No. There are ACLs to protect them, see https://ntfy.sh/docs/config/#access-control https://ntfy.sh/docs/config/#access-control But for all other topics, yes.

No. There are ACLs to protect them, see https://ntfy.sh/docs/config/#access-control https://ntfy.sh/docs/config/#access-control But for all other topics, yes. Knowing the topic means you can publish to it.