TL;DR
An intrusion detection system (IDS) monitors network and host traffic around the clock, flagging suspicious activity via signature- or anomaly-based detection — but it cannot stop attacks on its own. Understanding where to place your IDS, how it differs from firewalls and intrusion prevention systems, and its known limitations (like decryption gaps and false-positive overload) is essential for building a resilient security posture. Emerging IDS solutions are beginning to leverage machine learning to reduce administrative burden and improve accuracy.
How does an IDS fit into your security strategy?
Unlike an intrusion prevention system (IPS), a traditional IDS cannot block or resolve threats on its own. Instead, it flags anomalies and alerts security teams or automated software to investigate and address the issue.
First developed in the 1980s, IDS technology continues to evolve alongside modern cyber threats. Below, we'll examine how an IDS operates, key placement strategies, and the core benefits and limitations to help you determine if it's the right fit for your security posture.How does intrusion detection work?
Out of all businesses open in the United States right now, 14 million are vulnerable to a hack. Large corporations are obviously at risk. But even smaller companies could be enticing to thieves and mischievous programmers. An IDS should help you spot a problem early before too much damage is done.
There are two main types of IDS.
- Network Intrusion Detection System (NIDS): Monitors everything that goes into or out of a device on the network.
- Host Intrusion Detection System (HIDS): Monitors an individual device (or host) within the network. It scans inbound and outbound traffic.
How does an IDS spot a problem within traffic patterns? Two main detection types are available. Your system might flag issues based on:
- Signatures. The IDS compares movement within your system to a vast database of known hacking techniques. In essence, the program attempts to determine if what's happening on your system right now has harmed someone in the past.
- Anomalies. The system compares action happening right now to what has happened in that same spot in the past. A sudden spike in activity, or a precipitous drop, could be innocent enough. But it could also be a sign of a problem.
No matter what type of IDS you have and the detection type you're using, the solution won't reside within the IDS. These programs can't halt traffic, close trapdoors, or clean up messes.
Just as a smoke detector can't put out a fire, an IDS can't stop an attack in progress. All these programs can do is alert you to a problem.
Where should an IDS be located?
Your network has plenty of entrances and exits. You need them so data can move in and out freely. But each one is a vulnerability, and if you have many, finding the right place to install your IDS can be tricky.
You can place your IDS:
- Behind the firewall. Every company, no matter the size or configuration, should have a firewall. Install an IDS just behind your firewall for close monitoring of traffic entering your system.
- Within your firewall. Integrate the two systems to ensure monitoring of attacks as they enter the network.
- On your network. Ensure that an attack within your server doesn't spread with this approach.
Analyze past attacks, along with your current risks, to determine which placement choice is right for you. In time, you may find that you must move the IDS for the highest level of protection.
How is an IDS different from other security methods?
Plenty of security systems exist, and while they often work together, keeping them separate in your mind isn't always easy.
An IDS is different from:
| System | Detects problems | Stops/prevents problems | Alerts you | Notes |
|---|---|---|---|---|
| Firewall | No | Yes (by rules) | No | Rules define who should come in and what should happen while there. A firewall simply follows the rules you define. |
| Intrusion detection system (IDS) | Yes | No | Yes | Sends anomalies to another program or a human to assess and address. Cannot halt traffic, close trapdoors, or clean up messes. |
| Intrusion prevention system (IPS) | Yes | Yes | Yes | Both finds problems and solves them. You may still get an alert when a problem appears, but the solution is already in play. |
| Intrusion detection and prevention system (IDPS) | Yes | Yes | Yes | Identifies problems, reports them, and works on preventing them from happening again. May point out flaws in your plans that leave you vulnerable to attack. |
Security programs come with plenty of acronyms, and it's easy to get them confused. But in general, think of an IDS as a useful tool you pair with your own smarts to protect your company. Think of the other products as tools that can help make your job a little easier.
What are the benefits and drawbacks of an IDS?
Hackers are prolific. In December of 2020 alone, 14 known hacks took place. In just one, hackers demanded $1 million in bitcoin.
Without proper defenses, an attack like this is likely. And if you're not monitoring traffic, the attack can last for months or even years. The longer the intruder stays in your system, the greater your risk of catastrophic damage.
What are the known limitations of an IDS?
But even with an IDS in place, a hacker can move through your elaborate web of protection via:
- Masking. Proxy servers make hiding the source of an attack very easy.
- Sharing. Hackers may spread the work among many devices and users. It's harder to see the damage at a glance.
- Splitting. A hacker may fragment packets to avoid detection.
Your IDS may also be subject to known limitations, such as:
- Outdated software. If the IDS compares your traffic to prior attacks that are months or years old, you may miss signals of newer versions.
- Decryption problems. Most systems can't assess encrypted packets.
- Poor practices. More than half of all small and mid-sized companies under attack close their doors within six months. They don't develop next steps to guide them when an attack is in progress. Without those rules, they can see they're facing a problem, but they have no idea how to solve it.
- Lack of human resources. Someone must be available to look over every alert, including false positives. An IDS produces reams and reams of data, and a human must look over every piece.
What are the benefits of using an IDS?
Even so, with hacks coming every 39 seconds, companies can't afford to ignore the benefits and focus solely on the risks. An IDS does provide a great deal of valuable data you can use to protect your company. If you don't use it, you are leaving the door wide open to hackers.
What does the future of IDS look like?
Companies realize the limitations of a standard IDS. Some are reacting to build bigger and better products for their customers.
How is IDS technology evolving?
In a year or two, new IDS solutions may come with a lower administrative burden. They may rely on machine learning to lower the risk of false positives, so staff has less to examine every day. And vendors may update them simultaneously, so the system always has access to up-to-date information about new challenges.
Learn more about the difference between IDS and IPS.
Frequently asked questions
Can an intrusion detection system (IDS) stop a cyberattack on its own?
No. An intrusion detection system (IDS) is a monitoring and alerting tool only — it cannot halt traffic, close vulnerabilities, or remediate damage. Like a smoke detector that signals a fire but cannot extinguish it, an IDS notifies a human or another system to take action. For automated response capabilities, organizations should consider an intrusion prevention system (IPS) or an intrusion detection and prevention system (IDPS).
What is the difference between signature-based and anomaly-based intrusion detection system (IDS) detection?
Signature-based detection compares current network activity against a database of known attack patterns, flagging anything that matches a previously documented threat. Anomaly-based detection instead establishes a baseline of normal behavior and alerts when activity deviates significantly — such as sudden traffic spikes or drops — which can catch novel threats not yet in any signature database.
Where is the best place to install an intrusion detection system (IDS) on a network?
Placement depends on your specific risk profile and past attack history. Common options include behind the firewall (to monitor inbound traffic closely), within the firewall (to catch attacks as they enter), or directly on the network (to prevent internal spread). Reviewing your historical attack data and current vulnerabilities is the best way to determine the right placement — and you may need to adjust it over time.
What are the main limitations of an intrusion detection system (IDS)?
Key limitations include outdated signature databases that may miss newer attack variants, an inability to inspect encrypted packets, and the generation of large volumes of alerts (including false positives) that require human review. Additionally, without a defined incident response plan, organizations may detect an attack but have no clear path to address it.
How do hackers evade intrusion detection systems (IDSs)?
Attackers use several techniques to avoid intrusion detection system (IDS) detection: routing traffic through proxy servers to mask the attack's origin, distributing malicious activity across many devices to obscure the pattern, and fragmenting data packets to slip past detection rules. These evasion methods highlight why an IDS should be one layer of a broader, multi-tool security strategy.
How is intrusion detection system (IDS) technology expected to evolve?
Emerging intrusion detection system (IDS) solutions are expected to incorporate machine learning to reduce false positives and lower the administrative burden on security teams. Vendors are also moving toward simultaneous, real-time updates so that systems always have access to the latest threat intelligence — addressing one of the most significant current limitations of signature-based detection.
References
14 Million U.S. Businesses Are at Risk of a Hacker Threat. (July 2017). CNBC.
Why Every Business Needs a Firewall. (November 2018). Phoenix Business Journal.
Significant Cyber Incidents. Center for Strategic and International Studies.
60 Percent of Companies Fail in 6 Months Because of This (It's Not What You Think). (May 2017). Inc.
Hackers Attack Every 39 Seconds. (February 2017). Security.
