Allowing third parties to access your AWS organization

Most modern business use hundreds of SaaS products to get the job done. In organizations that deliver software services themselves, many of these third parties are integrated into the development and operation of the software and that often means they need some amount of access to AWS. Here are several ways to give them access.

The best strategies use IAM roles to avoid the need for long-lived access keys. If available, choose these over strategies that use IAM users.

Special-purpose IAM roles in certain accounts

Simplistic integrations with AWS will require you to create an IAM role in each account the third party needs to access, which is easy enough with substrate role create. The third party should provide you with their account number or the role ARN they’ll be using. Create an assume-role policy like this and commit it someplace in your Substrate repository, replacing the placeholder with the principal your third-party provided:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": ["<third-party-AWS-account-number-or-role-ARN>"]
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

Then use substrate role create to create a role in some or all of your AWS accounts and provide the file you just created as an extra assume-role policy:

substrate role create --role <RoleName> [account selection flags] --assume-role-policy <filename>

Special-purpose IAM role that can assume roles

More advanced integrations with AWS, ones that are aware of AWS Organizations, may be able to use sts:AssumeRole to move around your organization automatically. Take advantage of this by creating two roles. The first one should be created as in the previous section, specifying -admin as the lone account selection flag. The second role should trust the first role. Once again create an assume-role policy like this and commit it someplace in your Substrate repository, replacing the placeholders with your Substrate account number and the first role’s name:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": ["arn:aws:iam::<Substrate-account-number>:role/<RoleName>"]
        ]
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

Then use substrate role create a second time to create a role in some or all of your AWS accounts and provide the file you just created as an extra assume-role policy:

substrate role create --role <AnotherRoleName> [account selection flags] --assume-role-policy <filename>

Reusing the Substrate-managed Auditor role

A great many integrations can be trusted with read-only access (specifically, the kind of read-only access that Substrate manages, without blanket s3:GetObject). In those cases, the substrate.Auditor.assume-role-policy.json file discussed along with auditing your Substrate-managed AWS organization offers a simple solution.

Reusing the Substrate-managed Administrator role

Sometimes you will want to grant third-parties administrative access (e.g. to allow a CI/CD system to terraform apply). In those cases, the substrate.Administrator.assume-role-policy.json file discussed in adding administrators to your AWS organization offers a simple solution.

Creating an IAM user

If no better option exists, you can create an IAM user for a third party. Here, due to the riskier credential management story, we recommend granting absolutely minimal permissions. In the example, we’ll use Circle CI, a service that (at the time of this writing) only supported AWS integration via IAM user.

  1. root-modules/admin/<quality>/global/<circle-ci>.tf: Add an IAM user in your Substrate account; allow it to sts:AssumeRole, constraining which roles it’s allowed to assume as you see fit
  2. root-modules/<domain>/<environment>/<quality>/global/admin-providers.tf: Add a provider referencing your Substrate account with alias = "admin" so that the IAM user in your Substrate account may assume roles in this other account
    • This isn’t strictly necessary — IAM users can of course be created in any AWS account — but it’s more convenient to centralize credential management in your Substrate account so it’s easier to manage multiple domains, environments, and/or qualities
  3. root-modules/<domain>/<environment>/<quality>/global/main.tf: Add aws.admin = aws.admin to the providers map passed to module "<domain>" so that the IAM role in step 6 may reference the IAM user from step 1 (substituting the domain, environment, and quality of the account in question)
  4. modules/<domain>/global/admin-providers.tf: Add provider "aws" { alias = "admin" } to tell Terraform to expect an aws.admin provider
  5. modules/<domain>/global/main.tf: Add a data source that looks up the Circle CI user in the Substrate account using the aws.admin provider so that the IAM role in step 6 may reference the IAM user from step 1 (substituting the domain of the account in question)
  6. modules/<domain>/global/main.tf: Add an IAM role that may be assumed by the Circle CI user in your Substrate account
  7. modules/<domain>/global/main.tf: Attach policies, inline or managed, to allow the service account to perform its duties
  8. substrate setup
  9. substrate account account update --domain <domain> --environment <environment> --quality <quality>
  10. aws iam create-access-key --user-name <circle-ci> and give the resulting access key to Circle CI

Again, this is risky, because you’re about to let this access key out of your control. Be sure you trust the third party. If you’ve established a SOC 2 compliance program (or are even considering it), this third party is now a subprocessor and you should be reviewing their security practices and, ideally, their SOC 2 report.