The answer is that GuardDuty is not generating findings for the compromised EC2 instance because the VPC Flow Logs are not being delivered to CloudWatch Logs. GuardDuty relies on analyzing network traffic data from VPC Flow Logs to detect anomalous behavior, such as outbound communication with a known malicious IP; if those logs are enabled but not actually streaming to CloudWatch Logs, GuardDuty has no network telemetry to analyze. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of GuardDuty’s data sources—specifically that it consumes VPC Flow Logs, DNS logs, and CloudTrail events, but flow logs must be actively delivered to CloudWatch Logs to be ingested. A common trap is assuming that simply enabling VPC Flow Logs is sufficient, or confusing CloudTrail (which logs API calls) with network traffic analysis. Remember the memory tip: “No flow, no show”—if flow logs aren’t flowing to CloudWatch, GuardDuty won’t show findings.
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 security engineer is troubleshooting an issue where Amazon GuardDuty is not generating findings for a specific EC2 instance that is known to be compromised. The instance is in a VPC with VPC Flow Logs enabled. What could be the reason for the lack of findings?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VPC Flow Logs are not being delivered to CloudWatch Logs.
Option C is correct because GuardDuty requires VPC Flow Logs to be enabled and delivered to CloudWatch Logs to analyze network traffic. Option A is wrong because CloudTrail logs management events, not network traffic. Option B is wrong because subnet flow logs are not a concept; flow logs are at the VPC, subnet, or ENI level. Option D is wrong because GuardDuty does not require AWS Config to be enabled.
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.
✗
CloudTrail is not enabled in the account.
Why it's wrong here
CloudTrail records API calls, not network traffic; GuardDuty uses network data from VPC Flow Logs.
✗
AWS Config is not enabled for the EC2 instance.
Why it's wrong here
GuardDuty does not use AWS Config data for threat detection.
✗
VPC Flow Logs are only enabled for the subnet, not the instance's ENI.
Why it's wrong here
VPC Flow Logs can be at VPC, subnet, or ENI level; any level that captures traffic to the instance works.
✓
VPC Flow Logs are not being delivered to CloudWatch Logs.
Why this is correct
GuardDuty analyzes VPC Flow Logs data from CloudWatch Logs; without delivery, it cannot generate findings.
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: VPC Flow Logs are not being delivered to CloudWatch Logs. — Option C is correct because GuardDuty requires VPC Flow Logs to be enabled and delivered to CloudWatch Logs to analyze network traffic. Option A is wrong because CloudTrail logs management events, not network traffic. Option B is wrong because subnet flow logs are not a concept; flow logs are at the VPC, subnet, or ENI level. Option D is wrong because GuardDuty does not require AWS Config to be enabled.
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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