The answer is to use Amazon EventBridge to invoke an AWS Lambda function that modifies the instance’s security group. This is correct because GuardDuty findings are automatically sent to EventBridge as events, which can then trigger a Lambda function to revoke inbound and outbound rules in the instance’s security group, effectively isolating the compromised EC2 instance without affecting other resources in the subnet. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of event-driven automation and the principle of least privilege—modifying a security group targets only the flagged instance, whereas changing a network ACL would impact the entire subnet. A common trap is choosing termination, which is too aggressive and risks data loss, or relying on CloudWatch Alarms, which cannot directly process GuardDuty findings. Memory tip: think “GuardDuty finds, EventBridge binds, Lambda rewinds the security group lines.”
SCS-C02 Security Logging and Monitoring Practice Question
This SCS-C02 practice question tests your understanding of security logging and monitoring. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 is using Amazon GuardDuty to monitor for malicious activity. The security team wants to automatically isolate an EC2 instance that is flagged for outbound communication with a known malicious IP address. Which approach is the most efficient and scalable?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use Amazon EventBridge to invoke an AWS Lambda function that modifies the instance's security group.
Option C is correct because GuardDuty can send findings to EventBridge, which can trigger a Lambda function to modify the instance's security group to isolate it. Option A is wrong because modifying the NACL would affect the entire subnet, not just the instance. Option B is wrong because terminating the instance is too drastic and may cause data loss. Option D is wrong because CloudWatch Alarms cannot directly trigger Lambda for GuardDuty findings.
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.
✗
Use a CloudWatch Alarm to directly invoke a Lambda function to isolate the instance.
Why it's wrong here
CloudWatch Alarms cannot directly invoke Lambda.
✗
Use AWS Config to automatically terminate the instance when a GuardDuty finding is reported.
Why it's wrong here
Termination is too drastic.
✓
Use Amazon EventBridge to invoke an AWS Lambda function that modifies the instance's security group.
Create a CloudWatch alarm on GuardDuty findings and modify the subnet's network ACL to block the traffic.
Why it's wrong here
Affects entire subnet.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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.
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: Use Amazon EventBridge to invoke an AWS Lambda function that modifies the instance's security group. — Option C is correct because GuardDuty can send findings to EventBridge, which can trigger a Lambda function to modify the instance's security group to isolate it. Option A is wrong because modifying the NACL would affect the entire subnet, not just the instance. Option B is wrong because terminating the instance is too drastic and may cause data loss. Option D is wrong because CloudWatch Alarms cannot directly trigger Lambda for GuardDuty findings.
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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Question Discussion
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