SNS topic with subscriptions from external account.

sns_subscribe.json

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Description": "Publish or be damned.",
  "Parameters": {
    "SubscriberPrincipals": {
      "Type": "CommaDelimitedList",
      "Description": "Account IDs to allow subscription from."
    }
  },
  "Resources": {
    "SNSTopic": {
      "Type": "AWS::SNS::Topic",
      "Properties": {}
    },
    "SNSTopicPolicy": {
      "Type": "AWS::SNS::TopicPolicy",
      "Properties": {
        "PolicyDocument": {
          "Version": "2012-10-17",
          "Id": "MyTopicSubscribePolicy",
          "Statement": [
            {
              "Sid": "Allow-other-account-to-subscribe-to-topic",
              "Effect": "Allow",
              "Principal": {
                "AWS": { "Ref" : "SubscriberPrincipals" }
              },
              "Action": "sns:Subscribe",
              "Resource": {
                "Ref": "SNSTopic"
              }
            }
          ]
        },
        "Topics": [
          {
            "Ref": "SNSTopic"
          }
        ]
      }
    }
  },
  "Outputs": {
    "SNSTopicTopicARN": {
      "Value": {
        "Ref": "SNSTopic"
      }
    }
  }
}