Starting today, you can pass user attributes in the AWS session when your workforce sign-in into the cloud using AWS Single Sign-On. This gives you the centralized account access management of AWS Single Sign-On and ABAC, with the flexibility to use AWS SSO, Active Directory, or an external identity provider as your identity source. To learn more about the advantages of ABAC policies on AWS, you may read my previous blog post on the subject.

Overview
On one side, system administrators configure user attributes on the AWS Single Sign-On identity repository, or the managed Active Directory. System administrators may also configure an external identity provider, such as Okta, OneLogin or Ping Identity to pass existing user attributes in the AWS sessions when their workforce federates into AWS. These attributes are known as session tags in AWS. On the other side, cloud administrators create fine-grained permissions policies such that your workforce get only access to cloud resources with matching resource tags.

Creating policies based on matching attributes instead of functional roles helps to reduce the number of distinct permissions and roles you must create and manage in your AWS environment. For example, when developers Bob from team red and Alice from team blue sign-in into AWS and assume the same AWS Identity and Access Management (IAM) role, they get distinct permissions to project resources tagged for their team. The identity system sends the team name attribute in the AWS session when Bob and Alice sign-in into AWS. The role’s permissions grant access to project resources with matching team name tags. Now, if Bob moves to team blue and system administrators update his team name in their identity provider directory, Bob automatically gets access to team blue’s project resources without requiring permissions updates in IAM.

How to Configure AWS SSO to Map User Attributes
Before to configure AWS SSO, there are two important points to highlight. First, ABAC will work with attributes from any identity source configured in AWS SSO : AWS SSO itself, a managed Active Directory, or an external identity provider. Second, there are two ways to pass attributes for access control to AWS SSO. Either you can pass attributes directly in the SAML assertion using the prefix https://aws.amazon.com/SAML/Attributes/AccessControl, or you can use attributes that are in the AWS SSO identity store. Those attributes are configured by your AWS SSO administrator for users created in AWS SSO, synchronized in from an Active Directory, or synchronized in from an external identity provider using automatic provisioning (SCIM).

For this demo, I choose to use an external identity provider and SCIM.

I can enable ABAC in AWS using AWS SSO with three steps:

Step 1: I configure my identity source with the associated user identities and attributes in the external identity provider. As of today, AWS SSO supports identity synchronization via SCIM with Azure AD, Okta, OneLogin, and Ping Identity. Check this page to get an up-to-date list. The specifics depend on each identity provider.

Step 2: I configure the SCIM attributes I want to use for access control using the new Access Control Attributes global setting in the AWS SSO console or API. This screen allows me to select attributes for access control from the identity source I configured in step 1.

Attributes for Access Control

Step 3: I author ABAC rules through permission sets and resource-based policies using the attributes I configured in Step 2. More about this in a minute.

Now, when my workforce federates into an AWS account using SSO, they get access to their AWS resources based on matching attributes.

Attributes are passed as session tags. They are passed as comma-separated key:value pairs. The total character length of all the attributes together must be less than or equal to 460 characters.

What Does a Policy Look Like?
I now can use user attributes in my permission sets using the aws:PrincipalTag condition key when creating access control rules. For example, I can tag all the resources in my organization with their respective department name, and use a single permission set that grants developers access only to their department resources. Now, whenever developers federate into the AWS account, AWS SSO creates a department session tag with the value received from the identity provider. The security policies allow them to only get access to the resources in their respective department. As the team adds more developers and resources to their project, I only have to tag resources with the correct department name. As a result, as the organization adds new resources and developers to departments, developers can only manage resources aligned to their department without needing any permission updates.

An ABAC SSO permission set policy might look like this:

{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": [ "ec2:DescribeInstances"], "Resource": "*" }, { "Effect": "Allow", "Action": ["ec2:StartInstances","ec2:StopInstances"], "Resource": "*", "Condition": { "StringEquals": { "ec2:ResourceTag/Department": "${aws:PrincipalTag/Department}" } } } ]
}

This policy allows anybody to DescribeInstances, but only users with a aws:PrincipalTag/Department tag’s value matching the EC2 instance ec2:ResourceTag/Department tag’s value are authorized to stop or to start instances.

I attach this policy to an AWS Account’s Permission Set. On the left part of the AWS Single Sign-On console, I click AWS Accounts and select the Permission sets tab. Then I click Create permission set. On the next screen, I select Create a customer permission set.

Create a custom permission set

I enter a name and description, I make sure Create a custom permissions policy is selected. Then I can copy/paste the previous policy allowing to start and stop EC2 instances when the department name tag value is equal to the person’s department name tag value.

Create Custom Policy for Permission Set

On the next screen, I enter some tags, then I review my configuration before clicking Create. Et voila, I am ready to go.

If you have existing federation configured with AWS Security Token Service, remember that external identity providers consider AWS SSO as a new application configuration. This means when you move from direct IAM federation to AWS SSO, you have to update your external identity provider configuration to connect with AWS SSO and to introduce attributes as session tags for this configuration.

Available Today
There is no additional charge to configure user attributes with AWS Single Sign-On. You can start to use it today in all AWS Regions where AWS SSO is available.

— seb

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