The correct answer is to configure the web server security group with an inbound HTTPS rule from 0.0.0.0/0 and the database security group with an inbound MySQL rule referencing the web server security group as the source. This is the best practice for security groups in a multi-tier architecture because it enforces the principle of least privilege: the web tier is publicly accessible only on port 443, while the database tier is completely isolated from the internet, accepting traffic solely from the specific security group attached to the web servers. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of security group chaining versus CIDR-based rules, a common trap where candidates mistakenly use the web server’s IP range instead of its security group ID, which is less flexible and breaks if instances are replaced. Remember the memory tip: “Web opens to the world on 443; DB only trusts the web SG.”
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 is designing a security group for a web application that must receive HTTPS traffic from the internet and send traffic to a backend database. The backend database is an Amazon RDS MySQL instance. What is the best practice for configuring the security groups?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Web server SG: inbound HTTPS from 0.0.0.0/0. Database SG: inbound MySQL from web server SG.
Option D is correct because the web server security group should allow inbound HTTPS from 0.0.0.0/0, and the database security group should allow inbound MySQL traffic only from the web server security group. Option A is wrong because using the CIDR of the web server is less flexible. Option B is wrong because allowing all traffic is insecure. Option C is wrong because the web server should not allow inbound MySQL.
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.
✓
Web server SG: inbound HTTPS from 0.0.0.0/0. Database SG: inbound MySQL from web server SG.
Why this is correct
Security group references are best practice for inter-tier communication.
Clue confirmation
The clue word "best" in the question point toward this answer.
Web server SG: inbound HTTPS from 0.0.0.0/0, outbound to database SG on port 3306. Database SG: inbound MySQL from web server CIDR block.
Why it's wrong here
Using CIDR is less dynamic than security group references.
✗
Web server SG: inbound HTTPS from 0.0.0.0/0, outbound all traffic. Database SG: inbound MySQL from 0.0.0.0/0.
Why it's wrong here
Allowing all outbound and inbound MySQL from anywhere is insecure.
✗
Web server SG: inbound HTTPS from 0.0.0.0/0, inbound MySQL from database SG. Database SG: outbound MySQL to web server SG.
Why it's wrong here
Web server should not receive MySQL traffic.
Common exam traps
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: Web server SG: inbound HTTPS from 0.0.0.0/0. Database SG: inbound MySQL from web server SG. — Option D is correct because the web server security group should allow inbound HTTPS from 0.0.0.0/0, and the database security group should allow inbound MySQL traffic only from the web server security group. Option A is wrong because using the CIDR of the web server is less flexible. Option B is wrong because allowing all traffic is insecure. Option C is wrong because the web server should not allow inbound MySQL.
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.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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