A CloudFormation custom resource backed by a Lambda function that allows access to an encrypted SSM parameter by the parameter name

Once deployed to a lambda function the custom resource can be used in CloudFormation templates to access SSM parameters.

---
AWSTemplateFormatVersion: "2010-09-09"
Description: "Some CloudFormation-Defined System"

Resources:
  # Define the Custom Resource to access SSM parameters
  SecretParameter:
    Type: AWS::CloudFormation::CustomResource
    Properties:
      ServiceToken: !Sub arn:aws:lambda:${AWS::Region}:${AWS::AccountId}:function:SSMParameterAccess
      ParameterName: "MySecretParameterName"

  # Use the parameter value in a resource definition
  AnotherResource:
    Secret: !GetAtt MySecretParameterName.ParameterValue

There is no explicit configuration of the code. The custom resource provides access to any SSM parameter store parameters that are present in the AWS account the resource is deployed to. The IAM policy in the SAM configuration allows ssm:GetParameter access. The resource must also have access to the key used to encrypt parameters. If the default AWS key is used access is in place by default. If another key is used IAM permission must be given to use this key.

The Go binary must be built before deployment with the correct compilation flags for Lambda execution

# Build Go binary for Lambda execution
GOOS=linux go build -ldflags="-s -w"

The Lambda stack is defined in a SAM configuration and can be built and deployed using the standard SAM commands. The S3 bucket that holds the deployment package can be any existing bucket.

# Stack creation
sam package --template-file sam.yaml --s3-bucket ${DeploymentBucketName} --output-template-file sam-pkg.yaml
sam deploy --template-file ./sam-pkg.yaml --stack-name ${StackName} --capabilities CAPABILITY_IAM

# Stack deletion
aws cloudformation delete-stack --stack-name ${StackName}

The code can be tested locally using the standard SAM commands or directly using a lambci docker container. A presigned S3 URL is needed to receive the response

docker run --rm -v "$PWD":/var/task \
    -e AWS_ACCESS_KEY_ID -e AWS_SECRET_ACCESS_KEY \
    -e AWS_SESSION_TOKEN -e AWS_SECURITY_TOKEN \
    -e AWS_REGION=eu-west-1 \
    lambci/lambda:go1.x aws-ssm-parameter-access \
    "{
        \"RequestType\" : \"Create\",
        \"ResponseURL\" : \"https://presignedS3URL/\",
        \"StackId\" : \"arn:aws:cloudformation:us-west-2:123456789012:stack/stack-name/guid\",
        \"RequestId\" : \"unique id for this create request\",
        \"ResourceType\" : \"Custom::TestResource\",
        \"LogicalResourceId\" : \"MyTestResource\",
        \"ResourceProperties\" : {
            \"ParameterName\" : \"ATestParameter\"
        }
    }"