Using VS Code extensions to help with AWS CloudFormation templates

Writing CloudFormation templates by hand is time consuming and error prone. Usually I know what is is that I’m trying to create and and know roughly what the options are, but remembering the exact syntax in json or YAML is near impossible.

VS Code has a number of extensions that can make this a lot easier. Tab code complete with plugins like “CloudFormation Snippets’ makes writing new templates incredibly quick and easy:

By default with this extension, type cfn then Tab to auto complete the skeleton of a CloudFormation template. If you have autocomplete on tab turned off you can turn it on in your VS Code settings, or manually use Ctrl-Space to trigger:

After pressing Tab you get an empty template:

As I’m writing a template for a new DynamoDB table, I enter dynamodb-table under Resources and Tab and it adds the skeleton ready to complete:

This saves time in having to look at what the required and optional attributes are, but unless I’m missing a feature it doesn’t have any auto completion to help select values for some attributes where valid values are from a list of options. For example, for DynamoDB BillingMode, the available options are PROVISIONED or PAY_PER_REQUEST. I can go look that up in the docs here, but it would be nice if it would offer tab complete for those too.

AWS CloudFormation example for S3 bucket

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

    Type: AWS::S3::Bucket
      BucketName: s3-bucket-name
        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=

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": [
"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.