Disable Automatic Mounting of Default Service Account Tokens

By default, Kubernetes automatically mounts a service account token into every pod. While the default service account has minimal permissions, custom service accounts with elevated RBAC roles become dangerous attack vectors when their tokens are auto-mounted.

The default service account in the cluster has no special permissions by default. An attacker stealing a default SA token can only:

Authenticate to the API server
Get denied on most operations since it’s not bound to any role with elevated permissions

The real risk is with custom service accounts that have been granted elevated permissions via RoleBindings or ClusterRoleBindings.

Anyone who has access to the pod will be able to fetch the default token using the command below:

TOKEN=$(cat /var/run/secrets/kubernetes.io/serviceaccount/token)
curl -k -H "Authorization: Bearer $TOKEN" \
  https://kubernetes.default.svc/api/v1/namespaces/default/secrets

{
  "kind": "Status",
  "apiVersion": "v1",
  "metadata": {},
  "status": "Failure",
  "message": "secrets is forbidden: User \"system:serviceaccount:default:default\" cannot list resource \"secrets\" in API group \"\" in the namespace \"default\"",
  "reason": "Forbidden",
  "details": {
    "kind": "secrets"
  },
  "code": 403
}

If you try accessing different resources (pods, deployments, configmaps, etc.), most requests will be denied with the default token.

Disabling Auto-Mounting

You can set automountServiceAccountToken: false on the default service account to prevent pods from automatically mounting the token:

kubectl apply -f - <<EOF
apiVersion: v1
kind: ServiceAccount
metadata:
  name: default
automountServiceAccountToken: false
EOF

When to Disable

Disable auto-mounting when the application:

Does not need to communicate with the Kubernetes API
Only needs external service access
Uses external identity providers