### CVE-2021-25735: Validating Admission Webhook does not observe some previous fields

A security issue was discovered in kube-apiserver that could allow node
updates to bypass a Validating Admission Webhook. You are only affected
by this vulnerability if you run a Validating Admission Webhook for Nodes
that denies admission based at least partially on the old state of the
Node object.

- Added support for CEL expressions to `v1alpha1 AuthorizationConfiguration` webhook `matchConditions`. ([#121223](https://github.com/kubernetes/kubernetes/pull/121223), [@ritazh](https://github.com/ritazh))

        * At Request audit level or higher, an annotation with key "patch.webhook.admission.k8s.io/round_{round idx}_index_{order idx}" get logged with the JSON payload logging the patch sent by a webhook for given request.

- Error messages returned in authentication webhook status responses are now correctly included in the apiserver log ([#73595](https://github.com/kubernetes/kubernetes/pull/73595), [@liggitt](https://github.com/liggitt))

	notifySubsystem           MetricSubsystem = "notify"
	lambdaSubsystem           MetricSubsystem = "lambda"
	auditSubsystem            MetricSubsystem = "audit"
	webhookSubsystem          MetricSubsystem = "webhook"
)

// MetricName are the individual names for the metric.
type MetricName string

const (
	authTotal         MetricName = "auth_total"
	canceledTotal     MetricName = "canceled_total"

    * federation-apiserver now supports NamespaceLifecycle admission control, which is enabled by default. Set the --admission-control flag on the server to change that.
* Configure webhook ([#30923](https://github.com/kubernetes/kubernetes/pull/30923), [@Q-Lee](https://github.com/Q-Lee))

- Ignore context canceled from validate and mutate webhook ([#120020](https://github.com/kubernetes/kubernetes/pull/120020), [@divyasri537](https://github.com/divyasri537)) [SIG API Machinery]

  * Multiple fixes to avoid hitting AWS rate limits, and to throttle if we do
  * Support for the EC2 Container Registry (currently in us-east-1 only)

With kube-up:

  * Automatically install updates on boot & reboot
  * Use optimized image based on Jessie by default
  * Add support for Ubuntu Wily
  * Master is configured with automatic restart-on-failure, via CloudWatch

* Reduce memory utilization of admission webhook metrics by removing resource related labels. ([#69895](https://github.com/kubernetes/kubernetes/pull/69895), [@jpbetz](https://github.com/jpbetz))

    * can be overridden by explicitly specifying the scope values as
    * comma-separated string in the "users[*].config.scopes" field in the
    * KUBECONFIG file.
* kube-apiserver is changed to use SSH tunnels for webhook iff the webhook is not directly routable from apiserver's network environment. ([#58644](https://github.com/kubernetes/kubernetes/pull/58644), [@yguo0905](https://github.com/yguo0905))