Web servers, VPN gateways, mail servers, remote desktop, management consoles and cloud storage are all examples. Anyone can try to connect to them, and automated scanners do so constantly.

These assets are where attackers start. A flaw in an internal system needs a way in first. A flaw in an internet-facing one is the way in. That is why they are patched first and watched most closely.

Search services such as Shodan and Censys index what answers, so a forgotten server is often found by an attacker before its owner remembers it. The ProxyLogon attacks in 2021 mostly hit Exchange servers that were reachable from the internet.

Maintain a list of everything that is exposed, including things that third parties run for you, compare it to scan results regularly and close anything that does not need to be public.