What Is Intrusion Detection System (IDS)?

Request Your Free Trial

In the toolkit to counter cyber threats, the Intrusion Detection System (commonly abbreviated as “IDS”) stands out as a cornerstone in cybersecurity defences. IDS plays an integral role in an organisation’s security posture, providing monitoring and detection capabilities that help protect against malicious activity and unauthorised access to system resources.

An IDS is a sophisticated device or software application that meticulously monitors network traffic or system activities for any signs of potential violations, unauthorised access, or malicious activities. Its primary function is to detect these anomalies, raise alarms, and often produce detailed logs to aid further analysis.

Think of it as a vigilant watchdog, continually scanning its surroundings and barking to alert the owner when it perceives a threat. By providing an early warning of suspicious activities, IDS helps organisations take timely action to mitigate risks and prevent breaches.

Cybersecurity Education and Training Begins Here

Start a Free Trial

Here’s how your free trial works:

  • Meet with our cybersecurity experts to assess your environment and identify your threat risk exposure
  • Within 24 hours and minimal configuration, we’ll deploy our solutions for 30 days
  • Experience our technology in action!
  • Receive report outlining your security vulnerabilities to help you take immediate action against cybersecurity attacks

Fill out this form to request a meeting with our cybersecurity experts.

Thank you for your submission.

How IDS Works

An Intrusion Detection System is a vigilant monitor that constantly oversees network traffic for any signs of unauthorised access or malicious activities. When such activities are detected, the IDS springs into action by alerting relevant authorities or personnel. Here’s a breakdown of the IDS mechanism:

  1. Monitoring and Analysis: The IDS continually examines network traffic flow while scrutinising activity for anything suspicious.
  2. Rule and Pattern Comparison: It utilises a database of predefined rules and patterns, acting as the IDS’s criteria for potentially suspicious or malicious behaviour.
  3. Alert Generation: When network activity resonates with any of these established criteria, the IDS raises a flag by alerting the system’s administrator or relevant authority.

Intrusion detection systems can be categorised based on their placement or methodology. Each approach takes a different function behind how the IDS operates.

By Placement:

  • Host-based IDS (HIDS): Tailored for individual hosts, HIDS is installed directly on the host computer or device. It zeroes in on the activities exclusive to that host.
  • Network-based IDS (NIDS): As the name implies, NIDS oversees the entire network’s traffic, ensuring no malicious activity goes unnoticed.


By Detection Methodology:

  • Signature-based IDS: This system references a known library of attack patterns or signatures. When a match is found, the system reacts accordingly.
  • Anomaly-based IDS: Rather than relying on known attack patterns, this system focuses on the “normal” behaviour of the network. Any deviations from this established norm raise suspicions.


It’s also vital to differentiate IDS from its proactive counterpart, the Intrusion Prevention System (IPS). While both monitor network traffic for potential threats, the primary focus of an IDS is detection and alerting. In contrast, an IPS takes a more active stance to prevent the detected threats from causing harm.

In addition to its detection capabilities, the potency of IDS lies in its ability to enhance security responses. It identifies hosts and devices within the network, examines the data carried by network packets, and traces it back to the origin of a potential attack. This comprehensive approach fortifies a network’s defence against malicious intents.

Why Is IDS Important?

An intrusion detection system is essential in identifying and preventing malicious activities or policy violations in each network. On a more granular level, the role of IDS carries tremendous importance for several reasons:

  • Proactive Monitoring: IDS provides continuous surveillance of network activities, enabling early detection of threats before they escalate.
  • Threat Insights: By identifying the type and source of an attack, IDS offers invaluable insights, allowing administrators to strengthen vulnerabilities.
  • Compliance: Many industries mandate network monitoring for data protection. IDS helps organisations adhere to these regulations.
  • Deterrence: The mere presence of IDS can deter potential attackers aware of its existence.
  • Forensic Analysis: After an attack, IDS logs can aid in understanding the attack’s nature and source, helping in post-event analysis and preventing future breaches.
  • Fast Response Time: Quick detection leads to swift action, reducing potential damage or data loss.
  • Confidence and Trust: When an IDS is in place, stakeholders, clients, and employees can have greater confidence in an organisation’s network security.

IDS is an integral early-warning system for networks that plays a pivotal role in any organisation’s cybersecurity strategy.

Types of IDS Detection

Intrusion Detection Systems (IDS) employ various detection techniques to identify suspicious activities within a network. While the first two (below) are the primary types of IDS detection, alternative methods are used for specific environments:

Signature-based Detection

As one of the most common detection methods, signature-based detection relies on a database of known attack patterns, often termed “signatures”. When incoming traffic matches one of these patterns, an alert is generated. While effective against known threats, it can’t detect new, previously unrecorded threats.

Anomaly-based Detection

Unlike signature-based systems, anomaly-based IDS focuses on establishing a baseline of “normal” network behaviour. If the incoming traffic deviates significantly from this baseline, it triggers an alert. This approach is beneficial for detecting new or unknown threats but can sometimes produce false positives.

Heuristic-based Detection

Heuristic-based IDS uses advanced algorithms and analytics to predict an attacker’s next move based on their behaviour patterns. It can adapt and learn from observed traffic, protecting against novel and evolving threats.

Stateful Protocol Analysis

This method involves understanding and tracking the state of network protocols in use. It identifies deviations that might indicate an attack by comparing observed events to pre-determined profiles of generally accepted definitions of benign activity.

Policy-based Detection

This type functions on a defined set of policies or rules the network administrator sets. Any activity that violates these policies triggers an alert. It’s a proactive approach requiring periodic policy updating to stay relevant.

Honeypot Detection

Not a traditional detection technique, honeypots are decoy systems that attract potential attackers. They divert the attacker from the actual systems and gather information about their methods. The insights from honeypots can inform other IDS about emerging threat patterns.

Understanding the different detection types is critical in selecting the proper IDS for specific network environments. The best approach often combines multiple detection methods to ensure a comprehensive protective layer against a wide array of threats.

Intrusion Detection Systems vs. Intrusion Prevention Systems

Intrusion Detection Systems (IDS) and Intrusion Prevention Systems (IPS) are essential network security tools designed to identify and combat malicious activities or policy breaches within a network. Their primary distinction lies in their respective reactions to perceived threats.

Functionality and Response:

  • IDS: Operates primarily as a surveillance mechanism, closely monitoring network traffic. Upon detecting suspicious or anomalous activity, it generates alerts, serving as a “listen-only” device without the capacity to autonomously intervene.
  • IPS: Acts more proactively. Beyond mere detection, an IPS reacts in real-time to ongoing threats by taking measures to halt them, ensuring they never reach their intended targets in the network.


Applications and Advantages:

  • IDS: Aside from its core detection functions, IDS is instrumental in quantifying and categorising types of attacks. This intelligence can empower organisations to bolster their security measures, pinpoint vulnerabilities, or rectify any configuration anomalies in their network devices.
  • IPS: As a predominantly preventive instrument, the IPS’s capabilities extend beyond mere threat detection. It actively seeks to block or mitigate any malicious actions, serving as a robust protective barrier against potential intrusions.


While IDS and IPS have distinct roles, they often function best when used in tandem. IDS ensures nothing slips through unnoticed, and IPS prevents detected threats from causing harm.

IDS vs. Firewalls

Intrusion Detection Systems and firewalls are both integral components of network security. However, they serve different purposes, primarily based on their functionality and response mechanism.


  • IDS: Primarily a monitoring tool, IDS scans the network for suspicious activities and alerts administrators when such activities are detected. It acts as a surveillance camera, constantly watching and reporting.
  • Firewalls: These are network barriers that filter incoming and outgoing traffic based on predefined rules. Think of them as gatekeepers, deciding which traffic can enter or exit a network.


Response Mechanism:

  • IDS: While IDS can detect and alert malicious traffic, it doesn’t inherently block it.
  • Firewalls: They proactively block traffic that doesn’t comply with the set rules, offering a first line of defence against potential threats.


While firewalls control the flow of traffic based on set parameters, IDS monitors the network to identify and alert on anomalies. For a robust security posture, using both together offers layered protection, with firewalls filtering unwanted traffic and IDS ensuring continuous monitoring.


While IDS is a specialised tool for detecting threats, Security Information and Event Management (SIEM) provides a comprehensive security data analysis and management platform. Each operates in different capacities within a network security framework.


  • IDS: Primarily focused on detecting suspicious or anomalous activities in a network, IDS alerts administrators once such activities are identified. It’s like a vigilant watchman, always on the lookout for potential threats.
  • SIEM: SIEM goes beyond just detection. It collects, centralises, and analyses logs and events from various sources in an IT environment. Think of it as an intelligence centre, consolidating data to offer a holistic view of the security landscape.



  • IDS: Its purview is generally restricted to detecting potential threats based on known patterns or anomalies.
  • SIEM: With its broader scope, SIEM not only detects but also correlates data, aids in forensic analysis, and supports compliance reporting.


SIEM operates as the main control centre, offering a 360-degree view of security status, trends, and threats. It’s the analytical and integrative counterpart to the IDS’s vigilant watch. Leveraging both in unison ensures rapid threat detection combined with in-depth insights and layered defence.

IDS Evasion Tactics

As intrusion detection systems evolve, so do the tactics of threat actors. Many hackers have devised techniques to bypass or evade detection by IDS. Understanding these methods is critical for reinforcing defences and maintaining robust security. Here are some commonly used IDS evasion techniques and their explanations:

  • Fragmentation: Hackers split malicious payloads into smaller packets or fragments. By fragmenting the malicious data into chunks that don’t appear harmful on their own, they can evade detection. Once inside the network, these fragments are reassembled to execute the attack.
  • Polymorphic Shellcode: Polymorphism involves altering the appearance of malicious code so that its signature changes, but its function remains the same. By doing this, hackers can make their code unrecognisable to signature-based IDS solutions.
  • Obfuscation: Threat actors use this technique to modify the attack payload in a way that the target computer will reverse, but the IDS will not. Obfuscation can be used to exploit the end host without alerting the IDS.
  • Encryption and Tunnelling: By encrypting the attack payload or tunnelling it through a legitimate protocol (like HTTP or DNS), attackers can mask their malicious traffic, making it hard for IDS to detect the hidden content.
  • Low-and-Slow Attacks: Some attackers spread their activities over extended periods or limit their request rates, effectively staying “under the radar”. These prolonged, low-frequency attacks can go unnoticed by IDS systems that detect rapid and overly suspicious actions.
  • Session Splicing: Similar to fragmentation, session splicing involves distributing malicious payloads across multiple sessions or TCP packets. The intent is to introduce the payload slowly and inconspicuously, avoiding detection triggers.

To combat these evasion tactics, organisations must regularly update and configure their IDS. Additionally, IDS should be integrated with other security tools, as combining multiple layers of security and maintaining vigilance can help mitigate the risk of such evasion techniques.

How Proofpoint Can Help

Proofpoint’s Emerging Threat Intelligence solutions deliver timely and accurate threat intelligence, which provides the backbone for supporting modern Intrusion Detection Systems. Equipped with the solution’s ET Pro Ruleset, organisations can leverage an advanced rule set that helps detect and block threats via their existing network security appliances.

Proofpoint’s fully verified intel provides deeper context and integrates seamlessly with security tools to enhance decision-making. Its threat intelligence feeds can be directly fed to SIEMs, firewalls, intrusion detection systems (IDS), intrusion protection systems (IPS), and authentication systems.

When integrated with IDS, Proofpoint’s Emerging Threat Intelligence can help improve the detection and prevention of malicious activities or policy violations in a network. Emerging Threat Intelligence also provides separate lists for IP addresses and domains, and subscribers get free use of their Splunk technology add-on.

For more information, contact Proofpoint.

Ready to Give Proofpoint a Try?

Start with a free Proofpoint trial.