What is Intrusion Detection System?

What Is Intrusion Detection?

Intrusion detection is the process of monitoring network traffic and system activity for signs of unauthorised access, misuse, or malicious behaviour. An Intrusion Detection System (IDS) acts like a security camera for your network -- it watches everything that happens and raises an alert when it spots something suspicious.

While a firewall acts as a gatekeeper that blocks unwanted traffic at the door, an IDS monitors what is happening inside the building. Even the best firewall cannot catch everything, and attackers who make it past the perimeter still need to be detected before they can do damage.

How Intrusion Detection Works

Intrusion detection systems analyse network traffic and system logs in real time, looking for patterns that indicate malicious activity. There are two main approaches to detection:

Signature-Based Detection

This approach compares network traffic against a database of known attack patterns, or signatures. When the system sees traffic that matches a known attack, it triggers an alert. Signature-based detection is highly effective against known threats but cannot catch entirely new attacks that do not yet have a signature.

It works much like antivirus software -- if the attack has been seen before and documented, it will be caught. If it is a brand-new technique, it may slip through.

Anomaly-Based Detection

This approach establishes a baseline of what normal network behaviour looks like and then flags anything that deviates significantly from that baseline. For example, if a workstation that normally sends 50 megabytes of data per day suddenly starts transmitting gigabytes, that anomaly would trigger an alert.

Anomaly-based detection can catch novel attacks that signature-based systems miss, but it also tends to produce more false positives, especially in environments where network patterns change frequently.

IDS vs IPS: What Is the Difference?

An Intrusion Detection System (IDS) monitors and alerts but does not take action to block threats. An Intrusion Prevention System (IPS) does everything an IDS does but can also automatically block or quarantine suspicious traffic in real time.

Many modern security products combine both capabilities. The choice between passive detection and active prevention depends on the organisation's risk tolerance and the sensitivity of the systems being protected. In payment environments, active prevention is generally preferred because the consequences of a breach are severe.

Why Intrusion Detection Matters for Businesses

No security measure is perfect, and breaches do happen despite best efforts. The critical factor is how quickly a breach is detected. Industry research consistently shows that the longer an attacker remains undetected in a network, the greater the damage. Organisations that detect breaches within days suffer significantly lower costs and data loss compared to those where attackers operate undetected for weeks or months.

For businesses that handle payment data, intrusion detection is not optional. PCI DSS Requirement 11.4 specifically requires the use of intrusion detection or prevention techniques to detect and alert on or prevent intrusions into the network. This applies to all traffic at the perimeter of the cardholder data environment as well as at critical points inside it.

Intrusion Detection in Telephone Payment Environments

Contact centres present interesting challenges for intrusion detection. The mix of voice traffic, data traffic, and integration between telephony systems and payment platforms creates a complex environment where distinguishing normal activity from suspicious behaviour requires careful tuning.

VoIP systems, in particular, can be targeted by attackers seeking to intercept call data, commit toll fraud, or use the telephony infrastructure as a stepping stone into the broader network. An IDS monitoring the contact centre environment needs to understand VoIP protocols and be able to detect anomalies specific to telephony traffic, such as unusual call patterns or unexpected changes in call routing.

When card data is removed from the voice channel -- for example, through DTMF suppression -- the intrusion detection picture becomes simpler. There is less sensitive data flowing through the telephony systems, which reduces the attack surface and makes it easier to define what normal behaviour looks like.

Practical Considerations

• An IDS is only useful if someone is watching the alerts. Many organisations deploy intrusion detection but do not have the staff or processes to respond to what it finds. Consider whether you have the resources to monitor and act on alerts, or whether a managed security service might be more appropriate
• Tune your IDS carefully. Out-of-the-box configurations often generate an overwhelming number of false positives, which leads to alert fatigue and real threats being missed
• Keep signatures updated. New vulnerabilities and attack techniques are discovered constantly, and your IDS needs current threat intelligence to remain effective
• Log everything. IDS alerts should be fed into a centralised logging and monitoring system where they can be correlated with other security events for a complete picture
• Test your IDS regularly. Run simulated attacks to verify that your detection systems are actually catching what they should