Access and identity options for Azure Kubernetes Service (AKS)

There are different ways to authenticate, control access/authorize and secure Kubernetes clusters. Using Kubernetes role-based access control (RBAC), you can grant users, groups, and service accounts access to only the resources they need. With Azure Kubernetes Service (AKS), you can further enhance the security and permissions structure by using Azure Active Directory and Azure RBAC. These approaches help you secure your cluster access and provide only the minimum required permissions to developers and operators.

This article introduces the core concepts that help you authenticate and assign permissions in AKS:

Kubernetes role-based access control (RBAC)

To provide granular filtering of the actions that users can do, Kubernetes uses role-based access control (RBAC). This control mechanism lets you assign users, or groups of users, permission to do things like create or modify resources, or view logs from running application workloads. These permissions can be scoped to a single namespace, or granted across the entire AKS cluster. With Kubernetes RBAC, you create roles to define permissions, and then assign those roles to users with role bindings.

For more information, see Using RBAC authorization.

Roles and ClusterRoles

Before you assign permissions to users with Kubernetes RBAC, you first define those permissions as a Role. Kubernetes roles grant permissions. There's no concept of a deny permission.

Roles are used to grant permissions within a namespace. If you need to grant permissions across the entire cluster, or to cluster resources outside a given namespace, you can instead use ClusterRoles.

A ClusterRole works in the same way to grant permissions to resources, but can be applied to resources across the entire cluster, not a specific namespace.

RoleBindings and ClusterRoleBindings

Once roles are defined to grant permissions to resources, you assign those Kubernetes RBAC permissions with a RoleBinding. If your AKS cluster integrates with Azure Active Directory, bindings are how those Azure AD users are granted permissions to perform actions within the cluster, see how in Control access to cluster resources using role-based access control and Azure Active Directory identities.

Role bindings are used to assign roles for a given namespace. This approach lets you logically segregate a single AKS cluster, with users only able to access the application resources in their assigned namespace. If you need to bind roles across the entire cluster, or to cluster resources outside a given namespace, you can instead use ClusterRoleBindings.

A ClusterRoleBinding works in the same way to bind roles to users, but can be applied to resources across the entire cluster, not a specific namespace. This approach lets you grant administrators or support engineers access to all resources in the AKS cluster.

Note

Any cluster actions taken by Microsoft/AKS are made with user consent under a built-in Kubernetes role aks-service and built-in role binding aks-service-rolebinding. This role enables AKS to troubleshoot and diagnose cluster issues, but can't modify permissions nor create roles or role bindings, or other high privilege actions. Role access is only enabled under active support tickets with just-in-time (JIT) access. Read more about AKS support policies.

Kubernetes service accounts

One of the primary user types in Kubernetes is a service account. A service account exists in, and is managed by, the Kubernetes API. The credentials for service accounts are stored as Kubernetes secrets, which allows them to be used by authorized pods to communicate with the API Server. Most API requests provide an authentication token for a service account or a normal user account.

Normal user accounts allow more traditional access for human administrators or developers, not just services, and processes. Kubernetes itself doesn't provide an identity management solution where regular user accounts and passwords are stored. Instead, external identity solutions can be integrated into Kubernetes. For AKS clusters, this integrated identity solution is Azure Active Directory.

For more information on the identity options in Kubernetes, see Kubernetes authentication.

Azure Active Directory integration

The security of AKS clusters can be enhanced with the integration of Azure Active Directory (AD). Built on decades of enterprise identity management, Azure AD is a multi-tenant, cloud-based directory, and identity management service that combines core directory services, application access management, and identity protection. With Azure AD, you can integrate on-premises identities into AKS clusters to provide a single source for account management and security.

Azure Active Directory integration with AKS clusters

With Azure AD-integrated AKS clusters, you can grant users or groups access to Kubernetes resources within a namespace or across the cluster. To obtain a kubectl configuration context, a user can run the az aks get-credentials command. When a user then interacts with the AKS cluster with kubectl, they're prompted to sign in with their Azure AD credentials. This approach provides a single source for user account management and password credentials. The user can only access the resources as defined by the cluster administrator.

Azure AD authentication is provided to AKS clusters with OpenID Connect. OpenID Connect is an identity layer built on top of the OAuth 2.0 protocol. For more information on OpenID Connect, see the Open ID connect documentation. From inside of the Kubernetes cluster, Webhook Token Authentication is used to verify authentication tokens. Webhook token authentication is configured and managed as part of the AKS cluster.

Webhook and API server

Webhook and API server authentication flow

As shown in the graphic above, the API server calls the AKS webhook server and performs the following steps:

  1. The Azure AD client application is used by kubectl to sign in users with OAuth 2.0 device authorization grant flow.
  2. Azure AD provides an access_token, id_token, and a refresh_token.
  3. The user makes a request to kubectl with an access_token from kubeconfig.
  4. Kubectl sends the access_token to APIServer.
  5. The API Server is configured with the Auth WebHook Server to perform validation.
  6. The authentication webhook server confirms the JSON Web Token signature is valid by checking the Azure AD public signing key.
  7. The server application uses user-provided credentials to query group memberships of the logged-in user from the MS Graph API.
  8. A response is sent to the APIServer with user information such as the user principal name (UPN) claim of the access token, and the group membership of the user based on the object ID.
  9. The API performs an authorization decision based on the Kubernetes Role/RoleBinding.
  10. Once authorized, the API server returns a response to kubectl.
  11. Kubectl provides feedback to the user.

Learn how to integrate AKS with AAD here.

Azure role-based access control (Azure RBAC)

Azure RBAC is an authorization system built on Azure Resource Manager that provides fine-grained access management of Azure resources.

Azure RBAC is designed to work on resources within your Azure subscription while Kubernetes RBAC is designed to work on Kubernetes resources within your AKS cluster.

With Azure RBAC, you create a role definition that outlines the permissions to be applied. A user or group is then assigned this role definition via a role assignment for a particular scope, which could be an individual resource, a resource group, or across the subscription.

For more information, see What is Azure role-based access control (Azure RBAC)?

There are two levels of access needed to fully operate an AKS cluster:

  1. Access the AKS resource in your Azure subscription. This process allows you to control things scaling or upgrading your cluster using the AKS APIs as well as pull your kubeconfig.
  2. Access to the Kubernetes API. This access is controlled either by Kubernetes RBAC (traditionally) or by integrating Azure RBAC with AKS for Kubernetes authorization

Azure RBAC to authorize access to the AKS resource

With Azure RBAC, you can provide your users (or identities) with granular access to AKS resources across one or more subscriptions. For example, you could have the Azure Kubernetes Service Contributor role that allows you to do actions like scale and upgrade your cluster. While another user could have the Azure Kubernetes Service Cluster Admin role that only gives permission to pull the Admin kubeconfig.

Alternatively you could give your user the general Contributor role, which would encompass the above permissions and every action possible on the AKS resource with the exception of managing permissions itself.

See more how to use Azure RBAC to secure the access to the kubeconfig file that gives access to the Kubernetes API here.

Azure RBAC for Kubernetes Authorization (Preview)

With the Azure RBAC integration, AKS will use a Kubernetes Authorization webhook server to enable you to manage permissions and assignments of Azure AD-integrated K8s cluster resources using Azure role definition and role assignments.

Azure RBAC for Kubernetes authorization flow

As shown on the above diagram, when using the Azure RBAC integration all requests to the Kubernetes API will follow the same authentication flow as explained on the Azure Active integration section.

But after that, instead of solely relying on Kubernetes RBAC for Authorization, the request is actually going to be authorized by Azure, as long as the identity that made the request exists in AAD. If the identity doesn't exist in AAD, for example a Kubernetes service account, then the Azure RBAC won't kick in, and it will be the normal Kubernetes RBAC.

In this scenario you could give users one of the four built-in roles, or create custom roles as you would do with Kubernetes roles but in this case using the Azure RBAC mechanisms and APIs.

This feature will allow you to, for example, not only give users permissions to the AKS resource across subscriptions but set up and give them the role and permissions that they will have inside each of those clusters that controls the access to the Kubernetes API. For example, you can grant the Azure Kubernetes Service RBAC Viewer role on the subscription scope and its recipient will be able to list and get all Kubernetes objects from all clusters, but not modify them.

Built-in roles

AKS provides the following four built-in roles. They are similar to the Kubernetes built-in roles but with a few differences like supporting CRDs. For the full list of actions allowed by each built in role please see here.

Role Description
Azure Kubernetes Service RBAC Viewer Allows read-only access to see most objects in a namespace. It doesn't allow viewing roles or role bindings. This role doesn't allow viewing Secrets, since reading the contents of Secrets enables access to ServiceAccount credentials in the namespace, which would allow API access as any ServiceAccount in the namespace (a form of privilege escalation)
Azure Kubernetes Service RBAC Writer Allows read/write access to most objects in a namespace. This role doesn't allow viewing or modifying roles or role bindings. However, this role allows accessing Secrets and running Pods as any ServiceAccount in the namespace, so it can be used to gain the API access levels of any ServiceAccount in the namespace.
Azure Kubernetes Service RBAC Admin Allows admin access, intended to be granted within a namespace. Allows read/write access to most resources in a namespace (or cluster scope), including the ability to create roles and role bindings within the namespace. This role doesn't allow write access to resource quota or to the namespace itself.
Azure Kubernetes Service RBAC Cluster Admin Allows super-user access to perform any action on any resource. It gives full control over every resource in the cluster and in all namespaces.

See how to leverage Azure RBAC for Kubernetes authorization here.

Next steps

For more information on core Kubernetes and AKS concepts, see the following articles: