Using Kubernetes Impersonate (sudo) for least-privilege

It has become very easy and simple to deploy Kubernetes services using the various cloud offerings like EKS or GKE, after you created your cluster and have the cluster-admin privileges to apply changes as you like. This model is great for development because you can start consuming Kubernetes services right away but this doesn’t work well for production clusters and gets more challenging when running PCI compliant workloads.

I want to explain a bit how to apply a least-privilege principle for Elastic Kubernetes Services (EKS) using the AWS integrated IAM. The diagram below is a simple example showing two IAM roles for admin and reader privileges for AWS resources. On the Kubernetes cluster the IAM roles are bound to the k8s cluster-admin and reader roles. The k8s sudoer role allows to impersonate cluster-admin privileges for cluster readers:

Normally you would add your DevOps team to the IAM reader role. This way the DevOps team has the default read permissions for AWS and Kubernetes resources but they can also elevate Kubernetes permissions to cluster-admin level when required without having full access to the AWS resources.

Let’s look at the EKS aws-auth ConfigMap where you need to define the IAM role mapping for admin and reader to internal Kubernetes groups:

apiVersion: v1
kind: ConfigMap
metadata:
  name: aws-auth
  namespace: kube-system
data:
  mapRoles: |
    - rolearn: arn:aws:iam::xxx:role/admin
      username: cluster-admin
      groups:
        - system:masters
    - rolearn: arn:aws:iam::xxx:role/reader
      username: cluster-reader
      groups:
        - cluster-reader
    - rolearn: arn:aws:iam::555555555555:role/devel-worker-nodes-NodeInstanceRole-74RF4UBDUKL6
      username: system:node:{{EC2PrivateDNSName}}
      groups:
        - system:bootstrappers
        - system:nodes
  mapUsers: |
    []

The system:masters group is a Kubernetes default role and rolebinding and requires no additional configuration. For the cluster-reader we need to apply a ClusterRole and a ClusterRoleBinding:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: cluster-reader
rules:
- apiGroups:
  - ""
  resources:
  - componentstatuses
  - nodes
  - nodes/status
  - persistentvolumeclaims/status
  - persistentvolumes
  - persistentvolumes/status
  - pods/binding
  - pods/eviction
  - podtemplates
  - securitycontextconstraints
  - services/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - admissionregistration.k8s.io
  resources:
  - mutatingwebhookconfigurations
  - validatingwebhookconfigurations
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - apps
  resources:
  - controllerrevisions
  - daemonsets/status
  - deployments/status
  - replicasets/status
  - statefulsets/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - apiextensions.k8s.io
  resources:
  - customresourcedefinitions
  - customresourcedefinitions/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - apiregistration.k8s.io
  resources:
  - apiservices
  - apiservices/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - autoscaling
  resources:
  - horizontalpodautoscalers/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - batch
  resources:
  - cronjobs/status
  - jobs/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - coordination.k8s.io
  resources:
  - leases
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - extensions
  resources:
  - daemonsets/status
  - deployments/status
  - horizontalpodautoscalers
  - horizontalpodautoscalers/status
  - ingresses/status
  - jobs
  - jobs/status
  - podsecuritypolicies
  - replicasets/status
  - replicationcontrollers
  - storageclasses
  - thirdpartyresources
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - events.k8s.io
  resources:
  - events
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - policy
  resources:
  - poddisruptionbudgets/status
  - podsecuritypolicies
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - rbac.authorization.k8s.io
  resources:
  - clusterrolebindings
  - clusterroles
  - rolebindings
  - roles
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - settings.k8s.io
  resources:
  - podpresets
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - storage.k8s.io
  resources:
  - storageclasses
  - volumeattachments
  - volumeattachments/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - scheduling.k8s.io
  resources:
  - priorityclasses
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - certificates.k8s.io
  resources:
  - certificatesigningrequests
  - certificatesigningrequests/approval
  - certificatesigningrequests/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - authorization.k8s.io
  resources:
  - localsubjectaccessreviews
  - selfsubjectaccessreviews
  - selfsubjectrulesreviews
  - subjectaccessreviews
  verbs:
  - create
- apiGroups:
  - authentication.k8s.io
  resources:
  - tokenreviews
  verbs:
  - create
- apiGroups:
  - ""
  resources:
  - podsecuritypolicyreviews
  - podsecuritypolicyselfsubjectreviews
  - podsecuritypolicysubjectreviews
  verbs:
  - create
- apiGroups:
  - ""
  resources:
  - nodes/metrics
  - nodes/spec
  verbs:
  - get
- apiGroups:
  - ""
  resources:
  - nodes/stats
  verbs:
  - create
  - get
- nonResourceURLs:
  - '*'
  verbs:
  - get
- apiGroups:
  - networking.k8s.io
  resources:
  - ingresses/status
  verbs:
  - get
- apiGroups:
  - networking.k8s.io
  resources:
  - ingresses/status
  verbs:
  - list
- apiGroups:
  - networking.k8s.io
  resources:
  - ingresses/status
  verbs:
  - watch
- apiGroups:
  - node.k8s.io
  resources:
  - runtimeclasses
  verbs:
  - get
- apiGroups:
  - node.k8s.io
  resources:
  - runtimeclasses
  verbs:
  - list
- apiGroups:
  - node.k8s.io
  resources:
  - runtimeclasses
  verbs:
  - watch
- apiGroups:
  - storage.k8s.io
  resources:
  - csidrivers
  verbs:
  - get
- apiGroups:
  - storage.k8s.io
  resources:
  - csidrivers
  verbs:
  - list
- apiGroups:
  - storage.k8s.io
  resources:
  - csidrivers
  verbs:
  - watch
- apiGroups:
  - storage.k8s.io
  resources:
  - csinodes
  verbs:
  - get
- apiGroups:
  - storage.k8s.io
  resources:
  - csinodes
  verbs:
  - list
- apiGroups:
  - storage.k8s.io
  resources:
  - csinodes
  verbs:
  - watch
- apiGroups:
  - operators.coreos.com
  resources:
  - clusterserviceversions
  - catalogsources
  - installplans
  - subscriptions
  - operatorgroups
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - packages.operators.coreos.com
  resources:
  - packagemanifests
  - packagemanifests/icon
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - packages.operators.coreos.com
  resources:
  - packagemanifests
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - ""
  resources:
  - namespaces
  verbs:
  - get
- apiGroups:
  - ""
  resources:
  - configmaps
  - endpoints
  - persistentvolumeclaims
  - pods
  - replicationcontrollers
  - replicationcontrollers/scale
  - serviceaccounts
  - services
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - ""
  resources:
  - bindings
  - events
  - limitranges
  - namespaces/status
  - pods/log
  - pods/status
  - replicationcontrollers/status
  - resourcequotas
  - resourcequotas/status
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - ""
  resources:
  - namespaces
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - apps
  resources:
  - controllerrevisions
  - daemonsets
  - deployments
  - deployments/scale
  - replicasets
  - replicasets/scale
  - statefulsets
  - statefulsets/scale
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - autoscaling
  resources:
  - horizontalpodautoscalers
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - batch
  resources:
  - cronjobs
  - jobs
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - extensions
  resources:
  - daemonsets
  - deployments
  - deployments/scale
  - ingresses
  - networkpolicies
  - replicasets
  - replicasets/scale
  - replicationcontrollers/scale
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - policy
  resources:
  - poddisruptionbudgets
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - networking.k8s.io
  resources:
  - networkpolicies
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - networking.k8s.io
  resources:
  - ingresses
  verbs:
  - get
- apiGroups:
  - networking.k8s.io
  resources:
  - ingresses
  verbs:
  - list
- apiGroups:
  - networking.k8s.io
  resources:
  - ingresses
  verbs:
  - watch
- apiGroups:
  - metrics.k8s.io
  resources:
  - pods
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - ""
  resources:
  - resourcequotausages
  verbs:
  - get
  - list
  - watch

After you created the ClusterRole you need to create the ClusterRoleBinding:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: cluster-reader
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-reader
subjects:
- apiGroup: rbac.authorization.k8s.io
  kind: Group
  name: cluster-reader

To give a cluster-reader impersonate permissions you need to create the sudoer ClusterRole with the right to impersonate system:admin:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: sudoer
rules:
- apiGroups:
  - ""
  resourceNames:
  - system:admin
  resources:
  - systemusers
  - users
  verbs:
  - impersonate
- apiGroups:
  - ""
  resourceNames:
  - system:masters
  resources:
  - groups
  - systemgroups
  verbs:
  - impersonate

Create the ClusterRoleBinding:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: sudoer
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: sudoer
subjects:
- apiGroup: rbac.authorization.k8s.io
  kind: Group
  name: cluster-reader

For a cluster-reader to impersonate and get cluster-admin privileges you use the following kubectl options –as-group and –as:

kubectl get nodes --as-group system:masters --as system:admin

You want to restrict the membership of the IAM admin role as much as possible as everyone should only use the read permissions to not accidentally delete Kubernetes or AWS resources.

Please share!


Also published on Medium.

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