- A
Stop the EC2 instance and launch a new one in a different subnet.
Why wrong: This is disruptive and does not prevent the attack from recurring; the new instance would likely be targeted again.
- B
Modify the security group of the EC2 instance to deny inbound SSH from the attacking IPs.
Why wrong: Security groups are stateful and apply at the instance level, but the ALB forwards traffic to the instance on the load balancer's private IP; blocking at the instance may not stop all attack vectors if the instance is also directly reachable.
- C
Create a network ACL rule on the subnet to deny inbound traffic from the attacking IPs.
Why wrong: Network ACLs are stateless and would block traffic to the ALB as well, potentially affecting legitimate users.
- D
Configure AWS WAF on the ALB to block the attacking IPs using an IP set rule.
AWS WAF can filter incoming requests to the ALB, blocking malicious IPs while allowing legitimate traffic. This can be automated via GuardDuty findings triggering a Lambda function.
Quick Answer
The answer is to configure AWS WAF on the ALB with an IP set rule to block the attacking IPs. This is the most effective approach because GuardDuty can automatically trigger remediation via CloudWatch Events and a Lambda function that updates the WAF IP set, stopping the SSH brute force at the network edge before traffic reaches the EC2 instance. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of integrating detective controls (GuardDuty) with preventive controls (WAF), and the common trap is choosing to modify security groups or NACLs—which would disrupt legitimate traffic from the ALB or fail to scale with the Auto Scaling group. The key insight is that the ALB is the single entry point, making WAF the ideal layer for blocking external IPs without affecting internal traffic. Memory tip: “WAF at the door, GuardDuty on the floor”—the WAF blocks at the entry point while GuardDuty monitors the instance.
SCS-C02 Security Logging and Monitoring Practice Question
This SCS-C02 practice question tests your understanding of security logging and monitoring. 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.
A company uses Amazon GuardDuty to monitor for threats. The security team receives a high-severity finding: 'UnauthorizedAccess:EC2/SSHBruteForce'. The finding indicates a single EC2 instance with a public IP is receiving SSH connection attempts from multiple external IPs. The instance is part of an Auto Scaling group and is fronted by an Application Load Balancer (ALB). The security team wants to block the attacking IPs without disrupting legitimate traffic. What is the MOST effective approach?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Configure AWS WAF on the ALB to block the attacking IPs using an IP set rule.
GuardDuty can automatically update AWS WAF web ACLs via CloudWatch Events and Lambda. Since the ALB is the entry point, blocking at the WAF level prevents attacks before they reach the instance.
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.
- ✗
Stop the EC2 instance and launch a new one in a different subnet.
Why it's wrong here
This is disruptive and does not prevent the attack from recurring; the new instance would likely be targeted again.
- ✗
Modify the security group of the EC2 instance to deny inbound SSH from the attacking IPs.
Why it's wrong here
Security groups are stateful and apply at the instance level, but the ALB forwards traffic to the instance on the load balancer's private IP; blocking at the instance may not stop all attack vectors if the instance is also directly reachable.
- ✗
Create a network ACL rule on the subnet to deny inbound traffic from the attacking IPs.
Why it's wrong here
Network ACLs are stateless and would block traffic to the ALB as well, potentially affecting legitimate users.
- ✓
Configure AWS WAF on the ALB to block the attacking IPs using an IP set rule.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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Security Logging and Monitoring — study guide chapter
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
Security Logging and Monitoring — This question tests Security Logging and Monitoring — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure AWS WAF on the ALB to block the attacking IPs using an IP set rule. — GuardDuty can automatically update AWS WAF web ACLs via CloudWatch Events and Lambda. Since the ALB is the entry point, blocking at the WAF level prevents attacks before they reach the instance.
What should I do if I get this SCS-C02 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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Same concept, more angles
1 more ways this is tested on SCS-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company uses Amazon GuardDuty for threat detection. The security team wants to automatically isolate an EC2 instance that is communicating with a known malicious IP address. Which combination of services should be used?
medium- A.GuardDuty -> AWS Config -> Lambda -> modify security group
- B.GuardDuty -> CloudWatch Alarm -> Lambda -> modify security group
- ✓ C.GuardDuty -> EventBridge -> Lambda -> modify security group
- D.GuardDuty -> AWS Shield -> modify security group
- E.GuardDuty -> AWS Systems Manager -> modify security group
Why C: Option B is correct because GuardDuty findings can be sent to EventBridge, which triggers a Lambda function to modify the security group. Option A is wrong because CloudWatch Alarms do not directly process GuardDuty findings. Option C is wrong because AWS Config is for configuration management. Option D is wrong because Systems Manager is for patch management. Option E is wrong because AWS Shield is for DDoS protection.
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Last reviewed: Jun 20, 2026
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