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CDK routines for easily assigning correct and minimal IAM permissions

Project description

AWS IAM Construct Library

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

    const role = new Role(this, 'MyRole', {
      assumedBy: new ServicePrincipal('sns.amazonaws.com')
    });

    role.addToPolicy(new PolicyStatement()
        .addAllResources()
        .addAction('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).

    const user = new User(this, 'MyUser', { password: SecretValue.plainText('1234') });
    const group = new Group(this, 'MyGroup');

    const policy = new Policy(this, 'MyPolicy');
    policy.attachToUser(user);
    group.attachInlinePolicy(policy);

Managed policies can be attached using xxx.attachManagedPolicy(arn):

const group = new Group(this, 'MyGroup');
group.attachManagedPolicy('arn:aws:iam::aws:policy/AdministratorAccess');

Configuring an ExternalId

If you need to create roles that will be assumed by 3rd parties, it is generally a good idea to require an ExternalId to assume them. Configuring an ExternalId works like this:

const role = new iam.Role(this, 'MyRole', {
  assumedBy: new iam.AccountPrincipal('123456789012'),
  externalId: '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:

const statement = new PolicyStatement();
statement.addServicePrincipal('cloudwatch.amazonaws.com');
statement.addServicePrincipal('ec2.amazonaws.com');
statement.addAwsPrincipal('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:

const role = new iam.Role(this, 'MyRole', {
  assumedBy: new iam.CompositePrincipal(
    new iam.ServicePrincipal('ec2.amazonawas.com'),
    new iam.AccountPrincipal('1818188181818187272')
  )
});

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.

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