CV0-004 Troubleshooting Practice Question
A company has a three-tier application in a cloud VPC: web servers in a public subnet, application servers in a private subnet, and database servers in a private subnet. The web servers can connect to the application servers, but the application servers cannot connect to the database servers. The security groups are configured as follows: - Web SG: inbound HTTP from 0.0.0.0/0, outbound all - App SG: inbound HTTP from Web SG, outbound all - DB SG: inbound MySQL from App SG, outbound all What is the most likely cause of the connectivity issue?
⚠ Common exam trap
The trap is focusing only on security groups because they are commonly misconfigured; the exam tests whether you remember that NACLs are stateless and can block traffic even when security groups are correct.
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
✓
The network access control list (NACL) on the database subnet is blocking inbound traffic from the application subnet.
The most likely cause is a network ACL (NACL) on the database subnet blocking inbound traffic from the application subnet. Security groups are stateful and allow return traffic, but NACLs are stateless and must explicitly allow both inbound and outbound traffic on ephemeral ports. If the NACL denies inbound MySQL (port 3306) from the app subnet's CIDR, the connection fails even though the security groups are correctly configured.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The database security group is missing an inbound rule for MySQL.
Why it's wrong here
The database security group already contains an inbound MySQL rule referencing the application security group, so this rule is present. It is tempting because a missing inbound rule is the usual cause of blocked database access, but the stem explicitly lists that rule as configured.
- ✗
The application security group is missing an outbound rule for MySQL.
Why it's wrong here
The application security group already has an outbound rule permitting all traffic, so no MySQL-specific egress rule is missing. It is tempting because outbound filtering is a common cause of connectivity failures, but here the outbound rule is permissive and the fault lies on the database side.
- ✓
The network access control list (NACL) on the database subnet is blocking inbound traffic from the application subnet.
Why this is correct
Security groups are stateful and already permit MySQL from the App SG, so the block must come from the stateless layer: the database subnet's NACL. Its inbound rules are evaluated independently of outbound, and a missing allow for the application subnet's CIDR on port 3306 drops the traffic.
- ✗
The web security group is blocking traffic to the database.
Why it's wrong here
The web security group's outbound rule permits all traffic, and web servers never initiate connections to the database tier in this architecture. It is tempting because the web tier sits in the public subnet, but security groups are stateful and the failing path is app-to-database, not web-to-database.
Visual reference
Go deeper
Related to this question
About these practice questions
This CV0-004 question is part of Courseiva's 834-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
This CV0-004 practice question is part of Courseiva's free CompTIA 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 CV0-004 exam.