A legacy dependency is an older tool, interface, or connection that an application or organization still relies on, even when newer options exist. Examples include retired APIs, outdated software libraries, and custom integrations that remain embedded in business workflows.
These dependencies matter because they can disrupt operations when a platform changes or retires an interface. They may also increase security risk if they are unsupported, poorly maintained, or granted broader access than necessary. In real environments, an application may stop working after an API retirement, leading administrators to restore the old interface. Defenders can reduce this risk by inventorying integrations, identifying their owners and permissions, testing migrations, and treating any re-enabled legacy service as a monitored exception with a clear removal plan.