- A
Add a security group rule to deny inbound traffic on port 22 from 0.0.0.0/0.
Why wrong: Security groups do not support deny rules; they only allow.
- B
Add a network ACL rule to deny inbound traffic on port 22 from 0.0.0.0/0 at the subnet level.
Network ACLs can deny traffic and are applied at the subnet boundary.
- C
Add a network ACL rule to allow inbound traffic on port 22 from 0.0.0.0/0 and then add a deny rule for the same traffic.
Why wrong: NACL rules are evaluated in order; a deny rule would work but allowing first is unnecessary and could be confusing.
- D
Add a security group rule to block inbound traffic on port 22 from 0.0.0.0/0 at the VPC level.
Why wrong: Security groups are not applied at the VPC level; they are attached to resources.
Question 451 of 1,748
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 needs to block SSH traffic (port 22) from the internet to all EC2 instances in a VPC. Which approach is the most secure and scalable?
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 inbound traffic on port 22 from 0.0.0.0/0 at the subnet level.
Option B is correct because network ACLs (NACLs) are stateless and operate at the subnet level, allowing you to explicitly deny inbound SSH traffic from 0.0.0.0/0. This approach is more secure and scalable than security group rules because NACLs can block traffic before it reaches the instance, and they support explicit deny rules, which security groups do not. Security groups only support allow rules, so you cannot add a deny rule to block SSH traffic; you must omit the allow rule, which is less explicit and can be accidentally overridden.
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 security group rule to deny inbound traffic on port 22 from 0.0.0.0/0.
Why it's wrong here
Security groups do not support deny rules; they only allow.
- ✓
Add a network ACL rule to deny inbound traffic on port 22 from 0.0.0.0/0 at the subnet level.
Why this is correct
Network ACLs can deny traffic and are applied at the subnet boundary.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Add a network ACL rule to allow inbound traffic on port 22 from 0.0.0.0/0 and then add a deny rule for the same traffic.
Why it's wrong here
NACL rules are evaluated in order; a deny rule would work but allowing first is unnecessary and could be confusing.
- ✗
Add a security group rule to block inbound traffic on port 22 from 0.0.0.0/0 at the VPC level.
Why it's wrong here
Security groups are not applied at the VPC level; they are attached to resources.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse security groups with network ACLs, assuming security groups can have explicit deny rules, when in fact only NACLs support deny rules and operate at the subnet level.
Trap categories for this question
Similar concept trap
NACL rules are evaluated in order; a deny rule would work but allowing first is unnecessary and could be confusing.
Detailed technical explanation
How to think about this question
Network ACLs are stateless, meaning they evaluate inbound and outbound rules independently, and they process rules in ascending order by rule number, stopping at the first matching rule. This allows you to place a deny rule with a lower rule number (e.g., 100) to explicitly block SSH traffic before any allow rules (e.g., 200) are evaluated. In contrast, security groups are stateful and only support allow rules, so to block SSH you must simply not include an allow rule for port 22, but this can be accidentally overridden if a broader allow rule (e.g., all traffic from a specific CIDR) is added later.
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 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
What to study next
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
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: Add a network ACL rule to deny inbound traffic on port 22 from 0.0.0.0/0 at the subnet level. — Option B is correct because network ACLs (NACLs) are stateless and operate at the subnet level, allowing you to explicitly deny inbound SSH traffic from 0.0.0.0/0. This approach is more secure and scalable than security group rules because NACLs can block traffic before it reaches the instance, and they support explicit deny rules, which security groups do not. Security groups only support allow rules, so you cannot add a deny rule to block SSH traffic; you must omit the allow rule, which is less explicit and can be accidentally overridden.
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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Last reviewed: Jul 4, 2026
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