kubernetes default service account

https://stackoverflow.com/questions/52995962/kubernetes-namespace-default-service-account#:~:text=The%20default%20permissions%20for%20a,those%20of%20an%20unauthenticated%20user.

A default service account is automatically created for each namespace.

kubectl get serviceaccount
NAME SECRETS AGE
default 1 1d

Service accounts can be added when required. Each pod is associated with exactly one service account but multiple pods can use the same service account.

A pod can only use one service account from the same namespace.

Service account are assigned to a pod by specifying the account’s name in the pod manifest. If you don’t assign it explicitly the pod will use the default service account.

The default permissions for a service account don't allow it to list or modify any resources. The default service account isn't allowed to view cluster state let alone modify it in any way.

By default, the default service account in a namespace has no permissions other than those of an unauthenticated user.

Therefore pods by default can’t even view cluster state. Its up to you to grant them appropriate permissions to do that.

kubectl exec -it test -n foo sh/# curl localhost:8001/api/v1/namespaces/foo/services { "kind": "Status", "apiVersion": "v1", "metadata": {
}, "status": "Failure", "message": "services is forbidden: User "system:serviceaccount:foo:default"cannot list resource "services"in API group ""in the namespace "foo"", "reason": "Forbidden", "details": { "kind": "services"}, "code": 403
as can be seen above the default service account cannot list services
but when given proper role and role binding like below

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  creationTimestamp: null
  name: foo-role
  namespace: foo
rules:
- apiGroups:
  - ""
  resources:
  - services
  verbs:
  - get
  - list

apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  creationTimestamp: null
  name: test-foo
  namespace: foo
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: foo-role
subjects:
- kind: ServiceAccount
  name: default
  namespace: foo

now i am able to list the resurce service

kubectl exec -it test -n foo sh
/ # curl localhost:8001/api/v1/namespaces/foo/services
{
  "kind": "ServiceList",
  "apiVersion": "v1",
  "metadata": {
    "selfLink": "/api/v1/namespaces/bar/services",
    "resourceVersion": "457324"
  },
  "items": []

Giving all your service accounts the clusteradmin ClusterRole is a bad idea. It is best to give everyone only the permissions they need to do their job and not a single permission more.

It’s a good idea to create a specific service account for each pod and then associate it with a tailor-made role or a ClusterRole through a RoleBinding.

If one of your pods only needs to read pods while the other also needs to modify them then create two different service accounts and make those pods use them by specifying the serviceaccountName property in the pod spec.

You can refer the below link for an in-depth explanation.
Service account example with roles
You can check kubectl explain serviceaccount.automountServiceAccountToken and edit the service account
kubectl edit serviceaccount default -o yaml

apiVersion: v1
automountServiceAccountToken: false
kind: ServiceAccount
metadata:
  creationTimestamp: 2018-10-14T08:26:37Z
  name: default
  namespace: default
  resourceVersion: "459688"
  selfLink: /api/v1/namespaces/default/serviceaccounts/default
  uid: de71e624-cf8a-11e8-abce-0642c77524e8
secrets:
- name: default-token-q66j4

Once this change is done whichever pod you spawn doesn't have a serviceaccount token as can be seen below.

kubectl exec tp -it bash
root@tp:/# cd /var/run/secrets/kubernetes.io/serviceaccount
bash: cd: /var/run/secrets/kubernetes.io/serviceaccount: No such file or directory