CVE-2023-2728: Bypassing enforce mountable secrets policy imposed by the ServiceAccount admission plugin

A security issue was discovered in Kubernetes where users may be able to launch containers that bypass the mountable secrets policy enforced by the ServiceAccount admission plugin when using ephemeral containers. The policy ensures pods running with a service account may only reference secrets specified in the service account’s secrets field. Kubernetes clusters are only affected if the ServiceAccount admission plugin and the *kubernetes.io/enforce-mountable-secrets* annotation are used together with ephemeral containers. This issue affects kube-apiserver. Clusters are impacted by this vulnerability if: 1. The ServiceAccount admission plugin is used. Most clusters should have this on by default as recommended in https://kubernetes.io/docs/reference/access-authn-authz/admission-controllers/#serviceaccount 2. The *kubernetes.io/enforce-mountable-secrets* annotation is used by a service account. This annotation is not added by default. 3. Pods using ephemeral containers.