- A
Stop the instance immediately.
Why wrong: Stopping the instance disrupts the running state and may interfere with forensic evidence such as memory or disk content.
- B
Modify the instance's security group to deny outbound traffic to the malicious IP.
Why wrong: Security groups cannot deny outbound traffic; they only allow. Modifying the security group to remove outbound rules would block all outbound traffic, not just to the malicious IP.
- C
Add a network ACL rule to deny outbound traffic from the instance's subnet to the malicious IP.
Correct. Network ACLs allow explicit deny rules and apply at the subnet level, immediately blocking outbound traffic to the specified IP while preserving the instance.
- D
Modify the route table to route traffic to the malicious IP to a blackhole.
Why wrong: Modifying the route table to blackhole traffic also blocks traffic to the IP but affects the entire subnet and is less granular than a network ACL rule; it is harder to revert.
Block Outbound Traffic from Compromised EC2 Instance to Malicious IP | AWS Security Specialty Explained
This SCS-C02 practice question tests your understanding of threat detection and incident response. 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. A key principle to apply: security Groups vs Network ACLs. 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 EC2 instance is sending outbound traffic to a known malicious IP address. The engineer needs to quickly block all traffic to that IP while preserving the instance for forensic analysis. Which approach is the most effective?
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
Add a network ACL rule to deny outbound traffic from the instance's subnet to the malicious IP.
Modifying the network ACL to deny outbound traffic to the malicious IP is the most effective approach. Network ACLs are stateless and can explicitly deny traffic, applying at the subnet level. This blocks the traffic immediately while preserving the instance for forensic analysis, as it does not require stopping the instance or affecting its state.
Key principle: Security Groups vs Network ACLs
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Stop the instance immediately.
Why it's wrong here
Stopping the instance disrupts the running state and may interfere with forensic evidence such as memory or disk content.
- ✗
Modify the instance's security group to deny outbound traffic to the malicious IP.
Why it's wrong here
Security groups cannot deny outbound traffic; they only allow. Modifying the security group to remove outbound rules would block all outbound traffic, not just to the malicious IP.
- ✓
Add a network ACL rule to deny outbound traffic from the instance's subnet to the malicious IP.
Why this is correct
Correct. Network ACLs allow explicit deny rules and apply at the subnet level, immediately blocking outbound traffic to the specified IP while preserving the instance.
Related concept
Security Groups vs Network ACLs
- ✗
Modify the route table to route traffic to the malicious IP to a blackhole.
Why it's wrong here
Modifying the route table to blackhole traffic also blocks traffic to the IP but affects the entire subnet and is less granular than a network ACL rule; it is harder to revert.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates often believe security groups can deny specific IP traffic, but security groups are allow-only. Network ACLs, being stateless, can explicitly deny traffic and are the correct tool for this scenario.
Detailed technical explanation
How to think about this question
Security groups are stateful, meaning that if you add an outbound deny rule for a specific IP, the security group automatically allows return traffic for any established connections, but new outbound connections to that IP are blocked immediately. Under the hood, security group rules are implemented as iptables or equivalent firewall rules on the hypervisor, and changes propagate within seconds without affecting the instance's running state. In a real-world incident response scenario, this approach allows the engineer to isolate the instance's communication while keeping it running for memory capture, disk imaging, or live analysis using tools like AWS Systems Manager Session Manager.
KKey Concepts to Remember
- Security Groups vs Network ACLs
- Explicit Deny
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 Groups vs Network ACLs
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.
Visual reference
Quick reference
Cloud Service Model Comparison
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
What to study next
Got this wrong? Here's your next step.
Review security Groups vs Network ACLs, then practise related SCS-C02 questions on the same topic to reinforce the concept.
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Threat Detection and Incident Response — study guide chapter
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
Threat Detection and Incident Response — This question tests Threat Detection and Incident Response — Security Groups vs Network ACLs.
What is the correct answer to this question?
The correct answer is: Add a network ACL rule to deny outbound traffic from the instance's subnet to the malicious IP. — Modifying the network ACL to deny outbound traffic to the malicious IP is the most effective approach. Network ACLs are stateless and can explicitly deny traffic, applying at the subnet level. This blocks the traffic immediately while preserving the instance for forensic analysis, as it does not require stopping the instance or affecting its state.
What should I do if I get this SCS-C02 question wrong?
Review security Groups vs Network ACLs, then practise related SCS-C02 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Security Groups vs Network ACLs
About these practice questions
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Same concept, more angles
1 more ways this is tested on SCS-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A security engineer notices that an EC2 instance is sending outbound traffic to a known malicious IP address. The instance is part of an Auto Scaling group behind an Application Load Balancer. The engineer needs to immediately stop the exfiltration while preserving forensic evidence. What is the BEST course of action?
medium- A.Detach the EBS volume from the instance.
- B.Terminate the EC2 instance immediately.
- C.Shut down the instance from within the OS.
- ✓ D.Remove the instance from the target group and apply a security group that denies all traffic.
Why D: Option D is correct because removing the instance from the target group immediately stops new traffic from the Application Load Balancer to the instance, while applying a security group that denies all outbound traffic (e.g., a custom security group with no outbound rules) halts any ongoing exfiltration without destroying the instance or its attached EBS volumes. This preserves the forensic evidence (memory, disk, logs) for later analysis, unlike termination or shutdown which could lose volatile data.
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Last reviewed: Jul 4, 2026
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