- A
Add a network ACL rule to deny outbound traffic to the suspicious IP address.
Why wrong: Adding a network ACL deny rule affects the entire subnet and is stateless, potentially impacting other instances and requiring explicit allow for return traffic. It does not immediately stop traffic for the specific instance without side effects.
- B
Terminate the instance immediately.
Why wrong: Terminating the instance would stop the traffic but also reduces application availability until a new instance is launched by Auto Scaling. This affects availability.
- C
Update the security group attached to the instance to deny outbound traffic to the suspicious IP address.
By removing the outbound allow rule for the suspicious IP from the security group, traffic is implicitly denied. Security groups are stateful and only support allow rules; to block traffic, you remove the allow rule.
- D
Detach the instance from the Auto Scaling group.
Why wrong: Detaching the instance from the Auto Scaling group does not stop the traffic; the instance continues running and sending traffic to the suspicious IP. It also affects scaling management.
SCS-C02 Security group Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: security group. 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 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
Update the security group attached to the instance to deny outbound traffic to the suspicious IP address.
Option C is correct because modifying the security group's outbound rules to remove the rule that allows traffic to the suspicious IP address effectively blocks the traffic at the instance level without terminating the instance. Security groups are stateful and allow-only; they do not support explicit deny rules. By removing the allow rule, the traffic is implicitly denied. This approach maintains application availability because the instance remains in the Auto Scaling group and continues to serve legitimate traffic.
Key principle: Security group
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
Adding a network ACL deny rule affects the entire subnet and is stateless, potentially impacting other instances and requiring explicit allow for return traffic. It does not immediately stop traffic for the specific instance without side effects.
- ✗
Terminate the instance immediately.
Why it's wrong here
Terminating the instance would stop the traffic but also reduces application availability until a new instance is launched by Auto Scaling. This affects availability.
- ✓
Update the security group attached to the instance to deny outbound traffic to the suspicious IP address.
Why this is correct
By removing the outbound allow rule for the suspicious IP from the security group, traffic is implicitly denied. Security groups are stateful and only support allow rules; to block traffic, you remove the allow rule.
Clue confirmation
The clue word "immediately / without restart" in the question point toward this answer.
Related concept
Security group
- ✗
Detach the instance from the Auto Scaling group.
Why it's wrong here
Detaching the instance from the Auto Scaling group does not stop the traffic; the instance continues running and sending traffic to the suspicious IP. It also affects scaling management.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates often assume security groups support explicit deny rules, when in fact they only allow rules. To block traffic, you must remove the corresponding allow rule rather than adding a deny rule. Additionally, network ACLs (Option A) support deny rules but are stateless and affect the entire subnet, potentially impacting availability.
Detailed technical explanation
How to think about this question
Security groups are stateful, meaning that if you allow inbound traffic, the outbound response is automatically allowed regardless of outbound rules; however, outbound rules can be used to explicitly deny traffic to specific IPs by not including them in allowed destinations. Under the hood, security group rules are implemented as iptables or similar firewall rules on the EC2 hypervisor, and changes take effect immediately without requiring instance reboot. In a real-world scenario, this approach is critical for incident response where you need to isolate a compromised instance while preserving its role in the Auto Scaling group for forensic analysis or gradual remediation.
KKey Concepts to Remember
- Security group
- Network ACL
- Auto Scaling group
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Security group
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review security group, then practise related SCS-C02 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
Infrastructure Security — This question tests Infrastructure Security — Security group.
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 C is correct because modifying the security group's outbound rules to remove the rule that allows traffic to the suspicious IP address effectively blocks the traffic at the instance level without terminating the instance. Security groups are stateful and allow-only; they do not support explicit deny rules. By removing the allow rule, the traffic is implicitly denied. This approach maintains application availability because the instance remains in the Auto Scaling group and continues to serve legitimate traffic.
What should I do if I get this SCS-C02 question wrong?
Review security group, then practise related SCS-C02 questions on the same topic to reinforce the concept.
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?
Security group
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Last reviewed: Jul 4, 2026
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