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AWS Identity and Access Management
User Guide

Roles Terms and Concepts

Here are some basic terms to help you get started with roles.

Role

A role is essentially a set of permissions that grant access to actions and resources in AWS. These permissions are attached to the role, not to an IAM user or group. Roles can be used by the following:

  • An IAM user in the same AWS account as the role

  • An IAM user in a different AWS account as the role

  • A web service offered by AWS such as Amazon Elastic Compute Cloud (Amazon EC2)

  • An external user authenticated by an external identity provider (IdP) service that is compatible with SAML 2.0 or OpenID Connect, or a custom-built identity broker.

Delegation

Delegation is granting permission to someone that allows access to resources that you control. This involves setting up a trust between the account that owns the resource (the trusting account), and the account that contains the users that need to access the resource (the trusted account). The trusted and trusting accounts can be any of the following:

  • The same account.

  • Two accounts that are both under your (organization's) control.

  • Two accounts owned by different organizations.

To delegate permission to access a resource, you create an IAM role that has two policies attached. The permissions policy grants the user of the role the needed permissions to carry out the desired tasks on the resource. The trust policy specifies which trusted accounts are allowed to grant its users permissions to assume the role. Keep in mind that you cannot specify a wildcard (*) as a principal in the role's trust policy. The trust policy on the role in the trusting account is one-half of the permissions. The other half is a permissions policy attached to the user in the trusted account that allows that user to switch to, or assume the role. A user who assumes a role temporarily gives up his or her own permissions and instead takes on the permissions of the role. When the user exits, or stops using the role, the original user permissions are restored. An additional parameter called external ID helps ensure secure use of roles between accounts that are not controlled by the same organization.

Federation

Federation is creating a trust relationship between an external identity provider and AWS. Users can sign in to a web identity provider, such as Login with Amazon, Facebook, Google, or any IdP that is compatible with OpenID Connect (OIDC). Users can also sign in to an enterprise identity system that is compatible with Security Assertion Markup Language (SAML) 2.0, such as Microsoft Active Directory Federation Services. When you use OIDC and SAML 2.0 to configure a trust relationship between these external identity providers and AWS, the user is assigned to an IAM role and receives temporary credentials that enable the user to access your AWS resources.

Policy

An IAM policy is a document in JSON format in which you define the permissions for a role. The document is written according to the rules of the IAM Policy Language.

When you create a role, you create two separate policies for it: a trust policy, which specifies who is allowed to assume the role (the trusted entity, or principal; see the next term), and the permissions policy, which defines what actions and resources the principal is allowed to use.

Principal

A principal is an entity in AWS that can perform actions and access resources. A principal can be an AWS account (the "root" user), an IAM user, or a role. You can grant permissions to access a resource in one of two ways:

  • You can attach a permissions policy to a user (directly, or indirectly through a group) or to a role.

  • For those services that support resource-based policies, you can identify the principal in the Principal element of a policy attached to the resource.

If you reference an AWS account as principal, it generally means any principal defined within that account.

Note

You cannot use a wildcard (*) in the Principal element in a role's trust policy.

Cross-account access

Granting access to resources in one account to a trusted principal in a different account is often referred to as cross-account access. Roles are the primary way to grant cross-account access. However, with some of the web services offered by AWS you can attach a policy directly to a resource (instead of using a role as a proxy). These are called resource-based policies, and you can use them to grant principals in another AWS account access to the resource. The following services support resource-based policies for the specified resources: Amazon Simple Storage Service (S3) buckets, Amazon Glacier vaults, Amazon Simple Notification Service (SNS) topics, and Amazon Simple Queue Service (SQS) queues. For more information, see How IAM Roles Differ from Resource-based Policies.