Kubernetes Security - Best Practice Guide

This document acts as a best practice guide to Kubernetes Security. K8s is a powerful platform which can be abused in many ways if not configured properly. Contributors to this guide are running Kubernetes in production and worked on several K8s projects to learn about security flaws the hard way.

Your cluster is as secure as the system running it

Before you start looking into Kubernetes security specifics you should start with your system running Kubernetes. Go through some guides for securing your OS in general.

Here are some to begin with:

• Red Hat Enterprise Linux 6 Security Guide

Private topology

If your infrastructure allows for private IP addresses you should host the cluster in a private subnet and only forward must have ports from your NAT gateway to your cluster. If you're running on a cloud provider like AWS this can be achieved through a private VPC.

Private topology with kops

kops create cluster --topology private --networking calico ...

Firewall ports

This is a general security best practice, never expose a port, which don't need exposure. IMHO defining port exposure should be done in the following order:

• Check if you can define a listen IP/interface to bind the service to, if possible 127.0.0.1/lo
• If a listen IP/interface is not possible firewall the port

Kubernetes processes like kubelet are opening a few ports on all network interfaces, which should be firewalled from public access. Those ports may "only" allow to query for sensitive information, but some of them allow straight full access to your cluster.

Health check ports are no security threat per se by the information they expose, but critical components like the network provider could be DoSed through an exposed health check port, which would affect the hole cluster.

Bastion host

Don't provide straight public SSH access to each Kubernetes node, use a bastion host setup where you expose SSH only on one specific host from which you SSH into all other hosts. There are quiet a few articles on how to do this, for example https://www.nadeau.tv/ssh-with-a-bastion-host/. Also SSH session recording as described in https://aws.amazon.com/blogs/security/how-to-record-ssh-sessions-established-through-a-bastion-host/ can be useful.

For general SSH hardening check Hardening OpenSSH and the OpenSSH chapter in Applied Crypto Hardening by bettercrypto.org.

API authorization mode

Some installers like kops will use the AlwaysAllow authorization mode for the cluster. This would grant any authenticated entity full cluster access. Instead RBAC should be used for Role-based access control. To find out what your current configuration is, check the --authorization-mode parameter of your kube-apiserver processes. More information on that topic at https://kubernetes.io/docs/admin/authorization/.

Note This doesn't affect the kubelet authorization mode. kubelet itself exposes an API to execute commands through which the Kubernetes API can be bypassed completely.

Kubelet authorization mode

Kubelet offers a command API used by kube-apiserver through which arbitrary commands can be executed on the specific node. On top of firewalling the port (10250/TCP) from public access, the kubelet settings --authorization-mode=Webhook and --anonymous-auth=false should be ensured.

Cloud provider host privileges

If you're running your cluster on a cloud provider, especially AWS you must know that by default every Pod inherits the privileges of it's Kubernetes node. This means, if you've policies in place allowing an EC2 instance to manipulate AWS resources your Pod has the except same power to do so too.

A fix is provided by the project kube2iam and kiam.

AWS Metadata API

For AWS only, you should definitely firewall access to the EC2 metdata API through which IAM role credentials can be queried. Find a fix for that issue at https://github.com/jtblin/kube2iam#iptables. You can check EC2 metadata API access from within a Pod by executing the following command which should fail.

curl -s http://169.254.169.254/latest/user-data

Auto mount default Service Account

The Admission Controller ensures that all Pods have a Service Account assigned by default, which is called "default". The credentials for this Service Account will be mounted into the containers file system running in the Pod unless the auto mounting feature is disabled. The mounted token can be used to query the Kubernetes API.

kubectl patch serviceaccount default -p "automountServiceAccountToken: false"

This will disable the auto mounting of the Service Account token and needs to be done on a per namespace basis.

Note For every new namespace, the Admission Controller will create the default Service Account. Changes to this Service Account need be applied accordingly.

Use Network Policies

Network Policies are firewall rules for Kubernetes. If you're using a network provider, which supports Network Policies you should definitely use them to secure internal cluster communication and external cluster access. By default there are no restrictions in place to limit Pods from communicating with each other.

Check Kubernetes Network Policy Recipes for an awesome starting point. If you're network provider doesn't support network policies, consider switching to one which does, check https://kubernetes.io/docs/concepts/cluster-administration/networking/.

Restrict "docker image pull"

Docker images are a completely uncontrolled environment. Everyone with access to the Docker socket or Kubernetes API can pull any image they like. Because of that many Kubernetes clusters secretly became Bitcoin miners, because of infected Docker images or Kubernetes security issues. The Docker plugin Docker Image policy plugin will help you with that problem. The plugin hooks into the internal Docker API and enforces a set of black and white list rules to restrict what images can be pulled.

Ultimately Docker is pulling an image, so securing Docker is considered a good approach but alternatively Kubernetes also provides a way. The AddmissionController provides the ImagePolicyWebhook through which a provided web service can intercept image pulls.

Kubernetes Dashboard

Typically, the kubernetes-dashboard plugin is granted a Service Account with full cluster access to be able to see and manage all aspects of the cluster.

TBD