SAP-C02 Design for New Solutions Practice Question

Exhibit

Refer to the exhibit.

CloudFormation template snippet:
```yaml
Resources:
  MyEC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: ami-12345678
      InstanceType: t2.micro
      SecurityGroups:
        - !Ref MySecurityGroup
  MySecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Allow SSH
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0
```

A solutions architect attempts to create this stack but receives an error: "Value of property SecurityGroups must be a list of strings". What is the likely cause?

⚠ Common exam trap

Candidates often confuse the SecurityGroups property with SecurityGroupIds, or assume that a single reference can be passed as a scalar, but CloudFormation strictly enforces the list type for SecurityGroups even when only one security group is used.

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 SecurityGroups property should be a list, but the YAML specifies a single reference incorrectly.

The error 'Value of property SecurityGroups must be a list of strings' occurs because the YAML template specifies the SecurityGroups property as a single string (e.g., !Ref MySecurityGroup) instead of a list of strings (e.g., [!Ref MySecurityGroup]). In AWS CloudFormation, the SecurityGroups property for an EC2 instance expects a list of security group IDs or names, even if only one security group is provided. The YAML syntax must wrap the reference in square brackets to form a list, or the template will fail validation.

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 SecurityGroups property should be a list, but the YAML specifies a single reference incorrectly.
Why this is correct
The error occurs when the SecurityGroups property is provided as a single string instead of a list. In CloudFormation, even one security group must be specified as a list, e.g., [!Ref MySecurityGroup].
✗
The security group ingress rule allows SSH from anywhere.
Why it's wrong here
The ingress rule allowing SSH from anywhere is a separate security group configuration, not the cause of the 'list of strings' error.
✗
There is a circular dependency between the EC2 instance and the security group.
Why it's wrong here
A circular dependency would cause a different error, such as a stack update failure, not the SecurityGroups list type error.
✗
The AMI ID is invalid.
Why it's wrong here
An invalid AMI ID would result in an error about the AMI, not about the SecurityGroups property type.

Visual reference

Go deeper

SAP-C02 Design for New Solutions Practice Question

Exhibit

Why each option matters

Option-by-option breakdown

Related to this question