A very clever bad guy hides inside a computer system for a long, long time without being seen. What two main security tools, one watching all the network roads and another watching what each single computer is doing, must work together to find and stop this hidden bad guy?

To find and stop a hidden bad guy who has been inside a computer system for a long time, two main security tools must work together: a network intrusion detection system (NIDS) and a host-based intrusion detection system (HIDS). The NIDS watches all the network roads, which means it inspects all the data traffic flowing in and out of the computer system across its network connections. Think of it like a security guard at the main entrance of a building, checking everyone and everything that comes and goes. The NIDS looks for suspicious patterns in this traffic, like unusual connection attempts, data exfiltration (data being stolen and sent out), or communication with known malicious servers. It operates by analyzing network packets, which are small chunks of data that travel across the network, and comparing them against a database of known attack signatures or against normal network behavior to spot anomalies. The HIDS, on the other hand, watches what each single computer is doing. This is like having security cameras inside each room of the building, monitoring the activity within. The HIDS is installed on individual computers (called hosts) and monitors their internal activities. This includes looking at system logs (records of events on the computer), file integrity (checking if important system files have been modified), running processes (what programs are currently active), and user activity. The HIDS can detect malicious activities that might not be visible from the network perspective, such as malware that has already infected a system and is operating internally. When these two systems work together, the NIDS can alert the HIDS to potential threats originating from the network, and the HIDS can confirm if those threats are actually manifesting on specific computers. Conversely, the HIDS can alert the NIDS to suspicious activity on a host that might indicate an ongoing attack the NIDS has not yet fully identified. This coordinated effort creates a comprehensive security posture, allowing for the early detection and effective stopping of persistent, hidden threats by correlating network-level suspicious events with host-level suspicious activities.