On December 9, 2021, a critical exploit has been announced and server software updates have been issued to mitigate a Remote Code Execution vulnerability. The best way to patch against this exploit is to update your server software to the latest version. Spigot & Paper versions 1.8.8 and newer have been updated, alongside Fabric (Loader 0.12.9+) and Forge (1.12.2+) respectively.
How to Update your Server Software
In your Game Control Panel, make sure to select the server you wish to manage and make sure it's fully stopped. Then, find Configuration > Advanced in the navigation bar at to the right side.
You'll see the Install Different Edition feature on this page. Make sure to select the correct Server Software Edition (eg. Paper/Forge/etc) and the matching Server Version. Finally, click on the Install Edition button and start your server.