New Docs page for API Server Bypass Risks

content/en/docs/concepts/security/api-server-bypass-risks.md

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+---
+title: Kubernetes API Server Bypass Risks
+description: >
+  Security architecture information relating to the API server and other components
+content_type: concept
+---
+
+<!-- overview -->
+
+The Kubernetes API server is the main point of entry to a cluster for external parties 
+(users and services) interacting with it. 
+
+As part of this role, the API server has several key built-in security controls, such as
+audit logging and {{< glossary_tooltip text="admission controllers" term_id="admission-controller" >}}. However, there are ways to modify the configuration
+or content of the cluster that bypass these controls.
+
+This page describes the ways in which the security controls built into the
+Kubernetes API server can be bypassed, so that cluster operators
+and security architects can ensure that these bypasses are appropriately restricted.
+
+<!-- body -->
+
+## Static Pods {#static-pods}
+
+The {{< glossary_tooltip text="kubelet" term_id="kubelet" >}} on each node loads and
+directly manages any manifests that are stored in a named directory or fetched from
+a specific URL as [*static Pods*](/docs/tasks/configure-pod-container/static-pod) in
+your cluster. The API server doesn't manage these static Pods. An attacker with write
+access to this location could modify the configuration of static pods loaded from that
+source, or could introduce new static Pods.
+
+Static Pods are restricted from accessing other objects in the Kubernetes API . For example,
+you can't configure a static Pod to mount a Secret from the cluster. However, these Pods can
+take other security sensitive actions, such as using `hostPath` mounts from the underlying
+node.
+
+By default, the kubelet creates a {{< glossary_tooltip text="mirror pod" term_id="mirror-pod">}}
+so that the static Pods are visible in the Kubernetes API. However, if the attacker uses an invalid
+namespace name when creating the Pod, it will not be visible in the Kubernetes API and can only
+be discovered by tooling that has access to the affected host(s).
+
+If a static Pod fails admission control, the kubelet won't register the Pod with the
+API server. However, the Pod still runs on the node. For more information, refer to
+[kubeadm issue #1541](https://github.com/kubernetes/kubeadm/issues/1541#issuecomment-487331701)). 
+
+### Mitigations {#static-pods-mitigations}
+
+- Only [enable the kubelet static Pod manifest functionality](/docs/tasks/configure-pod-container/static-pod/#static-pod-creation)
+   if required by the node.
+- If a node uses the static Pod functionality, restrict filesystem access to the static Pod manifest directory
+   or URL to users who need the access.
+- Restrict access to kubelet configuration parameters and files to prevent an attacker setting
+   a static Pod path or URL.
+- Regularly audit and centrally report all access to directories or web storage locations that host
+   static Pod manifests and kubelet configuration files.
+
+## The kubelet API {#kubelet-api}
+
+The kubelet provides an HTTP API that is typically exposed on TCP port 10250 on cluster
+worker nodes. The API might also be exposed on control plane nodes depending on the Kubernetes
+distribution in use. Direct access to the API allows for disclosure of information about
+the pods running on a node, the logs from those pods, and execution of commands in
+every container running on the node.
+
+When Kubernetes cluster users have RBAC access to `Node` object sub-resources, that access
+serves as authorization to interact with the kubelet API. The exact access depends on
+which sub-resource access has been granted, as detailed in [kubelet authorization](https://kubernetes.io/docs/reference/access-authn-authz/kubelet-authn-authz/#kubelet-authorization).
+
+Direct access to the kubelet API is not subject to admission control and is not logged
+by Kubernetes audit logging. An attacker with direct access to this API may be able to
+bypass controls that detect or prevent certain actions.
+
+The kubelet API can be configured to authenticate requests in a number of ways. 
+By default, the kubelet configuration allows anonymous access. Most Kubernetes providers
+change the default to use webhook and certificate authentication. This lets the control plane
+ensure that the caller is authorized to access the `nodes` API resource or sub-resources.
+The default anonymous access doesn't make this assertion with the control plane.
+
+### Mitigations
+
+- Restrict access to sub-resources of the `nodes` API object using mechanisms such as
+   [RBAC](/docs/reference/access-authn-authz/rbac/). Only grant this access when required,
+   such as by monitoring services.
+- Restrict access to the kubelet port. Only allow specified and trusted IP address
+   ranges to access the port.
+- [Ensure that kubelet authentication is set to webhook or certificate mode](/docs/reference/access-authn-authz/kubelet-authn-authz/#kubelet-authentication). 
+- Ensure that the unauthenticated "read-only" Kubelet port is not enabled on the cluster.
+
+## The etcd API
+
+Kubernetes clusters use etcd as a datastore. The `etcd` service listens on TCP port 2379.
+the only clients that need access are the Kubernetes API server and any backup tooling
+that you use. Direct access to this API allows for disclosure or modification of any
+data held in the cluster.
+
+Access to the etcd API is typically managed by client certificate authentication.
+Any certificate issued by a certificate authority that etcd trusts allows full access
+to the data stored inside etcd.
+
+Direct access to etcd is not subject to Kubernetes admission control and is not logged
+by Kubernetes audit logging. An attacker who has read access to the API server's
+etcd client certificate private key (or can create a new trusted client certificate) can gain
+cluster admin rights by accessing cluster secrets or modifying access rules. Even without
+elevating their Kubernetes RBAC privileges, an attacker who can modify etcd can retrieve any API object
+or create new workloads inside the cluster.
+
+Many Kubernetes providers configure 
+etcd to use mutual TLS (both client and server verify each other's certificate for authentication).
+There is no widely accepted implementation of authorization for the etcd API, although
+the feature exists. Since there is no authorization model, any certificate
+with client access to etcd can be used to gain full access to etcd. Typically, etcd client certificates
+that are only used for health checking can also grant full read and write access.
+
+### Mitigations {#etcd-api-mitigations}
+
+- Ensure that the certificate authority trusted by etcd is used only for the purposes of
+  authentication to that service.
+- Control access to the private key for the etcd server certificate, and to the API server's
+  client certificate and key.
+- Consider restricting access to the the etcd port at a network level, to only allow access
+  from specified and trusted IP address ranges.
+
+
+## Container runtime socket {#runtime-socket}
+
+On each node in a Kubernetes cluster, access to interact with containers is controlled 
+by the container runtime (or runtimes, if you have configured more than one). Typically,
+the container runtime exposes a Unix socket that the kubelet can access. An attacker with
+access to this socket can launch new containers or interact with running containers.
+
+At the cluster level, the impact of this access depends on whether the containers that
+run on the compromised node have access to Secrets or other confidential
+data that an attacker could use to escalate privileges to other worker nodes or to
+control plane components.
+
+### Mitigations {#runtime-socket-mitigations}
+
+- Ensure that you tightly control filesystem access to container runtime sockets. 
+   When possible, restrict this access to the `root` user.
+- Isolate the kubelet from other components running on the node, using
+   mechanisms such as Linux kernel namespaces.
+- Ensure that you restrict or forbid the use of [`hostPath` mounts](/docs/concepts/storage/volumes/#hostpath)
+   that include the container runtime socket, either directly or by mounting a parent
+   directory. Also `hostPath` mounts must be set as read-only to mitigate risks
+   of attackers bypassing directory restrictions.
+- Restrict user access to nodes, and especially restrict superuser access to nodes.