What is an Intrusion Detection System (IDS)

What is Intrusion Detection System (IDS)?

 

An Intrusion Detection System (IDS) is a cybersecurity tool designed to monitor a network or a system for suspicious activity or policy violations. It works by analysing network traffic or system logs to detect signs of potential threats, such as unauthorised access, malware, or cyberattacks. When an IDS detects suspicious activity, it typically generates an alert for security administrators to investigate, allowing them to respond before significant damage is done.

 

Key Functions of IDS:

 

• Monitoring: Continuously monitors network traffic or system behaviour for any signs of malicious activity
• Detection: Uses either signature-based or anomaly-based methods to detect known threats or abnormal behaviour that could indicate an attack
• Alerting: Once a potential threat is identified, the IDS alerts security personnel to act

 

Why Intrusion Detection Still Matters in a Zero Trust, AI-Era World

 

In today’s threat landscape—defined by AI-driven attacks, lateral movement, and ransomware-as-a-service—perimeter defences alone no longer cut it. IT leaders face increasing pressure to demonstrate security ROI while ensuring operational resilience. Intrusion Detection Systems (IDS) have evolved beyond their legacy roots; they now play a critical role in threat visibility, risk containment, and breach forensics. Whether integrated into modern SOCs or deployed at the edge, IDS delivers vital telemetry that accelerates response and strengthens your organisation’s cyber resilience strategy.

 

 

Types of Intrusion Detection Systems

 

There are several types of Intrusion Detection Systems (IDS), each designed to protect different aspects of a network or host system. The two most common types of IDS are:

 

Host-Based Intrusion Detection System (HIDS)

 

Host-Based Intrusion Detection System (HIDS) monitors activity on a specific device or host, such as a server or workstation. It analyses log files, file integrity, system calls, and other host-level events to detect unauthorised activity or malicious behaviour. Key features:

 

• Monitors file changes and log entries
• Can detect insider threats or unauthorised system changes
• Best for securing individual hosts

 

Network-Based Intrusion Detection System (NIDS)

 

A Network-Based Intrusion Detection System (NIDS) monitors and analyses network traffic in real-time. It inspects data packets transmitted across a network to identify malicious activities, such as attacks targeting network infrastructure. NIDS is generally deployed at strategic points within the network, such as near firewalls or gateways, to observe incoming and outgoing traffic. Key features:

 

• Analyses network traffic at various points within the network
• Detects large-scale attacks like Denial of Service (DoS) and malware propagation
• Effective for identifying external threats

 

Protocol-Based IDS (PIDS)

 

A Protocol-Based IDS (PIDS) is specifically designed to monitor and analyse the behaviour of specific protocols (e.g., HTTP, FTP, etc.). It is typically deployed near a server and examines traffic for violations of protocol rules or unusual behaviour. Key features:

 

• Detects attacks targeting application-layer protocols
• Best for web servers, email servers, or database systems

 

Application-Based IDS (APIDS)

 

An Application-Based IDS (APIDS) monitors specific applications rather than the entire host or network. It analyses application logs and input/output activity to identify suspicious or unauthorised interactions with the application. Key features:

 

• Best suited for protecting high-risk or critical applications
• Detects attacks at the application layer, such as SQL injection or buffer overflow attacks

 

Anomaly-Based IDS

 

An Anomaly-Based IDS creates a baseline of normal network or system behaviour and flags any deviations as suspicious. It uses statistical models or machine learning techniques to detect unusual patterns that may indicate an intrusion. Key features:

 

• Can detect new, unknown attacks or zero-day exploits
• More prone to false positives, as normal behaviour can sometimes vary unexpectedly
• Best suited for environments where traffic patterns are predictable

 

Why This Matters to IT Leaders?

 

Not all attacks trigger alarms — especially the ones that bypass EDR, firewalls, or even SIEM. IDS acts as a second line of visibility, often detecting lateral movement, reconnaissance, and data staging phases before exfiltration or encryption occurs. For cyber leaders, it's not about whether you have IDS — it’s about how well it integrates into your detection stack and whether it feeds actionable intelligence into your response pipeline.

 

Why Are Intrusion Detection Systems Important?

 

They play a vital role in safeguarding networks, systems, and data from increasingly sophisticated cyber threats. As attackers continuously evolve their techniques, IDS helps in identifying and responding to threats before they can cause significant damage. Here are several reasons why IDS is important:

 

1. Early Threat Detection: IDS provides real-time monitoring of network traffic and system activities, enabling organisations to detect malicious activity early. This early detection is critical for preventing intrusions from escalating into full-scale attacks. For instance, detecting unauthorised access attempts or abnormal traffic patterns can help stop attacks before they compromise sensitive systems or data.
2. Enhances Security Visibility: IDS provides enhanced visibility into what is happening within a network or host. By continuously monitoring traffic, IDS helps identify vulnerabilities and gaps in security that might otherwise go unnoticed. It allows security teams to track activities in real-time, giving them a better understanding of the network’s behaviour.
3. Prevents Damage from Insider Threats: Not all threats come from external attackers—insider threats, such as disgruntled employees or contractors with access to sensitive information, pose a significant risk. A Host-Based Intrusion Detection System (HIDS) can detect unusual activities on specific devices or hosts, such as unauthorised access to sensitive files or changes to system configurations, helping to mitigate insider threats.

 

 

Evasion techniques for IDS

 

Attackers often use various techniques to evade detection. These techniques exploit limitations or weaknesses in IDS mechanisms, allowing attackers to bypass security monitoring.

 

1. Fragmentation: Attackers can split malicious data into small, fragmented packets that are transmitted over the network. By fragmenting the payload, it becomes more challenging for the IDS to reassemble and analyse the full content of the malicious traffic.
2. Encryption: Encrypted traffic can conceal malicious payloads from the IDS because the IDS typically inspects unencrypted data. Attackers can hide malicious code inside encrypted traffic, making it invisible to traditional IDS, which relies on inspecting the payload for known signatures or patterns.
3. IP Address Spoofing: Attackers alter the source IP address in the packet header, making it appear as though the traffic is coming from a trusted or non-malicious source.
4. Traffic exhaustion: Attackers may attempt to overwhelm the IDS itself by flooding it with traffic or requests, consuming the IDS’s processing power. This may cause the system to slow down or crash, reducing the system’s ability to monitor and detect malicious activity.

 

 

Turn Threat Visibility into Actionable Defence

 

Speak with our advisors to explore how IDS fits into your overall detection and response strategy—whether you're building a Zero Trust architecture, maturing your SOC, or preparing for AI-driven threats. Find out how Ensign can help you align intrusion detection with business risk, compliance needs, and operational resilience.

 

Or explore our Managed Security Services to see how we help organisations stay ahead of evolving threats with 24/7 monitoring, real-time response, and continuous threat hunting.