firebase - FCM Security: Prevent multiple senders from pushing notifications to all devices?

Question

As part of our solution, we want to deploy an FCM "app server" at each of our customer sites. Each customer site has their own users with their own devices using our app. However, we want to make sure that if one of the customer sites is compromised, an attacker could not abuse the FCM "app server" (e.g. by sending notifications to all devices at all customer sites).

Instead of sharing credentials between all customer sites, we are thinking of generating a unique server key for each customer site. That way if one customer site is compromised, we can disable that server key and stop any more FCM notifications from being sent.

Question: Can we be sure that an attacker cannot send global notifications to all devices?

• Assuming an attacker has a server key and access to one customer site "app-server", can they get a list of all the registered devices?
• Is there a default notification "topic" that is sent to all devices? (e.g. /topic/all or /topic/global). If so, can we disable that default topic?

Answer 1

Waitting for answers