The running configuration is the live set of settings currently active on a device. On routers, switches, firewalls, and other network gear, it defines what the system is doing right now: interfaces, routes, tunnels, access controls, and management access. It is different from saved or committed configuration records, which may describe what operators intended to apply.
This matters in cyber security because attackers often try to change live behavior without leaving an obvious record, or they exploit gaps between intended and actual state. Defenders compare the running configuration with approved baselines, change tickets, and commit history to spot state drift. An unexplained tunnel, route, or policy in the running configuration can reveal unauthorized access, hidden persistence, or simple operational error. Regular validation of running state helps reveal blind spots before traffic is redirected or exposed.