Secure CDK deployments with IAM permission boundaries

The AWS Cloud Development Kit (CDK) accelerates cloud development by allowing developers to use common programming languages when modelling their applications. To take advantage of this speed, developers need to operate in an environment where permissions and security controls don’t slow things down, and in a tightly controlled environment this is not always the case. Of particular concern is the scenario where a developer has permission to create AWS Identity and Access Management (IAM) entities (such as users or roles), as these could have permissions beyond that of the developer who created them, allowing for an escalation of privileges. This approach is typically controlled through the use of permission boundaries for IAM entities, and in this post you will learn how these boundaries can now be applied more effectively to CDK development – allowing developers to stay secure and move fast.

Applying custom permission boundaries to CDK deployments

When the CDK deploys a solution, it assumes a AWS CloudFormation execution role to perform operations on the user’s behalf. This role is created during the bootstrapping phase by the AWS CDK Command Line Interface (CLI). This role should be configured to represent the maximum set of actions that CloudFormation can perform on the developers behalf, while not compromising any compliance or security goals of the organisation. This can become complicated when developers need to create IAM entities (such as IAM users or roles) and assign permissions to them, as those permissions could be escalated beyond their existing access levels. Taking away the ability to create these entities is one way to solve the problem. However, doing this would be a significant impediment to developers, as they would have to ask an administrator to create them every time. This is made more challenging when you consider that security conscious practices will create individual IAM roles for every individual use case, such as each AWS Lambda Function in a stack. Rather than taking this approach, IAM permission boundaries can help in two ways – first, by ensuring that all actions are within the overlap of the users permissions and the boundary, and second by ensuring that any IAM entities that are created also have the same boundary applied. This blocks the path to privilege escalation without restricting the developer’s ability to create IAM identities. With the latest version of the AWS CLI these boundaries can be applied to the execution role automatically when running the bootstrap command, as well as being added to IAM entities that are created in a CDK stack.

To use a permission boundary in the CDK, first create an IAM policy that will act as the boundary. This should define the maximum set of actions that the CDK application will be able to perform on the developer’s behalf, both during deployment and operation. This step would usually be performed by an administrator who is responsible for the security of the account, ensuring that the appropriate boundaries and controls are enforced. Once created, the name of this policy is provided to the bootstrap command. In the example below, an IAM policy called “developer-policy” is used to demonstrate the command.

cdk bootstrap --custom-permissions-boundary developer-policy

Once this command runs, a new bootstrap stack will be created (or an existing stack will be updated) so that the execution role has this boundary applied to it. Next, you can ensure that any IAM entities that are created will have the same boundaries applied to them. This is done by either using a CDK context variable, or the permissionBoundary attribute on those resources. To explain this in some detail, let’s use a real world scenario and step through an example that shows how this feature can be used to restrict developers from using the AWS Config service.

Installing or upgrading the AWS CDK CLI

Before beginning, ensure that you have the latest version of the AWS CDK CLI tool installed. Follow the instructions in the documentation to complete this. You will need version 2.54.0 or higher to make use of this new feature. To check the version you have installed, run the following command.

cdk --version

Creating the policy

First, let’s begin by creating a new IAM policy. Below is a CloudFormation template that creates a permission policy for use in this example. In this case the AWS CLI can deploy it directly, but this could also be done at scale through a mechanism such as CloudFormation Stack Sets. This template has the following policy statements:

1. Allow all actions by default – this allows you to deny the specific actions that you choose. You should carefully consider your approach to allow/deny actions when creating your own policies though.
2. Deny the creation of users or roles unless the “developer-policy” permission boundary is used. Additionally limit the attachment of permissions boundaries on existing entities to only allow “developer-policy” to be used. This prevents the creation or change of an entity that can escalate outside of the policy.
3. Deny the ability to change the policy itself so that a developer can’t modify the boundary they will operate within.
4. Deny the ability to remove the boundary from any user or role
5. Deny any actions against the AWS Config service

Here items 2, 3 and 4 all ensure that the permission boundary works correctly – they are controls that prevent the boundary being removed, tampered with, or bypassed. The real focus of this policy in terms of the example are items 1 and 5 – where you allow everything, except the specific actions that are denied (creating a deny list of actions, rather than an allow list approach).

Resources:
  PermissionsBoundary:
    Type: AWS::IAM::ManagedPolicy
    Properties:
      PolicyDocument:
        Statement:
          # ----- Begin base policy ---------------
          # If permission boundaries do not have an explicit allow
          # then the effect is deny
          - Sid: ExplicitAllowAll
            Action: "*"
            Effect: Allow
            Resource: "*"
          # Default permissions to prevent privilege escalation
          - Sid: DenyAccessIfRequiredPermBoundaryIsNotBeingApplied
            Action:
              - iam:CreateUser
              - iam:CreateRole
              - iam:PutRolePermissionsBoundary
              - iam:PutUserPermissionsBoundary
            Condition:
              StringNotEquals:
                iam:PermissionsBoundary:
                  Fn::Sub: arn:${AWS::Partition}:iam::${AWS::AccountId}:policy/developer-policy
            Effect: Deny
            Resource: "*"
          - Sid: DenyPermBoundaryIAMPolicyAlteration
            Action:
              - iam:CreatePolicyVersion
              - iam:DeletePolicy
              - iam:DeletePolicyVersion
              - iam:SetDefaultPolicyVersion
            Effect: Deny
            Resource:
              Fn::Sub: arn:${AWS::Partition}:iam::${AWS::AccountId}:policy/developer-policy
          - Sid: DenyRemovalOfPermBoundaryFromAnyUserOrRole
            Action: 
              - iam:DeleteUserPermissionsBoundary
              - iam:DeleteRolePermissionsBoundary
            Effect: Deny
            Resource: "*"
          # ----- End base policy ---------------
          # -- Begin Custom Organization Policy --
          - Sid: DenyModifyingConfig
            Effect: Deny
            Action: config:*
            Resource: "*"
          # -- End Custom Organization Policy --
        Version: "2012-10-17"
      Description: "Bootstrap Permission Boundary"
      ManagedPolicyName: developer-policy
      Path: /

Save the above locally as developer-policy.yaml and then you can deploy it with a CloudFormation command in the AWS CLI:

aws cloudformation create-stack --stack-name DeveloperPolicy \
        --template-body file://developer-policy.yaml \
        --capabilities CAPABILITY_NAMED_IAM

Creating a stack to test the policy

To begin, create a new CDK application that you will use to test and observe the behaviour of the permission boundary. Create a new directory with a TypeScript CDK application in it by executing these commands.

mkdir DevUsers && cd DevUsers
cdk init --language typescript

Once this is done, you should also make sure that your account has a CDK bootstrap stack deployed with the cdk bootstrap command – to start with, do not apply a permission boundary to it, you can add that later an observe how it changes the behaviour of your deployment. Because the bootstrap command is not using the --cloudformation-execution-policies argument, it will default to arn:aws:iam::aws:policy/AdministratorAccess which means that CloudFormation will have full access to the account until the boundary is applied.

cdk bootstrap

Once the command has run, create an AWS Config Rule in your application to be sure that this works without issue before the permission boundary is applied. Open the file lib/dev_users-stack.ts and edit its contents to reflect the sample below.

import * as cdk from 'aws-cdk-lib';
import { ManagedRule, ManagedRuleIdentifiers } from 'aws-cdk-lib/aws-config';
import { Construct } from "constructs";

export class DevUsersStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    new ManagedRule(this, 'AccessKeysRotated', {
      configRuleName: 'access-keys-policy',
      identifier: ManagedRuleIdentifiers.ACCESS_KEYS_ROTATED,
      inputParameters: {
        maxAccessKeyAge: 60, // default is 90 days
      },
    });
  }
}

Next you can deploy with the CDK CLI using the cdk deploy command, which will succeed (the output below has been truncated to show a summary of the important elements).

❯ cdk deploy
✨  Synthesis time: 3.05s
✅  DevUsersStack
✨  Deployment time: 23.17s

Stack ARN:
arn:aws:cloudformation:ap-southeast-2:123456789012:stack/DevUsersStack/704a7710-7c11-11ed-b606-06d79634f8d4

✨  Total time: 26.21s

Before you deploy the permission boundary, remove this stack again with the cdk destroy command.

❯ cdk destroy
Are you sure you want to delete: DevUsersStack (y/n)? y
DevUsersStack: destroying... [1/1]
✅ DevUsersStack: destroyed

Using a permission boundary with the CDK test application

Now apply the permission boundary that you created above and observe the impact it has on the same deployment. To update your booststrap with the permission boundary, re-run the cdk bootstrap command with the new custom-permissions-boundary parameter.

cdk bootstrap --custom-permissions-boundary developer-policy

After this command executes, the CloudFormation execution role will be updated to use that policy as a permission boundary, which based on the deny rule for config:* will cause this same application deployment to fail. Run cdk deploy again to confirm this and observe the error message.

❌ Deployment failed: Error: Stack Deployments Failed: Error: The stack
named DevUsersStack failed creation, it may need to be manually deleted 
from the AWS console: 
  ROLLBACK_COMPLETE: 
    User: arn:aws:sts::123456789012:assumed-role/cdk-hnb659fds-cfn-exec-role-123456789012-ap-southeast-2/AWSCloudFormation
    is not authorized to perform: config:PutConfigRule on resource: access-keys-policy with an explicit deny in a
    permissions boundary

This shows you that the action was denied specifically due to the use of a permissions boundary, which is what was expected.

Applying permission boundaries to IAM entities automatically

Next let’s explore how the permission boundary can be extended to IAM entities that are created by a CDK application. The concern here is that a developer who is creating a new IAM entity could assign it more permissions than they have themselves – the permission boundary manages this by ensuring that entities can only be created that also have the boundary attached. You can validate this by modifying the stack to deploy a Lambda function that uses a role that doesn’t include the boundary. Open the file lib/dev_users-stack.ts again and edit its contents to reflect the sample below.

import * as cdk from 'aws-cdk-lib';
import { PolicyStatement } from "aws-cdk-lib/aws-iam";
import {
  AwsCustomResource,
  AwsCustomResourcePolicy,
  PhysicalResourceId,
} from "aws-cdk-lib/custom-resources";
import { Construct } from "constructs";

export class DevUsersStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    new AwsCustomResource(this, "Resource", {
      onUpdate: {
        service: "ConfigService",
        action: "putConfigRule",
        parameters: {
          ConfigRule: {
            ConfigRuleName: "SampleRule",
            Source: {
              Owner: "AWS",
              SourceIdentifier: "ACCESS_KEYS_ROTATED",
            },
            InputParameters: '{"maxAccessKeyAge":"60"}',
          },
        },
        physicalResourceId: PhysicalResourceId.of("SampleConfigRule"),
      },
      policy: AwsCustomResourcePolicy.fromStatements([
        new PolicyStatement({
          actions: ["config:*"],
          resources: ["*"],
        }),
      ]),
    });
  }
}

Here the AwsCustomResource is used to provision a Lambda function that will attempt to create a new config rule. This is the same result as the previous stack but in this case the creation of the rule is done by a new IAM role that is created by the CDK construct for you. Attempting to deploy this will result in a failure – run cdk deploy to observe this.

❌ Deployment failed: Error: Stack Deployments Failed: Error: The stack named 
DevUsersStack failed creation, it may need to be manually deleted from the AWS 
console: 
  ROLLBACK_COMPLETE: 
    API: iam:CreateRole User: arn:aws:sts::123456789012:assumed-
role/cdk-hnb659fds-cfn-exec-role-123456789012-ap-southeast-2/AWSCloudFormation
    is not authorized to perform: iam:CreateRole on resource:
arn:aws:iam::123456789012:role/DevUsersStack-
AWS679f53fac002430cb0da5b7982bd2287S-1EAD7M62914OZ
    with an explicit deny in a permissions boundary

The error message here details that the stack was unable to deploy because the call to iam:CreateRole failed because the boundary wasn’t applied. The CDK now offers a straightforward way to set a default permission boundary on all IAM entities that are created, via the CDK context variable core:permissionsBoundary in the cdk.json file.

{
  "context": {
     "@aws-cdk/core:permissionsBoundary": {
       "name": "developer-policy"
     }
  }
}

This approach is useful because now you can import constructs that create IAM entities (such as those found on Construct Hub or out of the box constructs that create default IAM roles) and have the boundary apply to them as well. There are alternative ways to achieve this, such as setting a boundary on specific roles, which can be used in scenarios where this approach does not fit. Make the change to your cdk.json file and run the CDK deploy again. This time the custom resource will attempt to create the config rule using its IAM role instead of the CloudFormation execution role. It is expected that the boundary will also protect this Lambda function in the same way – run cdk deploy again to confirm this. Note that the deployment updates from CloudFormation show that this time the role creation succeeds this time, and a new error message is generated.

❌ Deployment failed: Error: Stack Deployments Failed: Error: The stack named
DevUsersStack failed creation, it may need to be manually deleted from the AWS 
console:
  ROLLBACK_COMPLETE: 
    Received response status [FAILED] from custom resource. Message returned: User:
    arn:aws:sts::123456789012:assumed-role/DevUsersStack-
AWS679f53fac002430cb0da5b7982bd2287S-84VFVA7OGC9N/DevUsersStack-
AWS679f53fac002430cb0da5b7982bd22872-MBnArBmaaLJp
    is not authorized to perform: config:PutConfigRule on resource: SampleRule with an explicit deny in a permissions boundary

In this error message you can see that the user it refers to is DevUsersStack-AWS679f53fac002430cb0da5b7982bd2287S-84VFVA7OGC9N rather than the CloudFormation execution role. This is the role being used by the custom Lambda function resource, and when it attempts to create the Config rule it is rejected because of the permissions boundary in the same way. Here you can see how the boundary is being applied consistently to all IAM entities that are created in your CDK app, which ensures the administrative controls can be applied consistently to everything a developer does with a minimal amount of overhead.

Cleanup

At this point you can either choose to remove the CDK bootstrap stack if you no longer require it, or remove the permission boundary from the stack. To remove it, delete the CDKToolkit stack from CloudFormation with this AWS CLI command.

aws cloudformation delete-stack --stack-name CDKToolkit

If you want to keep the bootstrap stack, you can remove the boundary by following these steps:

1. Browse to the CloudFormation page in the AWS console, and select the CDKToolit stack.
2. Select the ‘Update’ button. Choose “Use Current Template” and then press ‘Next’
3. On the parameters page, find the value InputPermissionsBoundary which will have developer-policy as the value, and delete the text in this input to leave it blank. Press ‘Next’ and the on the following page, press ‘Next’ again
4. On the final page, scroll to the bottom and check the box acknowledging that CloudFormation might create IAM resources with custom names, and choose ‘Submit’

With the permission boundary no longer being used, you can now remove the stack that created it as the final step.

aws cloudformation delete-stack --stack-name DeveloperPolicy

Conclusion

Now you can see how IAM permission boundaries can easily be integrated in to CDK development, helping ensure developers have the control they need while administrators can ensure that security is managed in a way that meets the needs of the organisation as well.

With this being understood, there are next steps you can take to further expand on the use of permission boundaries. The CDK Security and Safety Developer Guide document on GitHub outlines these approaches, as well as ways to think about your approach to permissions on deployment. It’s recommended that developers and administrators review this, and work to develop and appropriate approach to permission policies that suit your security goals.

Additionally, the permission boundary concept can be applied in a multi-account model where each Stage has a unique boundary name applied. This can allow for scenarios where a lower-level environment (such as a development or beta environment) has more relaxed permission boundaries that suit troubleshooting and other developer specific actions, but then the higher level environments (such as gamma or production) could have the more restricted permission boundaries to ensure that security risks are more appropriately managed. The mechanism for implement this is defined in the security and safety developer guide also.

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