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Question 1,741 of 1,750

Incident and Event Response →hardMultiple ChoiceObjective-mapped

ECS Fargate ECR AccessDenied — VPC Endpoint Private DNS

This DOP-C02 practice question tests your understanding of incident and event response. 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 company runs a containerized microservices application on Amazon ECS with Fargate launch type. The application uses an Application Load Balancer to route traffic to the ECS service. Recently, the DevOps team noticed that the ECS service is failing to deploy new tasks during a rolling update. The CloudWatch Logs for the ECS service show that new tasks are failing to start because they cannot pull the container image from Amazon ECR. The error message indicates 'AccessDenied' when attempting to pull the image. The task execution role has the necessary permissions, and the image URI is correct. The VPC has a VPC endpoint for ECR configured. The security group for the tasks allows outbound traffic to the VPC endpoint. What is the MOST likely cause of the access denied error?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The task execution role does not have the 'ecr:GetAuthorizationToken' permission.
Why wrong: The error is 'AccessDenied' but the team verified the role has necessary permissions; the issue is network-related.
B
The security group for the ALB does not allow inbound traffic to the ECS tasks.
Why wrong: The ALB security group controls traffic to tasks; the error is about pulling images, not about incoming traffic.
C
The VPC endpoint for ECR does not have 'Private DNS names enabled' selected.
When private DNS is not enabled, the DNS resolution for ECR endpoints defaults to public IPs, which may not be reachable from the VPC, causing access denied.
D
The task role does not have the 'ecr:BatchGetImage' permission.
Why wrong: The task role is used by the application, not for pulling images; the task execution role handles image pulls.

Full breakdown with real-world context →

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 VPC endpoint for ECR does not have 'Private DNS names enabled' selected.

The correct answer is C. For ECS tasks using Fargate to pull images from ECR via a VPC endpoint, the private DNS names must be enabled on the endpoint. If not enabled, the task's DNS resolution of the ECR repository URL returns a public IP, which may be blocked by security groups or route tables, causing an 'AccessDenied' error despite correct IAM permissions. Option A is incorrect because 'ecr:GetAuthorizationToken' is needed for authentication, but the error occurs after authentication (the task execution role has permissions). Option B is irrelevant as the ALB security group does not affect image pulling. Option D is incorrect because the task role is for application-level permissions, not for pulling images; that is handled by the task execution role.

Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.

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 task execution role does not have the 'ecr:GetAuthorizationToken' permission.
Why it's wrong here
The error is 'AccessDenied' but the team verified the role has necessary permissions; the issue is network-related.
✗
The security group for the ALB does not allow inbound traffic to the ECS tasks.
Why it's wrong here
The ALB security group controls traffic to tasks; the error is about pulling images, not about incoming traffic.
✓
The VPC endpoint for ECR does not have 'Private DNS names enabled' selected.
Why this is correct
When private DNS is not enabled, the DNS resolution for ECR endpoints defaults to public IPs, which may not be reachable from the VPC, causing access denied.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Authentication checks who the user is.
✗
The task role does not have the 'ecr:BatchGetImage' permission.
Why it's wrong here
The task role is used by the application, not for pulling images; the task execution role handles image pulls.

Common exam traps

Common exam trap: authentication is not authorization

Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.

Detailed technical explanation

How to think about this question

This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.

KKey Concepts to Remember

Authentication checks who the user is.
Authorization controls what the user is allowed to do after login.
Privilege levels affect access to EXEC and configuration commands.
AAA, TACACS+ and RADIUS can separate login success from command access.

TExam Day Tips

Do not assume successful login means full administrative access.
Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
Separate login problems from permission problems before choosing the answer.

Key takeaway

Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.

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.

Visual reference

What to study next

Got this wrong? Here's your next step.

Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related DOP-C02 questions on access control and AAA configuration.

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FAQ

Questions learners often ask

What does this DOP-C02 question test?

Incident and Event Response — This question tests Incident and Event Response — Authentication checks who the user is..

What is the correct answer to this question?

The correct answer is: The VPC endpoint for ECR does not have 'Private DNS names enabled' selected. — The correct answer is C. For ECS tasks using Fargate to pull images from ECR via a VPC endpoint, the private DNS names must be enabled on the endpoint. If not enabled, the task's DNS resolution of the ECR repository URL returns a public IP, which may be blocked by security groups or route tables, causing an 'AccessDenied' error despite correct IAM permissions. Option A is incorrect because 'ecr:GetAuthorizationToken' is needed for authentication, but the error occurs after authentication (the task execution role has permissions). Option B is irrelevant as the ALB security group does not affect image pulling. Option D is incorrect because the task role is for application-level permissions, not for pulling images; that is handled by the task execution role.

What should I do if I get this DOP-C02 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Authentication checks who the user is.

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