Security Countermeasures Against Attacks

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If there’s one thing that’s abundant in the IT world, it is attacks and attackers. Attackers lurk almost everywhere. If you have computer systems, you can’t escape them. However, you can be proactive in identifying the different types of attacks and take steps to prevent them, or at least prevent their effectiveness. It’s important to realize that effective countermeasures exist for all of the attacks.

Attackers are actively working on beating the countermeasures. As they do, security professionals create additional countermeasures and the attackers try to beat them. The battle continues daily. If you’re planning to take the Security+ exam, you should have a basic understanding of various types of attacks from different sources and have some insight into preventing many of them.

For example, can you answer this question?

Q. Some protocols include timestamps and sequence numbers. What types of attacks do these components help protect against?

A. Smurf

B. Replay

C. Flood guards

D. Salting

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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Replay Attacks

A replay attack is one where an attacker replays data that was already part of a communication session. In a replay attack, a third party attempts to impersonate a client that is involved in the original session. Replay attacks can occur on both wired and wireless networks.

Security Countermeasures Against Attacks

As an example, Maggie and Bart may initiate a session with each other. During the communication, each client authenticates with the other by passing authentication credentials to the other system. Hacker Harry intercepts all the data, including the credentials, and later initiates a conversation with Maggie pretending to be Bart. When Maggie challenges hacker Harry, he sends Bart’s credentials.

Many protocols use timestamps and sequence numbers to thwart replay attacks. For example, Kerberos helps prevent replay attacks with timestamped tickets.

Remember this

Replay attacks capture data in a session with the intent of later impersonating one of the parties in the session. Timestamps and sequence numbers are effective countermeasures against replay attacks.

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Xmas Attacks

The Xmas attack, also called a Christmas tree attack, is a type of port scan used to identify underlying details of an operating system. For example, it can help determine if the scanned system is running a Microsoft-based operating system or a Linux-based operating system.

A typical port scan attempts to learn what ports are open on a system. Based on what ports are open, the port scanner can detect what services and protocols are running on a system. For example, if port 80 is open, it’s very likely that the Hypertext Transfer Protocol (HTTP) protocol is running on the system because the well-known port for HTTP is port 80.

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However, the Xmas attack goes farther than a typical port scan. It has several bits set in the packet header and is reminiscent of lights lit in a Christmas tree. As least someone thought it looked like a Christmas tree and decided to name it a Christmas tree attack, or Xmas attack.

More importantly, the Xmas attack sets specific flags within the TCP packet header. Different operating systems respond to these flags in specific ways. Attackers can analyze the response and determine the operating system of the remote system in addition to what ports are open. In many cases, the attacker can even determine the version of the responding system.

The Xmas attack is often used as reconnaissance in an overall attack. It doesn’t cause damage itself. However, attackers use the information they gain from the Xmas attack to launch other attacks. Most intrusion detection systems (IDSs) and intrusion prevention systems (IPSs) can detect these attacks.

CompTIA Security+ Study Guide

The 501 Version of the Study Guide

The CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide is an update to the top-selling SY0-201, SY0-301, and SY0-401 study guides, which have helped thousands of readers pass the exam the first time they took it.  It includes the same elements readers raved about in the previous three versions.

Each of the eleven chapters presents topics in an easy to understand manner and includes real-world examples of security principles in action.

You’ll understand the important and relevant security topics for the Security+ exam, without being overloaded with unnecessary details. Additionally, each chapter includes a comprehensive review section to help you focus on what’s important.



Over 300 realistic practice test questions with in-depth explanations will help you test your comprehension and readiness for the exam. The book includes:

  • A 75 question pre-test
  • A 75 question post-test
  • Practice test questions at the end of every chapter.

Each practice test question includes a detailed explanation to help you understand the content and the reasoning behind the question. You’ll be ready to take and pass the exam the first time you take it.

If you plan to pursue any of the advanced security certifications, this guide will also help you lay a solid foundation of security knowledge. Learn this material, and you’ll be a step ahead for other exams. This SY0-501 study guide is for any IT or security professional interested in advancing in their field, and a must-read for anyone striving to master the basics of IT security.

Kindle edition also available.

Man-in-the-Middle Attacks

A man-in-the-middle (MITM) attack is a form of active interception or active eavesdropping. It uses a separate computer that accepts traffic from each party in a conversation and forwards the traffic between the two. The two computers are unaware of the MITM computer, and it can interrupt the traffic at will or insert malicious code.

For example, imagine that Maggie and Bart are exchanging information with their two computers over a network. If hacker Harry can launch an MITM attack from a third computer, he will be able to intercept all traffic. Maggie and Bart still receive all the information, so they are unaware of the attack. However, hacker Harry also receives all the information. Because the MITM computer can control the entire conversation, it is easy to insert malicious code and send it to the computers. The ARP Poisoning Attacks blog post shows how ARP poisoning can be used to launch an MITM attack.

Kerberos helps prevent man-in-the-middle attacks with mutual authentication. It doesn’t allow a malicious system to insert itself in the middle of the conversation without the knowledge of the other two systems.


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  • Comparing Threats, Vulnerabilities, and Common Attacks (full content from Chapter 6 of the study guide including the exam topic review and 15 practice test questions)
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Q. Some protocols include timestamps and sequence numbers. These components help protect against what type of attacks?

A. Smurf

B. Replay

C. Flood guards

D. Salting

Answer is B. Timestamps and sequence numbers act as countermeasures against replay attacks.

Blocking directed broadcasts prevents smurf attacks.

Flood guards protect against SYN (synchronize) attacks.

Salting protects against brute force attacks on passwords.

See Chapter 7 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information identifying advanced attacks.

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