This CAS-004 practice question tests your understanding of security operations. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
```
# auth.log excerpt
Mar 15 10:23:45 server sshd[1234]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:46 server sshd[1235]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:47 server sshd[1236]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:48 server sshd[1237]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:49 server sshd[1238]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:50 server sshd[1239]: Failed password for invalid user admin from 192.168.1.100 port 22 ssh2
Mar 15 10:23:51 server sshd[1240]: Failed password for admin from 192.168.1.100 port 22 ssh2
Mar 15 10:23:52 server sshd[1241]: Failed password for admin from 192.168.1.100 port 22 ssh2
```
Based on the auth.log exhibit, what is the MOST appropriate immediate action to mitigate this attack?
Refer to the exhibit.
```
# auth.log excerpt
Mar 15 10:23:45 server sshd[1234]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:46 server sshd[1235]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:47 server sshd[1236]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:48 server sshd[1237]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:49 server sshd[1238]: Failed password for root from 192.168.1.100 port 22 ssh2
Mar 15 10:23:50 server sshd[1239]: Failed password for invalid user admin from 192.168.1.100 port 22 ssh2
Mar 15 10:23:51 server sshd[1240]: Failed password for admin from 192.168.1.100 port 22 ssh2
Mar 15 10:23:52 server sshd[1241]: Failed password for admin from 192.168.1.100 port 22 ssh2
```
A
Disable root login and remove the admin account.
Why wrong: These are long-term hardening steps but won't stop the ongoing attack immediately.
B
Block the entire 192.168.1.0/24 subnet at the firewall.
Why wrong: Blocking the whole subnet may affect other legitimate hosts.
C
Configure fail2ban to block the IP address after a threshold of failed attempts.
fail2ban automatically blocks the attacking IP, stopping the attack.
D
Change the SSH port to a non-standard port.
Why wrong: Changing the port may reduce automated scans but won't prevent this specific attack from the same IP.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure fail2ban to block the IP address after a threshold of failed attempts.
Option C is correct. The log shows a brute-force attack against SSH. Installing fail2ban will dynamically block the IP after multiple failures. Option A is too broad and may block legitimate users from that subnet. Option B doesn't address the immediate attack. Option D is good practice but does not stop the ongoing attack immediately.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Disable root login and remove the admin account.
Why it's wrong here
These are long-term hardening steps but won't stop the ongoing attack immediately.
✗
Block the entire 192.168.1.0/24 subnet at the firewall.
Why it's wrong here
Blocking the whole subnet may affect other legitimate hosts.
✓
Configure fail2ban to block the IP address after a threshold of failed attempts.
Why this is correct
fail2ban automatically blocks the attacking IP, stopping the attack.
Changing the port may reduce automated scans but won't prevent this specific attack from the same IP.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related CAS-004 subnetting questions on CIDR, address ranges, and subnet selection.
Security Operations — This question tests Security Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure fail2ban to block the IP address after a threshold of failed attempts. — Option C is correct. The log shows a brute-force attack against SSH. Installing fail2ban will dynamically block the IP after multiple failures. Option A is too broad and may block legitimate users from that subnet. Option B doesn't address the immediate attack. Option D is good practice but does not stop the ongoing attack immediately.
What should I do if I get this CAS-004 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related CAS-004 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This CAS-004 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CAS-004 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.