When Forrester’s John Kindervag first wrote about the concept of the Zero Trust Model (ZTM) it almost seemed too paranoid. ZTM posits that attackers are so successful in penetrating networks that a network architect should consider each and every device—from the Internet, to the firewall, to the switch, to the server—to be potentially compromised. Therefore nodes in the network should not implicitly trust each other: all connections should be encrypted, and only the endpoints should trust each other (after authentication).
ZTM seemed paranoid at the time because that’s not how traditional network architecture worked at all; architects established a security perimeter so that nodes on the safe side of that perimeter could freely communicate.