What is an Intrusion Detection System (IDS)?

What an IDS does and how it works

An IDS can be a software application installed on endpoints, a dedicated hardware appliance connected to the network, or a cloud-based service. Whatever the form, the workflow is the same: it captures and monitors traffic or system activity, compares what it sees against detection logic, and triggers an alert when something matches a known threat or deviates from normal. Those alerts go to administrators or, more commonly, into a centralized security tool for investigation. The IDS also logs incidents, and those logs feed back into tuning the system, for example by adding new signatures or refining the model of normal behavior.

A defining trait of most IDS deployments is that they sit out-of-band. Rather than placing itself directly in the traffic path, an IDS typically taps a copy of the traffic stream through a TAP or SPAN port. That way legitimate traffic never has to wait for analysis, so the IDS can inspect deeply without becoming a bottleneck. Anomalous patterns get examined at the protocol and application layers, which lets an IDS flag events such as DNS poisoning, malformed packets, or unusual port scans.

IDS detection methods

How an IDS decides what is suspicious comes down to its detection method. There are two primary approaches, plus a hybrid and a couple of less common techniques.

Signature-based detection

This method compares network packets against a database of attack signatures, the unique characteristics or byte sequences associated with a specific threat, such as a known malware variant. If a packet matches a signature, the IDS flags it. Signature-based detection is fast, accurate, and easy to understand for known threats, but it has a clear blind spot: a brand-new attack with no existing signature can slip right past it. This is why signature databases must be updated constantly with fresh threat intelligence.

Anomaly-based detection

Instead of matching known patterns, anomaly-based detection uses machine learning to build and continually refine a baseline of normal network activity, then flags anything that deviates, for instance a process suddenly using far more bandwidth than usual, or a device opening an unexpected port. Its big advantage is catching unknown attacks and zero-day exploits that signature-based systems miss. The trade-off is a higher rate of false positives, since even benign but unusual behavior, such as a user accessing a sensitive resource for the first time, can trigger an alert.

Hybrid and less common methods

A hybrid IDS combines signature-based and anomaly-based detection to get the best of both: reliable detection of known threats plus the ability to spot novel ones. Two less common methods also exist. Reputation-based detection blocks or flags traffic from IP addresses and domains tied to known malicious activity. Stateful protocol analysis focuses on protocol behavior, for example spotting a denial-of-service attack when a single IP makes many simultaneous connection requests in a short window.

Types of Intrusion Detection Systems

Beyond how it detects, an IDS is categorized by where it sits and what it monitors. The two most common types are NIDS and HIDS, with a few specialized variants.

• Network Intrusion Detection System (NIDS): Deployed at strategic points across the network to monitor inbound and outbound traffic for all connected devices. NIDS are often placed just behind firewalls at the perimeter to catch anything that breaks through, and sometimes behind internal firewalls to spot insider threats or lateral movement. They usually run out-of-band, analyzing copies of packets.
• Host-based Intrusion Detection System (HIDS): Installed on a specific endpoint such as a laptop, server, or router. It monitors that device's traffic, system logs, file integrity, and running processes, often by taking periodic snapshots of critical operating system (OS) files and flagging unexpected changes. HIDS adds protection around high-value assets and can catch malware spreading from a compromised host.
• Protocol-based (PIDS): Typically installed on a web server to monitor and analyze the protocol between users and the server, such as HTTP or HTTPS.
• Application protocol-based (APIDS): Works at the application layer, monitoring application-specific protocols, for example sitting between a web server and a SQL database to detect SQL injection.
• Hybrid IDS: Combines host and network data for a more complete view.

Teams frequently combine NIDS and HIDS: the NIDS watches overall network traffic while the HIDS hardens critical individual systems.

IDS vs. IPS

The most common point of confusion is the difference between an IDS and an IPS. Both inspect traffic for threats, but they differ in placement and in what they do when they find something.

In practice, many organizations do not choose one or the other. The two capabilities are frequently combined into a single Intrusion Detection and Prevention System (IDPS) that detects, logs, alerts, and automatically responds.

IDS vs. firewall

An IDS and a firewall are complementary, not competing. A firewall is an active gatekeeper: it faces the network boundary and uses predefined rules to allow or block traffic based on ports, addresses, and protocols. It prevents unauthorized traffic from crossing, but it will not alert you to an attack that originates inside the network or that arrives over an allowed port. An IDS is a passive observer: it does not filter or block, but it watches traffic in motion, recognizes suspicious behavior a firewall would wave through, and raises an alert so responders can act. This is why IDS sensors often sit right behind firewalls, catching whatever slips past. Many next-generation firewalls now bundle IDS and IPS functions directly.

IDS evasion techniques

Attackers actively try to slip past an IDS, which creates an ongoing back-and-forth as vendors update their tools. The most common evasion techniques include:

• Fragmentation: Splitting a malicious payload into small packets, sometimes delayed or sent out of order, so the signature is obscured and the IDS struggles to reassemble and recognize the attack.
• Flooding: Overwhelming the IDS with a high volume of traffic, often spoofed User Datagram Protocol (UDP) or Internet Control Message Protocol (ICMP), so the real attack is camouflaged in the noise or the detector fails open.
• Spoofing: Faking IP addresses or DNS records to make malicious traffic appear to come from a trusted source.
• Obfuscation and encoding: Altering or encoding a payload (for example with Base64 or hex) so it stays functional but becomes hard for a signature-based IDS to read.
• Encryption: Hiding an attack inside encrypted traffic that the IDS cannot inspect without the decryption key.
• Operator fatigue: Deliberately generating a flood of alerts to distract and exhaust the response team so real activity goes unnoticed.

Benefits and challenges of an IDS

The benefits are significant:

• Extra layer of protection: An IDS catches threats that bypass primary defenses like firewalls, so even if a frontline control fails, teams are still alerted.
• Early detection: Immediate alerts enable faster response and reduce dwell time.
• Better network visibility: Helps identify configuration bugs, weaknesses, and unexpected traffic patterns.
• Compliance support: Standards such as Payment Card Industry Data Security Standard (PCI-DSS) call for intrusion detection, and IDS logs document that requirement is met.

The challenges are just as real, and they are why an IDS is rarely deployed alone:

• False positives: An IDS can flag benign activity as malicious, and a flood of low-value alerts leads directly to alert fatigue.
• False negatives: A genuine attack mistaken for legitimate traffic passes unnoticed, which is the more dangerous failure mode.
• Resource intensity: An IDS needs regular tuning and signature updates to stay effective.
• Encrypted traffic: An IDS cannot inspect traffic it cannot read without the decryption key.
• No blocking capability: An IDS tells you something is wrong; it does not stop it.

Where an IDS fits: SIEM, IPS, and firewalls

An IDS is not a standalone answer. It is designed to be one part of a layered security architecture, and it delivers the most value when tightly integrated with the tools around it. IDS alerts are commonly funneled into a SIEM, where they are combined with data from other sources, enriched with threat intelligence, de-duplicated, and prioritized, which is what turns a noisy stream of alerts into something a team can actually act on. IDS and IPS are often merged into a single IDPS for detection plus automated response. And IDS sits alongside firewalls to catch what the perimeter misses. An IDS generates signals, but it takes correlation and response to turn those signals into security outcomes.

Expert insight: a standalone IDS produces alerts, not answers

A classic IDS is a detection sensor, and a good one, but on its own it hands a security team a pile of alerts and very little else. The two problems that sink legacy IDS deployments are false positives and the alert fatigue they create, and neither is solved by better signatures alone. They are solved by correlation, context, and response, which is the layer Sekoia is built to provide. This is also a space Sekoia knows from the inside as a European vendor: the open-source Suricata engine that powers much of the modern NIDS and network-detection ecosystem has deep European roots, and network telemetry is a first-class input to how Sekoia detects threats.

Rather than treat IDS output as a finished product, Sekoia treats network and IDS-style telemetry as one of the more than 300 integrations feeding the Sekoia unified SOC platform. Inside Sekoia Defend, those raw network signals are correlated with endpoint, identity, and cloud data and matched against roughly 1,000 detection rules mapped to MITRE ATT&CK, so a lone IDS alert becomes a high-confidence detection with the surrounding story attached. Those detections are continuously enriched by native Sekoia Intelligence Cyber Threat Intelligence (CTI), produced by an in-house Threat Detection & Research (TDR) team, which drives false positives down and helps analysts catch the lateral movement a perimeter IDS would miss. As a European provider, Sekoia also offers a data sovereignty posture that large US generalists rarely match, paired with automated incident response so a network signal can trigger containment in minutes. An IDS is a valuable sensor, but detection and response is what actually stops the attack.

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