The immediate step to contain an SSH brute force attack on EC2 is to update the security group to deny inbound SSH from the attacker’s IP address. This is correct because security groups act as a stateful virtual firewall for EC2 instances, and by adding a deny rule for the specific attacker IP on port 22, you block the brute force attempts at the network perimeter without affecting legitimate traffic from other sources. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of the principle of least privilege and the difference between security groups and network ACLs—a common trap is to immediately terminate the instance or modify the NACL, which would disrupt all traffic or require additional rules. Remember, security groups are stateful and support allow rules only, so you must explicitly deny by removing the existing allow rule for SSH from 0.0.0.0/0 and adding a more specific allow for trusted IPs, or by using a separate deny rule if your VPC design permits. Memory tip: “Block the IP, not the instance” to contain the threat while keeping the service running.
SCS-C02 Threat Detection and Incident Response Practice Question
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. 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 receives an AWS GuardDuty finding for 'UnauthorizedAccess:EC2/SSHBruteForce'. The affected EC2 instance has a public IP and is in a public subnet. What is the IMMEDIATE step to contain the threat?
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 to deny inbound SSH from the attacker's IP address.
Option D is correct because updating the security group to deny inbound SSH from the attacker's IP address immediately blocks the brute force attempts at the instance level without disrupting legitimate traffic from other sources. Security groups are stateful and act as a virtual firewall for EC2 instances, so denying the specific attacker IP for port 22 (SSH) stops the attack while preserving the instance's availability and other services.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 EC2 instance to stop the brute force attempts.
Why it's wrong here
Stopping an instance may change its public IP.
✗
Change the security group name to 'Quarantine' and remove all rules.
Why it's wrong here
Renaming does not change rules.
✗
Modify the network ACL to deny all inbound traffic from the attacker's IP address range.
Why it's wrong here
NACL affects all instances in the subnet and is less granular.
✓
Update the security group to deny inbound SSH from the attacker's IP address.
Why this is correct
Immediately blocks the attacker while allowing other traffic.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse security groups (stateful, instance-level) with network ACLs (stateless, subnet-level) and choose option C, thinking a subnet-wide block is more effective, but it is slower and impacts all instances in the subnet, whereas a security group update is immediate and targeted.
Detailed technical explanation
How to think about this question
Security groups operate at the instance level and are stateful, meaning that if you deny inbound SSH from a specific IP, the return traffic is automatically allowed without needing an explicit outbound rule. In contrast, network ACLs are stateless and require separate inbound and outbound rules for each direction, making them less suitable for rapid, targeted containment. A real-world scenario where this matters is during a coordinated brute force attack from multiple IPs—updating the security group to deny each attacker IP individually preserves the instance's functionality while blocking the threat.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Threat Detection and Incident Response — This question tests Threat Detection and Incident Response — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Update the security group to deny inbound SSH from the attacker's IP address. — Option D is correct because updating the security group to deny inbound SSH from the attacker's IP address immediately blocks the brute force attempts at the instance level without disrupting legitimate traffic from other sources. Security groups are stateful and act as a virtual firewall for EC2 instances, so denying the specific attacker IP for port 22 (SSH) stops the attack while preserving the instance's availability and other services.
What should I do if I get this SCS-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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