The correct answer is to modify the inbound SSH rule in the security group to source 203.0.113.0/24. This is correct because security groups are stateful and act as a virtual firewall for EC2 instances; to restrict SSH access to EC2 by IP using a security group, you simply update the existing rule’s source CIDR from 0.0.0.0/0 to the company’s specific public IP range, and since security groups evaluate all rules before making a decision, the more specific allowed range takes effect without needing additional rules. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of security group rule modification versus NACL behavior—a common trap is thinking you need to delete the old rule first or add a deny rule, but security groups only support allow rules, so you directly edit the source. Remember the memory tip: “Edit, don’t delete—security groups only say yes, so narrow the source to pass the test.”
SCS-C02 Infrastructure Security 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. 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 has a security group that allows inbound SSH from 0.0.0.0/0. The security team wants to restrict access to only the company's public IP range 203.0.113.0/24. What change should be made?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Modify the inbound SSH rule in the security group to source 203.0.113.0/24.
Option B is correct because security groups are stateful and act as a virtual firewall for instances. To restrict inbound SSH access from 0.0.0.0/0 to only the company's public IP range, you must modify the existing inbound rule's source CIDR from 0.0.0.0/0 to 203.0.113.0/24. This change directly updates the allowed source IP range, and since security groups evaluate all rules before making a decision, the more specific allowed range will take effect without needing additional rules.
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.
✗
Add a network ACL rule to deny SSH from 0.0.0.0/0.
Why it's wrong here
NACL is different; security group still allows all.
✓
Modify the inbound SSH rule in the security group to source 203.0.113.0/24.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the stateless behavior of network ACLs with the stateful behavior of security groups, leading them to incorrectly believe that adding a deny rule in a network ACL can override a security group's allow rule for the same traffic.
Detailed technical explanation
How to think about this question
Security groups are stateful, meaning if you allow inbound SSH from a specific source, the corresponding outbound return traffic is automatically allowed regardless of outbound rules. In contrast, network ACLs are stateless and require explicit rules for both inbound and outbound traffic; this distinction is critical when designing defense-in-depth. A common real-world scenario is when an administrator mistakenly adds a network ACL deny rule thinking it will override a security group allow rule, but because security groups are evaluated before network ACLs for inbound traffic (in a VPC), the security group rule still permits the traffic.
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.
Infrastructure Security — This question tests Infrastructure Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Modify the inbound SSH rule in the security group to source 203.0.113.0/24. — Option B is correct because security groups are stateful and act as a virtual firewall for instances. To restrict inbound SSH access from 0.0.0.0/0 to only the company's public IP range, you must modify the existing inbound rule's source CIDR from 0.0.0.0/0 to 203.0.113.0/24. This change directly updates the allowed source IP range, and since security groups evaluate all rules before making a decision, the more specific allowed range will take effect without needing additional rules.
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.
About these practice questions
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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 company wants to allow its developers to SSH into EC2 instances only from the corporate network IP range (203.0.113.0/24). Which configuration should be used to enforce this restriction?
easy
A.Configure a network ACL on the subnet to allow inbound SSH from the corporate range and deny all other inbound traffic.
B.Use AWS Systems Manager Session Manager to connect to instances instead of SSH.
C.Add an IAM policy that allows `ec2:RunInstances` only if the request includes the corporate IP.
✓ D.Add a security group rule that allows inbound SSH (port 22) from the corporate IP range.
Why D: Option C is correct because a security group rule can restrict inbound SSH to the specific IP range. Option A is wrong because IAM policies do not control network access. Option B is wrong because NACLs are stateless and require separate inbound and outbound rules. Option D is wrong because Systems Manager Session Manager does not use SSH.
Variation 2. A company has a security group rule that allows inbound traffic from 0.0.0.0/0 on port 22. The security engineer wants to restrict SSH access to only the company's public IP range (203.0.113.0/24). What is the correct way to update the security group rule?
easy
A.Remove the existing inbound rule and do not add any new rule; SSH access will be denied by default.
✓ B.Modify the existing inbound rule to change the source from 0.0.0.0/0 to 203.0.113.0/24.
C.Add a new inbound rule with source 203.0.113.0/24 and the security group will automatically deny all other traffic.
D.Change the outbound rules to restrict traffic.
Why B: Option A is correct because you can modify the CIDR of an existing rule. Option B is wrong because security groups don't have a default deny rule; you need to remove the old rule. Option C is wrong because security groups are stateful. Option D is wrong because removing the rule without adding a new one would block all SSH.
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Question Discussion
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