The answer is to update the security group attached to the EC2 instance to deny outbound traffic to the suspicious IP address. This is correct because security groups act as a virtual firewall at the instance level, and modifying them is immediate, stateful, and reversible—meaning you can block specific outbound traffic without terminating the instance or affecting the application’s availability. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of granular, instance-level controls versus broader network controls like NACLs, which are stateless and apply to entire subnets. A common trap is choosing to detach the instance from the Auto Scaling group, which does not stop traffic, or terminating the instance, which breaks availability. Remember the memory tip: “Security groups are surgical, NACLs are blunt”—when you need to stop outbound traffic from a single EC2 instance without collateral damage, always reach for the security group.
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. 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 security engineer notices that an Amazon EC2 instance is sending suspicious outbound traffic to an unknown IP address. The instance is part of an Auto Scaling group. The engineer needs to immediately stop the traffic without affecting the availability of the application. What should the engineer do?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "immediately / without restart"
Why it matters: Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Update the security group attached to the instance to deny outbound traffic to the suspicious IP address.
Option B is correct because modifying the security group to deny outbound traffic to the specific IP is immediate and reversible. Option A is wrong because terminating the instance would affect availability. Option C is wrong because a NACL would affect all instances in the subnet. Option D is wrong because detaching the instance from the ASG does not stop the traffic.
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.
✗
Add a network ACL rule to deny outbound traffic to the suspicious IP address.
Why it's wrong here
NACLs apply to the entire subnet, affecting other instances and potentially causing availability issues.
✗
Terminate the instance immediately.
Why it's wrong here
Terminating the instance will affect application availability if the ASG does not replace it quickly.
✓
Update the security group attached to the instance to deny outbound traffic to the suspicious IP address.
Why this is correct
Security group rules are stateful; adding a deny rule for the specific IP will block the traffic immediately.
Clue confirmation
The clue word "immediately / without restart" in the question point toward this answer.
Detaching does not stop the traffic; the instance continues to run.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Update the security group attached to the instance to deny outbound traffic to the suspicious IP address. — Option B is correct because modifying the security group to deny outbound traffic to the specific IP is immediate and reversible. Option A is wrong because terminating the instance would affect availability. Option C is wrong because a NACL would affect all instances in the subnet. Option D is wrong because detaching the instance from the ASG does not stop the traffic.
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.
Are there clue words in this question I should notice?
Yes — watch for: "immediately / without restart". Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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