Minecraft versions ago will break on your next upgrade, and nobody will fix it. That is a maintenance cost you are choosing to take on.
- Read recent reviews or issues, not the description
The description is written by the author. The issue tracker is written by people it broke. Fifteen open issues about the same crash is information the description does not carry.
- Check what it says it needs
A chat formatting plugin that wants to make outbound web requests deserves a question. Not every unusual permission is malicious, but every one should have an explanation you can find.
- Install it on a test instance first
If the plugin is significant, run it somewhere that is not your live world for a day. This is what splitter slots are useful for beyond networks.
A paid plugin offered for free has been modified by someone, and the modification is the point. What gets added is usually a backdoor that gives its author console access to every server that installs it — which means your worlds, your configs, and any database credentials in them. There is a separate article on this and it is not exaggerating.
Every plugin is a supply chain you are trusting: the author, their account security, and whoever they might sell the project to. Six plugins is six of those. Forty is forty, and you will not be watching all of them.
If you think one is malicious
Stop the server rather than removing the plugin while it runs. Take a copy of the whole server for evidence, remove the plugin, rotate every credential the server had — database passwords, RCON, SFTP, your panel password — and check your operator list and the activity log for changes you did not make.