How do I pass values between nested stacks within the same parent stack in AWS CloudFormation?

Resolution

The following resolution uses the AWS::CloudFormation::Stack resources NestedStackA and NestedStackB that are part of the same parent stack that's named RootStack. You're passing a value from NestedStackA to NestedStackB. NestedStackA created the S3 bucket resource, and NestedStackB created the S3 bucket policy that's attached to the S3 bucket.

Complete the following steps:

1. In the Outputs section of your CloudFormation template for NestedStackA, include the value that you want to share.
   JSON:

   {
       "Resources": {
           "S3Bucket": {
               "Type": "AWS::S3::Bucket",
               "DeletionPolicy": "Retain",
               "Properties": {
                   "BucketName": "DOC-EXAMPLE-BUCKET"
               }
           }
       }, 
       "Outputs": {
           "BucketNameOutput": {
               "Value": { "Ref" : "S3Bucket" },
               "Description": "You can refer to any resource from the template."
           }
       }
   }

   YAML:

   Resources:
     S3Bucket:
       Type: 'AWS::S3::Bucket'
       DeletionPolicy: Retain
       Properties:
         BucketName: DOC-EXAMPLE-BUCKET
   Outputs:
     BucketNameOutput:
       Value: !Ref S3Bucket
       Description: You can refer to any resource from the template.

   Note: In the preceding templates, replace DOC-EXAMPLE-BUCKET with your bucket name. The Outputs section of the preceding templates returns the bucket name from !Ref.

2. In the Parameters section of the CloudFormation template for NestedStackB, include a parameter to use the S3 bucket name from the output of NestedStackA.
   JSON:

   {
       "Parameters": {
           "BucketNameValueParameter": {
               "Type": "String",
               "Description": "The shared bucket name value from nestedStackA that will be passed to this parameter from the parent stack."
           }
       },
       "Resources": {
           "SampleBucketPolicy": {
               "Type": "AWS::S3::BucketPolicy",
               "Properties": {
                   "Bucket": {
                       "Ref": "BucketNameValueParameter"
                   },
                   "PolicyDocument": {
                       "Version": "2012-10-17",
                       "Statement": [
                           {
                               "Action": [
                                   "s3:GetObject"
                               ],
                               "Effect": "Allow",
                               "Resource": {
                                   "Fn::Join": [
                                       "",
                                       [
                                           "arn:aws:s3:::",
                                           {
                                               "Ref": "DOC-EXAMPLE-BUCKET"
                                           },
                                           "/*"
                                       ]
                                   ]
                               },
                               "Principal": "*",
                               "Condition": {
                                   "StringLike": {
                                       "aws:Referer": [
                                           "http://www.example.com/*",
                                           "http://example.net/*"
                                       ]
                                   }
                               }
                           }
                       ]
                   }
               }
           }
       }
   }
   

   YAML:

   Parameters:
     BucketNameValueParameter:
       Type: String
       Description: >-
         The shared bucket name value from nestedStackA that will be passed to this
         parameter from the parent stack.
   Resources:
     SampleBucketPolicy:
       Type: 'AWS::S3::BucketPolicy'
       Properties:
         Bucket: !Ref BucketNameValueParameter
         PolicyDocument:
           Version: 2012-10-17
           Statement:
             - Action:
                 - 's3:GetObject'
               Effect: Allow
               Resource: !Join 
                 - ''
                 - - 'arn:aws:s3:::'
                   - !Ref DOC-EXAMPLE-BUCKET
                   - /*
               Principal: '*'
               Condition:
                 StringLike:
                   'aws:Referer':
                     - 'http://www.example.com/*'
                     - 'http://example.net/*'

3. To pass a value between NestedStackA and NestedStackB, configure RootStack to use the Fn::GetAtt function in the Parameter section for NestedStackB. Use the logical ID of NestedStackA and the bucket name output value in the Outputs.NestedStackOutputName format.
   JSON:

   {
     "AWSTemplateFormatVersion" : "2010-09-09",
     "Resources" : {
       "NestedStackA" : {
         "Type" : "AWS::CloudFormation::Stack",
         "Properties" : {
           "TemplateURL" : "https://s3.amazonaws.com/<pathway to .template for NestedStack A>"
         }
       }, 
       "NestedStackB" : {
         "Type" : "AWS::CloudFormation::Stack",
         "Properties" : {
           "TemplateURL" : "https://s3.amazonaws.com/<pathway to .template for NestedStack B>",
           "Parameters" : {
             "BucketNameValueParameter" : {
               "Fn::GetAtt": [
                 "NestedStackA",
                 "Outputs.BucketNameOutput"
               ]
             }
           }
         }
       }
     }
   }

   YAML:

   AWSTemplateFormatVersion: 2010-09-09
   Resources:
     NestedStackA:
       Type: 'AWS::CloudFormation::Stack'
       Properties:
         TemplateURL: 'https://s3.amazonaws.com/<pathway to .template for NestedStack A>'
     NestedStackB:
       Type: 'AWS::CloudFormation::Stack'
       Properties:
         TemplateURL: 'https://s3.amazonaws.com/<pathway to .template for NestedStack B>'
         Parameters:
           BucketNameValueParameter: !GetAtt 
             - NestedStackA
             - Outputs.BucketNameOutput

Related information

Embed stacks within other stacks using nested stacks

AWS CloudFormation template snippets