Infrastructure dependency is reliance on an external network, facility, supplier, or leased service that your own systems need in order to keep operating. The dependency may be physical, such as a fiber route or power feed, or logical, such as transit, DNS, cloud hosting, or managed connectivity. If that outside resource changes, fails, or is withdrawn, service continuity can be affected even when no system has been hacked.
In cyber security, this matters because resilience is not only about stopping intruders; it is also about surviving disruption. Attackers can exploit weak dependencies by targeting providers, supply chains, or shared infrastructure to create outages, while defenders must map critical external links, test backup paths, and plan for contract endings, maintenance windows, and capacity loss. Good dependency management means knowing what is essential, what can be replaced quickly, and which single points of failure would interrupt routing, availability, or recovery.