Intrusion Detection Systems (IDS): A Second Line of Defense in Cybersecurity

Introduction

With the rapid growth of the internet and digital connectivity, organisations face an ever-expanding landscape of cyber threats. From denial-of-service attacks to sophisticated penetration attempts, adversaries exploit system vulnerabilities within seconds. Traditional security measures, such as firewalls, provide a first line of defence by controlling access to networks. However, they cannot guarantee complete protection. This is where Intrusion Detection Systems (IDS) play a vital role by serving as the second line of defence, monitoring, detecting, and responding to suspicious activities.

What is an IDS?

An Intrusion Detection System (IDS) is a hardware or software solution that automates the process of monitoring system or network events and analysing them for signs of malicious activity. Much like a burglar alarm for a house, IDSs alert administrators when intruders attempt to breach a system. They detect both external attacks and misuse by authorised users who may exceed their privileges.

Key functions of IDS include:

• Monitoring system events and user behaviour

• Identifying deviations from normal activity

• Detecting known attack patterns (misuse detection)

• Recognising abnormal behaviour (anomaly detection)

• Alerting administrators in real-time

• Assisting in enforcing organisational security policies

IDS vs. Firewalls

Firewalls act as gatekeepers, controlling which traffic can enter or leave a network. IDS, on the other hand, functions as a monitoring system that identifies whether those gates have been bypassed or attacked. While firewalls prevent intrusions, IDS tools detect ongoing or successful breaches. A well-secured organization relies on both, ensuring that vulnerabilities overlooked by one are covered by the other.

Why IDS is Needed

The necessity of IDS arises from the limitations of preventive security tools. Attackers may exploit loopholes in firewalls, launch denial-of-service attempts, or probe systems with malicious intent. IDS helps by:

• Increasing the perceived risk of detection for attackers

• Identifying attacks not blocked by other measures

• Documenting ongoing threats to the organisation

• Supporting incident response and forensic analysis

• Acting as a quality check on existing security mechanisms

Components of an IDS

Every IDS comprises three fundamental components:

• Information Source – System logs, network traffic, or application activities that provide monitoring data.

• Analysis Engine – Processes the data to identify signs of intrusions through misuse or anomaly detection techniques.

• Response Mechanism – Takes action after detection, ranging from alerts (passive) to blocking traffic or logging off attackers (active).

Types of IDS IDSs can be classified based on architecture, data source, or analysis method:

• Network-Based IDS (NIDS): Monitors traffic across network segments.

• Host-Based IDS (HIDS): Operates on individual systems, analysing logs and user activity.

• Application-Based IDS (AIDS): Monitors transactions within specific applications.

• Misuse Detection Systems: Use predefined attack signatures to detect known threats.

• Anomaly Detection Systems: Identify unusual patterns compared to normal system behaviour.

Deployment and Challenges

Effective IDS deployment requires careful strategy, as placement determines visibility into potential attacks. Common sensor locations include behind firewalls, on critical subnets, or at network backbones. However, IDSs also face limitations: they may not detect brand-new attacks, can struggle under heavy traffic loads, and require skilled human oversight to interpret alerts accurately.

Conclusion

In today’s interconnected world, firewalls alone are insufficient to secure an organization’s digital infrastructure. Intrusion Detection Systems provide an essential second layer of defense, ensuring that suspicious activity is identified and addressed promptly. By combining IDS with firewalls, authentication, encryption, and strong security policies, organizations can significantly reduce risks and safeguard their most valuable digital assets.