Sceptre and IAM

Inevitably, when working with CloudFormation, IAM permissions are relevant. By default, Sceptre uses the permissions of the user executing Sceptre commands when performing any of its actions. Also, by default, CloudFormation uses the permissions of the user executing its actions when performing actions on individual resources within those stacks.

This means that, by default, if the user executing Sceptre has admin-level permissions, there won’t be any issue executing any actions. But this is often not a viable option. Organizations usually have few users with these permissions. Furthermore, there are legitimate reasons for not wanting to grant users (or CI/CD systems like Jenkins) admin-level permissions.

Of course, admin-level AWS permissions aren’t essential; At the end of the day, Sceptre and CloudFormation actually only need the permissions necessary to perform the required actions for a given project on the resources that will be managed by them. The actual range of permissions required tends to be a much narrower scope than admin-level.

Permissions Configurations

There are three main configurations for Sceptre that can modify default permissions behavior to provide more flexibility, control, and safety within an organization: role_arn, iam_role, and profile. These can be applied in a very targeted way, on a stack by stack basis or can be applied broadly to a whole StackGroup.

role_arn

This is the CloudFormation service role that will be attached to a given CloudFormation stack. This IAM role needs to be able to be assumed by CloudFormation, and must provide all the necessary permissions for all create/read/update/delete operations for all resources defined in that stack. If CloudFormation can assume this role, the user executing Sceptre does not need those permissions, only iam:PassRole permissions on that role to give it to CloudFormation.

There are a few very important things to note about using a CloudFormation service role:

  • You cannot remove a role from a stack once you’ve added it. You’d have to delete and rebuild the stack without it if you wanted to remove it. You may, however, replace one role with another on the stack.

  • Once the role has been added to a stack, it will always be used for all actions; It doesn’t matter who is executing them; those permission cannot be overridden without modifying the role itself or replacing it with another role.

  • You cannot delete a stack with a service role unless that role has permissions to delete those resources. This means that an admin might need to add deletion permissions to that role before that stack can be removed.

Applying a service role to a stack is a very effective way to grant (and limit) the scope of permissions that a stack can utilize, but the rigidity of using it might prove overly burdensome, depending on the use case.

For more information on using CloudFormation service roles, see the AWS documentation.

As a resolvable property, Sceptre allows you to use a resolver to populate the role_arn for a Stack or StackGroup Config. This means you could define that role within your project, output its ARN, and then reference it using !stack_output.

iam_role

This is a role that Sceptre will assume when taking any actions on the Stack. It is not a service role for CloudFormation. Instead, this is simply a role that the current user assumes to execute any Sceptre actions on that stack. This has some benefits over a CloudFormation service role:

  • This is not permanently attached to the Stack after you’ve used it once. If you ever don’t want to assume this role, you could comment it out or remove it from the Stack Config and Sceptre simply won’t use it. This is useful if the user executing Sceptre already has the right permissions to take those actions. In other words, it doesn’t lock you in (unlike using role_arn).

  • CloudFormation can continue to use it’s default behavior of executing Stack actions with the permissions of the current user, but it interprets the current user to hold the indicated iam_role, which could grant additional permissions.

Using the iam_role configuration on a Stack or StackGroup Config allows the user to temporarily “step into” a different set of permissions in order to execute Sceptre actions on Stack(s) in the project without having to permanently hold those permissions.

In order to use an iam_role on a Sceptre Stack Config, that role needs to have an AssumeRolePolicyDocument that allows the current user to assume it and permissions to perform all deployment actions on the stack and all its resources.

As a resolvable property, Sceptre allows you to use a resolver to populate the iam_role for a Stack or StackGroup Config. This means you could define that role within your project, output its ARN, and then reference it using !stack_output.

profile

This is different from role_arn and iam_role, as both of those cause CloudFormation or Sceptre to assume a different role with different permissions than the permissions the current user has.

In contrast, the profile is simply an instruction for the underlying AWS SDK to reference that profile in the user’s local AWS configuration. It indicates which set of credentials to use when operating on a given Stack or StackGroup. There is no call to AWS STS to assume a role temporarily.

Utilizing the profile configuration is identical to setting the AWS_PROFILE environment variable, which has the same effect.

Tips for working with Sceptre, IAM, and a CI/CD system

  • Rather than giving your CI/CD system blanket, admin-level permissions, you can define an IAM role with Sceptre to use for deploying the rest of your infrastructure, outputing its ARN in the template. Then, in the rest of your project’s stacks, you can set the iam_role using !stack_output to get that role’s arn. This will mean your CI/CD system will temporarily “step into” that role when using Sceptre to interact with those specific stacks. It will also establish a dependency on your deployment role stack with every other stack in your project.

  • You can constrain the permissions of that deployment role by using:

    • An AssumeRolePolicyDocument that only allows the CI/CD system to assume it (instead of just anyone in your organization).

    • A PermissionsBoundary that guards sensitive/critical infrastructure and which must be on any roles created/updated by the deployment role. See AWS documentation on permission boundaries for more information.

    • The Path on IAM roles and managed policies to namespace your resources. Since that path is a part of the ARN structure on roles and managed policies, you can constrain the IAM-related permissions of the deployment role to only certain paths, preventing the deployment role from elevating its own permissions or modifying unrelated roles and policies.

    • Using aws:CalledVia and aws:CalledViaFirst conditions matching against "cloudformation.amazonaws.com" to ensure that the deployment role can only execute changes via CloudFormation and not on its own. Note: Some actions are taken by Sceptre directly and not via cloudformation (see the section below on this). Those actions should not have a CalledVia condition applied.

  • If you define your deployment role (and any other related resources) using Sceptre and then reference it on all other stacks using iam_role: !stack_output ..., this means that your CI/CD system will not be able to deploy changes to the deployment role or its resources, but that every deployment will depend on those. This is good! It means that, so long as those resources remain unchanged, automated deployment can proceed without issue. It also means that the scope of powers held by the deployment role needs to be reviewed by and manually deployed by a user with admin-level permissions. But after that manual deployment, your CI/CD system should be empowered to deploy all the other stacks in your project (so long as the deployment role has the full scope of permissions needed to do those deployments).

Basic permissions that Sceptre requires

There are certain permissions that Sceptre requires to perform even its most basic operations. These include:

For Basic operations:

  • cloudformation:CreateStack

  • cloudformation:DeleteStack

  • cloudformation:DescribeStackEvents

  • cloudformation:DescribeStackResource

  • cloudformation:DescribeStackResources

  • cloudformation:DescribeStacks

  • cloudformation:GetStackPolicy

  • cloudformation:GetTemplate

  • cloudformation:GetTemplateSummary

  • cloudformation:ListStackResources

  • cloudformation:ListStacks

  • cloudformation:SetStackPolicy

  • cloudformation:TagResource

  • cloudformation:UntagResource

  • cloudformation:UpdateStack

  • cloudformation:UpdateTerminationProtection

  • cloudformation:ValidateTemplate

If using change sets:

  • cloudformation:CreateChangeSet

  • cloudformation:DeleteChangeSet

  • cloudformation:DescribeChangeSet

  • cloudformation:ExecuteChangeSet

  • cloudformation:ListChangeSets

If using a template bucket:

  • s3:CreateBucket

  • s3:PutObject

If using a cloudformation service role:

  • iam:PassRole