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CDK routines for easily assigning correct and minimal IAM permissions

Project description

AWS Identity and Access Management Construct Library

---

cfn-resources: Stable

cdk-constructs: Stable


Define a role and add permissions to it. This will automatically create and attach an IAM policy to the role:

# Example automatically generated. See https://github.com/aws/jsii/issues/826
role = Role(self, "MyRole",
    assumed_by=ServicePrincipal("sns.amazonaws.com")
)

role.add_to_policy(PolicyStatement(
    resources=["*"],
    actions=["lambda:InvokeFunction"]
))

Define a policy and attach it to groups, users and roles. Note that it is possible to attach the policy either by calling xxx.attachInlinePolicy(policy) or policy.attachToXxx(xxx).

# Example automatically generated. See https://github.com/aws/jsii/issues/826
user = User(self, "MyUser", password=cdk.SecretValue.plain_text("1234"))
group = Group(self, "MyGroup")

policy = Policy(self, "MyPolicy")
policy.attach_to_user(user)
group.attach_inline_policy(policy)

Managed policies can be attached using xxx.addManagedPolicy(ManagedPolicy.fromAwsManagedPolicyName(policyName)):

# Example automatically generated. See https://github.com/aws/jsii/issues/826
group = Group(self, "MyGroup")
group.add_managed_policy(ManagedPolicy.from_aws_managed_policy_name("AdministratorAccess"))

Granting permissions to resources

Many of the AWS CDK resources have grant* methods that allow you to grant other resources access to that resource. As an example, the following code gives a Lambda function write permissions (Put, Update, Delete) to a DynamoDB table.

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
fn = lambda_.Function(...)
table = dynamodb.Table(...)

table.grant_write_data(fn)

The more generic grant method allows you to give specific permissions to a resource:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
fn = lambda_.Function(...)
table = dynamodb.Table(...)

table.grant(fn, "dynamodb:PutItem")

The grant* methods accept an IGrantable object. This interface is implemented by IAM principlal resources (groups, users and roles) and resources that assume a role such as a Lambda function, EC2 instance or a Codebuild project.

You can find which grant* methods exist for a resource in the AWS CDK API Reference.

Roles

Many AWS resources require Roles to operate. These Roles define the AWS API calls an instance or other AWS service is allowed to make.

Creating Roles and populating them with the right permissions Statements is a necessary but tedious part of setting up AWS infrastructure. In order to help you focus on your business logic, CDK will take care of creating roles and populating them with least-privilege permissions automatically.

All constructs that require Roles will create one for you if don't specify one at construction time. Permissions will be added to that role automatically if you associate the construct with other constructs from the AWS Construct Library (for example, if you tell an AWS CodePipeline to trigger an AWS Lambda Function, the Pipeline's Role will automatically get lambda:InvokeFunction permissions on that particular Lambda Function), or if you explicitly grant permissions using grant functions (see the previous section).

Opting out of automatic permissions management

You may prefer to manage a Role's permissions yourself instead of having the CDK automatically manage them for you. This may happen in one of the following cases:

  • You don't like the permissions that CDK automatically generates and want to substitute your own set.
  • The least-permissions policy that the CDK generates is becoming too big for IAM to store, and you need to add some wildcards to keep the policy size down.

To prevent constructs from updating your Role's policy, pass the object returned by myRole.withoutPolicyUpdates() instead of myRole itself.

For example, to have an AWS CodePipeline not automatically add the required permissions to trigger the expected targets, do the following:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
role = iam.Role(self, "Role",
    assumed_by=iam.ServicePrincipal("codepipeline.amazonaws.com"),
    # custom description if desired
    description="This is a custom role..."
)

codepipeline.Pipeline(self, "Pipeline",
    # Give the Pipeline an immutable view of the Role
    role=role.without_policy_updates()
)

# You now have to manage the Role policies yourself
role.add_to_policy(iam.PolicyStatement(
    action=[],
    resource=[]
))

Using existing roles

If there are Roles in your account that have already been created which you would like to use in your CDK application, you can use Role.fromRoleArn to import them, as follows:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
role = iam.Role.from_role_arn(self, "Role", "arn:aws:iam::123456789012:role/MyExistingRole",
    # Set 'mutable' to 'false' to use the role as-is and prevent adding new
    # policies to it. The default is 'true', which means the role may be
    # modified as part of the deployment.
    mutable=False
)

Configuring an ExternalId

If you need to create Roles that will be assumed by third parties, it is generally a good idea to require an ExternalId to assume them. Configuring an ExternalId works like this:

# Example automatically generated. See https://github.com/aws/jsii/issues/826
role = iam.Role(self, "MyRole",
    assumed_by=iam.AccountPrincipal("123456789012"),
    external_ids=["SUPPLY-ME"]
)

Principals vs Identities

When we say Principal, we mean an entity you grant permissions to. This entity can be an AWS Service, a Role, or something more abstract such as "all users in this account" or even "all users in this organization". An Identity is an IAM representing a single IAM entity that can have a policy attached, one of Role, User, or Group.

IAM Principals

When defining policy statements as part of an AssumeRole policy or as part of a resource policy, statements would usually refer to a specific IAM principal under Principal.

IAM principals are modeled as classes that derive from the iam.PolicyPrincipal abstract class. Principal objects include principal type (string) and value (array of string), optional set of conditions and the action that this principal requires when it is used in an assume role policy document.

To add a principal to a policy statement you can either use the abstract statement.addPrincipal, one of the concrete addXxxPrincipal methods:

  • addAwsPrincipal, addArnPrincipal or new ArnPrincipal(arn) for { "AWS": arn }
  • addAwsAccountPrincipal or new AccountPrincipal(accountId) for { "AWS": account-arn }
  • addServicePrincipal or new ServicePrincipal(service) for { "Service": service }
  • addAccountRootPrincipal or new AccountRootPrincipal() for { "AWS": { "Ref: "AWS::AccountId" } }
  • addCanonicalUserPrincipal or new CanonicalUserPrincipal(id) for { "CanonicalUser": id }
  • addFederatedPrincipal or new FederatedPrincipal(federated, conditions, assumeAction) for { "Federated": arn } and a set of optional conditions and the assume role action to use.
  • addAnyPrincipal or new AnyPrincipal for { "AWS": "*" }

If multiple principals are added to the policy statement, they will be merged together:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
statement = PolicyStatement()
statement.add_service_principal("cloudwatch.amazonaws.com")
statement.add_service_principal("ec2.amazonaws.com")
statement.add_arn_principal("arn:aws:boom:boom")

Will result in:

{
  "Principal": {
    "Service": [ "cloudwatch.amazonaws.com", "ec2.amazonaws.com" ],
    "AWS": "arn:aws:boom:boom"
  }
}

The CompositePrincipal class can also be used to define complex principals, for example:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
role = iam.Role(self, "MyRole",
    assumed_by=iam.CompositePrincipal(
        iam.ServicePrincipal("ec2.amazonaws.com"),
        iam.AccountPrincipal("1818188181818187272"))
)

The PrincipalWithConditions class can be used to add conditions to a principal, especially those that don't take a conditions parameter in their constructor. The principal.withConditions() method can be used to create a PrincipalWithConditions from an existing principal, for example:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
principal = iam.AccountPrincipal("123456789000").with_conditions(StringEquals={"foo": "baz"})

NOTE: If you need to define an IAM condition that uses a token (such as a deploy-time attribute of another resource) in a JSON map key, use CfnJson to render this condition. See this test for an example.

The WebIdentityPrincipal class can be used as a principal for web identities like Cognito, Amazon, Google or Facebook, for example:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
principal = iam.WebIdentityPrincipal("cognito-identity.amazonaws.com").with_conditions(
    StringEquals={"cognito-identity.amazonaws.com:aud": "us-east-2:12345678-abcd-abcd-abcd-123456"},
    ForAnyValue:StringLike={"cognito-identity.amazonaws.com:amr": "unauthenticated"}
)

Parsing JSON Policy Documents

The PolicyDocument.fromJson and PolicyStatement.fromJson static methods can be used to parse JSON objects. For example:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
policy_document = {
    "Version": "2012-10-17",
    "Statement": [{
        "Sid": "FirstStatement",
        "Effect": "Allow",
        "Action": ["iam:ChangePassword"],
        "Resource": "*"
    }, {
        "Sid": "SecondStatement",
        "Effect": "Allow",
        "Action": "s3:ListAllMyBuckets",
        "Resource": "*"
    }, {
        "Sid": "ThirdStatement",
        "Effect": "Allow",
        "Action": ["s3:List*", "s3:Get*"
        ],
        "Resource": ["arn:aws:s3:::confidential-data", "arn:aws:s3:::confidential-data/*"
        ],
        "Condition": {"Bool": {"aws:_multi_factor_auth_present": "true"}}
    }
    ]
}

new_policy_document = PolicyDocument.from_json(policy_document)

OpenID Connect Providers

OIDC identity providers are entities in IAM that describe an external identity provider (IdP) service that supports the OpenID Connect (OIDC) standard, such as Google or Salesforce. You use an IAM OIDC identity provider when you want to establish trust between an OIDC-compatible IdP and your AWS account. This is useful when creating a mobile app or web application that requires access to AWS resources, but you don't want to create custom sign-in code or manage your own user identities. For more information about this scenario, see About Web Identity Federation and the relevant documentation in the Amazon Cognito Identity Pools Developer Guide.

The following examples defines an OpenID Connect provider. Two client IDs (audiences) are will be able to send authentication requests to https://openid/connect.

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
provider = OpenIdConnectProvider(self, "MyProvider",
    url="https://openid/connect",
    clients=["myclient1", "myclient2"]
)

You can specify an optional list of thumbprints. If not specified, the thumbprint of the root certificate authority (CA) will automatically be obtained from the host as described here.

Once you define an OpenID connect provider, you can use it with AWS services that expect an IAM OIDC provider. For example, when you define an Amazon Cognito identity pool you can reference the provider's ARN as follows:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
cognito.CfnIdentityPool(self, "IdentityPool",
    open_id_connect_provider_aRNs=[provider.open_id_connect_provider_arn]
)

The OpenIdConnectPrincipal class can be used as a principal used with a OpenIdConnectProvider, for example:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
provider = OpenIdConnectProvider(self, "MyProvider",
    url="https://openid/connect",
    clients=["myclient1", "myclient2"]
)
principal = iam.OpenIdConnectPrincipal(provider)

Features

  • Policy name uniqueness is enforced. If two policies by the same name are attached to the same principal, the attachment will fail.
  • Policy names are not required - the CDK logical ID will be used and ensured to be unique.
  • Policies are validated during synthesis to ensure that they have actions, and that policies attached to IAM principals specify relevant resources, while policies attached to resources specify which IAM principals they apply to.

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