PAS-C01 Operations and Maintenance Practice Question
A company uses AWS Systems Manager Patch Manager to patch a fleet of EC2 instances. After a recent patching operation, some instances failed with the error 'Unable to retrieve patch baseline'. The instances are in a private subnet with a VPC endpoint for SSM. What is the MOST likely cause?
⚠ Common exam trap
Test-takers frequently assume the error is due to missing IAM permissions (Option D) or instance registration (Option C), but the specific error message 'Unable to retrieve patch baseline' is a network connectivity error, not an authorization error, and the VPC endpoint security group misconfiguration is the classic cause in private subnet scenarios.
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 SSM does not have a security group that allows HTTPS outbound to the endpoint
The error 'Unable to retrieve patch baseline' indicates that the EC2 instances cannot communicate with the AWS Systems Manager service to fetch the patch baseline configuration. Since the instances are in a private subnet with a VPC endpoint for SSM, the most likely cause is that the VPC endpoint's security group does not allow HTTPS outbound traffic (port 443) to the endpoint itself. Without this outbound rule, the SSM Agent on the instance cannot establish the TLS connection required to retrieve the patch baseline from the SSM API.
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 VPC endpoint for SSM does not have a security group that allows HTTPS outbound to the endpoint
Why this is correct
Missing outbound HTTPS from the instance to the SSM endpoint prevents communication.
- ✗
The CodeDeploy agent on the instances is outdated
Why it's wrong here
Patch Manager uses SSM Agent, not CodeDeploy Agent.
- ✗
The instances are not registered as managed instances with the SSM ManagedInstance role
Why it's wrong here
If they are already managed, this error is not related to registration.
- ✗
The instances do not have an IAM instance profile attached
Why it's wrong here
If the instance were missing an IAM role, it would not appear in the managed instances list at all.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PAS-C01 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 PAS-C01 exam.