Lease expiration is the point at which a contract ends and the right to use a resource, path, or service changes. In networking, that resource may be a leased fiber route, a telecom circuit, cloud capacity, a domain name, or a security certificate. When the term ends, access can stop, be renewed, or be handed over to another party.
In cyber security, lease expiration matters because many critical controls depend on assets that are not fully owned. If an expired lease is not tracked, traffic can be rerouted unexpectedly, availability can drop, or a defender may lose visibility into a segment of the network. Attackers also benefit from missed expirations: they may wait for an old domain, certificate, or service contract to lapse and then use the gap to impersonate systems, hijack traffic, or trigger outages. Good defense means maintaining inventories, renewal alerts, fallback routes, and tested replacement plans.