- A
Isolate the instance by modifying its security group to deny all traffic.
Correct. Isolation stops communication while preserving the instance for forensic analysis.
- B
Terminate the instance immediately to stop the threat.
Why wrong: Termination may destroy evidence; isolation allows investigation.
- C
Ignore the alert because auto-scaling groups are ephemeral.
Why wrong: Ignoring could leave the threat active; auto-scaling does not mean it's safe.
- D
Update the GuardDuty threat list to ignore that IP.
Why wrong: Whitelisting a malicious IP is inappropriate; the instance should be investigated.
Question 459 of 1,000
CS0-003 Security Operations Practice Question
This CS0-003 practice question tests your understanding of security operations. 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 cloud security analyst is investigating an alert from AWS GuardDuty that indicates an EC2 instance is communicating with a known malicious IP address. The instance is part of an auto-scaling group. What is the best immediate action?
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
✓
Isolate the instance by modifying its security group to deny all traffic.
Isolating the instance by removing it from the security group or using a quarantine VPC prevents further communication while preserving forensic data.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Isolate the instance by modifying its security group to deny all traffic.
Why this is correct
Correct. Isolation stops communication while preserving the instance for forensic analysis.
Related concept
Standard ACLs match source addresses.
- ✗
Terminate the instance immediately to stop the threat.
Why it's wrong here
Termination may destroy evidence; isolation allows investigation.
- ✗
Ignore the alert because auto-scaling groups are ephemeral.
Why it's wrong here
Ignoring could leave the threat active; auto-scaling does not mean it's safe.
- ✗
Update the GuardDuty threat list to ignore that IP.
Why it's wrong here
Whitelisting a malicious IP is inappropriate; the instance should be investigated.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
- Standard ACLs match source addresses.
- Extended ACLs can match source, destination, protocol and ports.
- The first matching ACL entry is used.
- There is usually an implicit deny at the end.
TExam Day Tips
- Check inbound versus outbound direction.
- Read the ACL from top to bottom.
- Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A SOC analyst notices unusual lateral movement in the network at 2 AM. The IR playbook dictates: identify and contain (isolate the affected machine), then eradicate (remove the malware), then recover (restore from backup), then document. Skipping containment before eradication risks the attacker regaining access. Questions like this test the sequence and rationale of incident response phases.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CS0-003 ACL questions on filtering logic and placement.
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FAQ
Questions learners often ask
What does this CS0-003 question test?
Security Operations — This question tests Security Operations — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Isolate the instance by modifying its security group to deny all traffic. — Isolating the instance by removing it from the security group or using a quarantine VPC prevents further communication while preserving forensic data.
What should I do if I get this CS0-003 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CS0-003 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Last reviewed: Jul 4, 2026
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