Implementing Defense In Depth Methods

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If you’re planning to take the Security+ exam you should have a basic understanding of network design elements and components. As an example, it’s important to understand  defense in depth methods.

For example, can you answer this question?

Q. An organization wants to provide protection against malware attacks. Administrators have installed antivirus software on all computers. Additionally, they implemented a firewall and an IDS on the network. Which of the following BEST identifies this principle?

A. Implicit deny

B. Layered security

C. Least privilege

D. Flood guard

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

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Layered Security/Defense in Depth

Layered security/defense in depth refers to the security practice of implementing several layers of protection. You can’t simply take a single action, such as implementing a firewall or installing antivirus software, and consider yourself protected. You must implement security at several different layers. This way, if one layer fails, you still have additional layers to protect you.

If you drive your car to a local Walmart, put a five-dollar bill on the dash, and leave the keys in the car and the car running, there is a very good chance the car won’t be there when you come out of the store. On the other hand, if you ensure nothing of value is visible from the windows, the car is locked, and it has an alarm system and stickers on the windows advertising the alarm system, it’s less likely that someone will steal it. Not impossible, but less likely.

You’ve probably heard this as “there is no silver bullet.” If you want to kill a werewolf, you can load your gun with a single silver bullet and it will find its mark. The truth is that there is no such thing as a silver bullet. (Of course, there is no such thing as a werewolf either.)

Applied to computers, it’s important to implement security at every step, every phase, and every layer. Information technology (IT) professionals can never rest on their laurels with the thought they have done enough and no longer need to worry about security.

It’s common to see several layers of protection on the network, such as a firewall, an intrusion detection system (IDS), and proxy servers for content filtering. The key is that they work together to provide several layers of security. Similarly, organizations often install antivirus software on all systems, but also attempt to detect and block malware from entering the network.

Purpose of a Firewall

A firewall filters traffic between networks and can filter both incoming and outgoing traffic. In other words, a firewall can ensure only specific types of traffic are allowed into your network and only specific types of traffic are allowed out of your network.

The purpose of a firewall in a network is similar to a firewall in a car. The firewall in a car is located between the engine and passenger compartment. If a fire starts in the engine compartment, the firewall will provide a layer of protection for passengers in the passenger compartment. Similarly, a firewall in a network will try to keep the bad traffic (often in the form of attackers) out of the network.

Of course, an engine has a lot of moving parts that can do damage to people if they accidentally reach into it while it’s running. The firewall in a car protects passengers from touching any of those moving parts. Similarly, a network can also block users from going to places that an administrator deems dangerous. For example, uneducated users could inadvertently download damaging files, but many firewalls can block potentially malicious downloads.

Firewalls start with a basic routing capability for packet filtering. More advanced firewalls go beyond simple packet filtering and include advanced content filtering. Unified Threat Management describes how some security appliances build on basic firewalls by adding in advanced filtering and inspection capabilities.

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Configuring NIDS 

A network-based intrusion detection system (NIDS) monitors activity on the network. An administrator installs NIDSs sensors on network devices such as routers and firewalls. These sensors gather information and report to a central monitoring server hosting a NIDS console.

A NIDS is not able to detect anomalies on individual systems or workstations unless the anomaly causes a significant difference in network traffic. Additionally, a NIDS is unable to decrypt encrypted traffic. In other words, it can only monitor and assess threats on the network from traffic sent in plaintext or nonencrypted traffic.

The following figure shows an example of a NIDS configuration. In the figure, sensors are located before the firewall, after the firewall, and on routers. These sensors collect and monitor network traffic on subnets within the network and report to the NIDS console. The NIDS provides overall monitoring and analysis and can detect attacks on the network.

Implementing Defense In Depth

NIDS sensors

The decision on where you want to place the sensors depends on what you want to measure. For example, the sensor on the Internet side of the firewall will see all the traffic. However, the sensor on the internal side of the firewall will only see traffic that passes through the firewall. In other words, the firewall will filter some attacks, and the internal sensor won’t see them.

If you want to see all attacks on your network, put a sensor on the Internet side. If you only want to see what gets through, put sensors internally only. If you want to see both, put sensors in both places.

Remember this

Layered security, or defense in depth, combines multiple layers of security, such as a firewall, an IDS, content filtering, and antivirus software.


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Q. An organization wants to provide protection against malware attacks. Administrators have installed antivirus software on all computers. Additionally, they implemented a firewall and an IDS on the network. Which of the following BEST identifies this principle?

A. Implicit deny

B. Layered security

C. Least privilege

D. Flood guard

Answer is B. Layered security (or defense in depth) implements multiple controls to provide several layers of protection. In this case, the antivirus software provides one layer of protection while the firewall and the intrusion detection system (IDS) provide additional layers.

Implicit deny blocks access unless it has been explicitly allowed.

Least privilege ensures that users are granted only the access they need to perform their jobs, and no more.

A flood guard attempts to block SYN Flood attacks.

See Chapter 1 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information on core security goals such as availability.

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