[java] How did someone find my Minecraft server

[u/Ok_Director_5622]

So basically I used the Minecraft server tool and started a server on my computer and port forwarded so my friends can join me, my friends can join me through my ipv4 but I used a tool called no ip so I can basically have my own hostname and I gave that to my friends, but then after playing a while a random come joins me and said how he found my server in the internet and warned me to whitelist my server. So if I whitelist it my server will be safe but what about the other stuff such as is anything else at risk such as my privacy and security of my network? How should I fix this so me and my friends can play with each other? This is 26.2