Prior to 1.17 and for existing nodes created by `kubeadm init` where kubelet client certificate rotation is desired, you must modify `/etc/kubernetes/kubelet.conf` to point to the PEM symlink for rotation:

- Added two client certificate metrics for exec auth:
    - `rest_client_certificate_expiration_seconds` a gauge reporting the lifetime of the current client certificate. Reports the time of expiry in seconds since January 1, 1970 UTC.

- Fixed an issue where the `vsphere` cloud provider would not trust a certificate if:
  - The issuer of the certificate was unknown (`x509.UnknownAuthorityError`)
  - The requested name did not match the set of authorized names (`x509.HostnameError`)

- kubeadm now includes the ability to specify certificate encryption and decryption keys for the upload and download certificate phases as part of the new v1beta2 kubeadm config format. ([#77012](https://github.com/kubernetes/kubernetes/pull/77012), [@rosti](https://github.com/rosti))

- The extension API server can now dynamically discover the requestheader CA certificate when the core API server doesn't use certificate based authentication for it's clients. ([#66394](https://github.com/kubernetes/kubernetes/pull/66394), [@rtripat](https://github.com/rtripat))

### SIG Autoscaling

#### TLS Bootstrapping
* [alpha] Rotation of the server TLS certificate on the kubelet. See [TLS bootstrapping - approval controller](https://kubernetes.io/docs/admin/kubelet-tls-bootstrapping/#approval-controller).

        * requests for a TLS client certificate for any node are approved if the CSR creator has `create` permission on the `certificatesigningrequests` resource and `nodeclient` subresource in the `certificates.k8s.io`...

* Allows extension API server to dynamically discover the requestheader CA certificate when the core API server doesn't use certificate based authentication for it's clients ([#66394](https://github.com/kubernetes/kubernetes/pull/66394), [@rtripat](https://github.com/rtripat))

- Kubeadm: add support for certificate chain validation. When using kubeadm in external CA mode, this allows an intermediate CA to be used to sign the certificates. The intermediate CA certificate must be appended to each signed certificate for this to work correctly. ([#97266](https://github.com/kubernetes/kubernetes/pull/97266), [@robbiemcmichael](https://github.com/robbiemcmichael))...

- Kubernetes components that accepted X.509 client certificate authentication now read the user UID from a certificate subject name RDN with object ID `1.3.6.1.4.1.57683.2`. An RDN with this object ID had to contain a string value and appear no more than once in the certificate subject. Reading the user UID from this RDN could be disabled by setting the beta feature gate `AllowParsingUserUIDFromCertAuth` to `false`(until...