The answer is to modify the security group attached to the compromised EC2 instance to revoke all outbound rules. This is the fastest action because security groups are stateful and act as a virtual firewall at the instance level, so removing the outbound rules immediately cuts all traffic from that specific instance without impacting any other resources in the VPC. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of the difference between security groups (instance-level, stateful) and network ACLs (subnet-level, stateless), with a common trap being to reach for a NACL change, which would affect all instances in the subnet. The key exam tip is to remember that for a single compromised instance, always modify its security group first, not the subnet-level controls. Memory tip: "SG for single, NACL for subnet" — security groups isolate one instance, while NACLs isolate an entire subnet.
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 company's security team discovers that an Amazon EC2 instance has been compromised and is sending outbound traffic to a known malicious IP address. The instance is in a VPC with a security group that allows all outbound traffic. What is the FASTEST way to stop the outbound traffic without affecting other instances?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Modify the security group attached to the instance to revoke all outbound rules.
Option D is correct because modifying the security group to deny outbound traffic will immediately affect that specific instance. Option A is wrong because terminating the instance is drastic and may cause data loss. Option B is wrong because modifying a network ACL affects all instances in the subnet. Option C is wrong because changing the route table affects all instances in the subnet.
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.
✗
Modify the network ACL of the subnet to deny outbound traffic to the malicious IP.
Why it's wrong here
Network ACL affects all instances in the subnet, not just the compromised one.
✗
Change the route table of the subnet to route traffic to a blackhole.
Why it's wrong here
Route table changes affect all instances in the subnet.
✗
Terminate the compromised EC2 instance immediately.
Why it's wrong here
Termination is irreversible and may not be the fastest if you need to preserve data.
✓
Modify the security group attached to the instance to revoke all outbound rules.
Why this is correct
Security group changes apply immediately to the instance and do not affect others.
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.
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: Modify the security group attached to the instance to revoke all outbound rules. — Option D is correct because modifying the security group to deny outbound traffic will immediately affect that specific instance. Option A is wrong because terminating the instance is drastic and may cause data loss. Option B is wrong because modifying a network ACL affects all instances in the subnet. Option C is wrong because changing the route table affects all instances in the subnet.
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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