AWS CloudFormation example for S3 bucket

Typical Cloudformation for an S3 bucket with block all public access enabled:

Resources:
  S3BucketExample:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: s3-bucket-name
      PublicAccessBlockConfiguration:
        BlockPublicAcls: true
        BlockPublicPolicy: true
        IgnorePublicAcls: true
        RestrictPublicBuckets: true

Using Serverless framework and AWS sts:assume-role to cross deploy to different AWS accounts

In order to assume a role in another account, the owning account needs to grant a ‘trust relationship’ to those allowed to assume the role. This can be done by referencing an IAM username or role for those in the other account that are allowed to assume this role.

You can do this in the Console using the Trust Relationship tab:

A Policy to grant access to to a specific IAM user looks like:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::ACCOUNT-ID:user/USER-ID"
      },
    "Action": "sts:AssumeRole"
    }
  ]
}

To assume this role, use the ‘aws sts assume-role’ cli command:

aws sts assume-role --role-arn arn:aws:iam::ACCOUNT-ID:role/ROLE-NAME --role-session-name SESSION-NAME

If this is successful, you’ll see a response that grants temporary values for the following AWS credentials that can be used after this point:

  • AccessKeyId
  • SecretAccessKey
  • SessionToken

The returned values can be used to set env vars to use with the CLI and other AWS SDK apps:

  • export AWS_ACCESS_KEY_ID=
  • export AWS_SECRET_ACCESS_KEY=
  • export AWS_SESSION_TOKEN=
  • export AWS_DEFAULT_REGION=

For Servlerless to deploy into another account, if you attempt a Serverless deploy at this point, you’ll see errors like:

User: arn:aws:sts::ACCOUNT-ID:assumed-role/ServerlessLambdaDeployRole/lambdadeploy is not authorized to perform: cloudformation:CreateStack on resource: arn:aws:cloudformation:us-east-1:TARGET-ACCOUNT-ID:stack/deploy-demo/*

In this case cloudformation:CreateStack is missing from the assumed role. If you incrementally attempt to find what additional permissions you’ll need to deploy, you’ll also need to add:

  • cloudformation:DescribeStackEvents
  • cloudformation:DescribeStackResource
  • cloudformation:ValidateTemplate
  • cloudformation:UpdateStack
  • cloudformation:DeleteStack
  • apigateway:POST
  • iam:CreateRole
  • iam:PutRolePolicy

ValidateTemplate appears to throw an error unless the Resource is for a wildcard of ‘*’ and not anything more specific, otherwise you’ll see this error:

Error: The CloudFormation template is invalid: User: arn:aws:sts::ACOUNT-ID:assumed-role/ServerlessLambdaDeployRole/lambdadeploy is not authorized to perform: cloudformation:ValidateTemplate

To grant permissions for ValidateTemplate specify a Resource of “*”

{
"Sid": "CreateCloudFormationStackValidate",
"Effect": "Allow",
"Action": [
"cloudformation:ValidateTemplate"
],
"Resource": "*"
}

The STS temporary credentials will expire after 1 hour, so if you see this error:

An error occurred (ExpiredToken) when calling the AssumeRole operation: The security token included in the request is expired

then you’ll need to rerun the ‘aws sts assume-role’ command again. If you previously set the session token in AWS_SESSION_TOKEN, you’ll need to set it back to blank (along with AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY) before you run the command again. When you get the refreshed values, remember to set the env vars with the updated values.

At this point, if you’ve run ‘aws sts assume-role’ and you’ve set the env vars for the returned temp credentials, you’ll be able to run a ‘serverless deploy’ and deploy into the other account where you’ve assumed this new role with the permissions to deploy. This should include permissions for creating a new Lambda and API Gateway, if you’re deploying anything else from your serverless config, you’ll need to add those permissions to the role you’re assuming.