Minecraft's mechanics are inherently weak, allowing for massive cheating.

I'm Japanese, so please excuse my use of Google Translate.

Griefers using EGG NET, Lumine, and other services have emerged. They can invade invitation-only worlds and may even steal operators. The situation could worsen. Take action immediately.

The attached images show the chat messages that were displayed continuously when the griefer appeared, and the content that appeared on the friend's world list after the griefing.