Restrict Automount SA Token

Description

Kubernetes automatically mounts ServiceAccount credentials in each Pod. The ServiceAccount may be assigned roles allowing Pods to access API resources. Blocking this ability is an extension of the least privilege best practice and should be followed if Pods do not need to speak to the API server to function. This policy ensures that mounting of these ServiceAccount tokens is blocked.

Risks

In Kubernetes, there are two key security layers: authentication and authorization (authz). Authentication ensures that the pod or user is who they claim to be, while authorization determines what actions they’re allowed to perform. When a pod is created, it automatically mounts a service account token, which allows it to interact with the Kubernetes API server for authentication purposes. However, by default, the pod doesn’t have permission (via authorization) to do much with that access.

The problem arises when an attacker gains control of a pod. Even though the attacker might not have initial permissions (due to the security of the authz layer), if they manage to bypass authorization, they can exploit the service account token to escalate privileges and perform unauthorized actions across the cluster. For example, an attacker could perform API requests like creating or deleting pods.

Kyverno Policy

Refer to the Nirmata curated policies - restrict-automount-sa-token.

Resource Example

Below is an example of a Deployment resource that has automountServiceAccountToken set to false.

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: busybox
  name: gooddeployment01
spec:
  replicas: 1
  selector:
    matchLabels:
      app: busybox
  strategy: {}
  template:
    metadata:
      labels:
        app: busybox
    spec:
      automountServiceAccountToken: false
      containers:
      - name: busybox
        image: busybox:1.35

Below is another example of a CronJob resource enforcing this policy.

apiVersion: batch/v1
kind: CronJob
metadata:
  labels:
  name: goodcronjob01
spec:
  schedule: "* * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: busybox
            image: busybox:1.35
          automountServiceAccountToken: false
          restartPolicy: OnFailure