Quick Answer
The answer is AWS WAF and Network ACLs (NACLs). AWS WAF is a web application firewall that uses web access control lists (ACLs) to inspect HTTP/S traffic and automatically block malicious IP addresses at the application layer of the network perimeter, often integrated with CloudFront or an Application Load Balancer. Network ACLs, on the other hand, are stateless virtual firewalls operating at the subnet level within a VPC, allowing you to create explicit deny rules for specific IP addresses, effectively blocking threats before they reach your instances. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of layered defense—distinguishing between services that operate at the edge (WAF) versus the VPC subnet boundary (NACL). A common trap is selecting Security Groups, but remember they are stateful and cannot explicitly deny traffic; they only allow rules. Memory tip: WAF blocks the web, NACL blocks the network—both deny at the perimeter, but at different layers.
This SCS-C02 practice question tests your understanding of threat detection and incident response. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Which TWO AWS services can be used to automatically block malicious IP addresses at the network perimeter? (Select TWO.)
Amazon Route 53
Why wrong: Route 53 is a DNS service, not a firewall.
Security Groups
Why wrong: Security Groups are allow-only, cannot explicitly deny an IP.
Network ACLs
Network ACLs can deny inbound traffic from specific IPs.
AWS WAF
WAF can block IP addresses via IP match conditions.
AWS Shield Advanced
Why wrong: Shield Advanced provides DDoS protection but does not block specific IPs.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Network ACLs
Network ACLs (NACLs) are stateless virtual firewalls that operate at the subnet level in a VPC. They can be configured with inbound and outbound rules to explicitly deny traffic from specific IP addresses, effectively blocking malicious IPs at the network perimeter before they reach the instances.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Amazon Route 53
Security Groups
Why it's wrong here
Security Groups are allow-only, cannot explicitly deny an IP.
Network ACLs
Why this is correct
Network ACLs can deny inbound traffic from specific IPs.
Related concept
Read the scenario before looking for a memorised answer.
AWS WAF
Why this is correct
WAF can block IP addresses via IP match conditions.
Related concept
Read the scenario before looking for a memorised answer.
AWS Shield Advanced
Why it's wrong here
Shield Advanced provides DDoS protection but does not block specific IPs.
Common exam traps
The trap here is that candidates often confuse Security Groups with Network ACLs, thinking Security Groups can block traffic at the network perimeter, but Security Groups are instance-level and cannot block traffic before it enters the subnet.
Detailed technical explanation
Network ACLs support both allow and deny rules, evaluated in order by rule number (lowest to highest), and are stateless, meaning return traffic must be explicitly allowed. In contrast, AWS WAF is a web application firewall that inspects HTTP/HTTPS requests and can block IPs via IP set rules, operating at the application layer (Layer 7) on CloudFront, ALB, or API Gateway. A real-world scenario: using NACLs to block known attacker IPs at the subnet level while WAF blocks malicious web requests at the application edge provides defense in depth.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Threat Detection and Incident Response — This question tests Threat Detection and Incident Response — Read the scenario before looking for a memorised answer..
The correct answer is: Network ACLs — Network ACLs (NACLs) are stateless virtual firewalls that operate at the subnet level in a VPC. They can be configured with inbound and outbound rules to explicitly deny traffic from specific IP addresses, effectively blocking malicious IPs at the network perimeter before they reach the instances.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SCS-C02 exam.
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