12 Kubernetes configuration best practices

Guest post by Ajmal Kohgadai, originally published on StackRox

By now most of us have heard about the role human error plays in causing data breaches. The Capital One breach from July is just the latest in a long line of security incidents that can trace their success back to a misconfigured infrastructure or security setting. As organizations accelerate their use of containers and Kubernetes and move their application development and deployment to cloud platforms, preventing avoidable misconfigurations in their environment becomes increasingly crucial.

Fortunately, most organizations understand that containers and Kubernetes are just like previous waves of infrastructure, where security starts with a securely configured infrastructure. In a recent survey of IT and security practitioners, respondents identified user-driven misconfigurations as their biggest concern for container security.

In this article, we will take a deep dive into key Kubernetes security configurations and recommended best practices you should follow.

It should be noted however that ensuring adherence to these best practices requires more than just knowing what they are. The level of success you have in consistently following these recommendations will also be determined by the degree to which you can automate the process of checking your environment for misconfigurations.

That’s because in a sprawling Kubernetes environment with several clusters spanning tens, hundreds, or even thousands of nodes, created by hundreds of different developers, manually checking the configurations is not feasible. And like all humans, developers can make mistakes – especially given that Kubernetes configuration options are complicated, security features are not enabled by default, and most of the community is learning how to effectively use components including Pod Security Policies and Security Context, Network Policies, RBAC, the API server, kubelet, and other Kubernetes controls.

As you and your teams come up to speed on all the details of Kubernetes security, follow these best practices to build a strong foundation:

1. Update Kubernetes to the latest version

If you haven’t already done so, update your Kubernetes deployments to the latest version (1.16), which includes several new and exciting features. Every new release is typically bundled with a host of different security features. Be sure to check out our blog post that highlights 7 reasons why you should upgrade Kubernetes to the latest version.

2. Use Pod Security Policies to prevent risky containers/Pods from being used

PodSecurityPolicy is a cluster-level resources available in Kubernetes (via kubectl) that is highly recommended. You must enable the PodSecurityPolicy admission controller to use it. Given the nature of admission controllers, you must authorize at least one policy – otherwise no pods will be allowed to be created in the cluster.

Pod Security Policies address several critical security use cases, including:

• Preventing containers from running with privileged flag – this type of container will have most of the capabilities available to the underlying host. This flag also overwrites any rules you set using CAP DROP or CAP ADD.
• Preventing sharing of host PID/IPC namespace, networking, and ports – this step ensures proper isolation between Docker containers and the underlying host
• Limiting use of volume types – writable hostPath directory volumes, for example, allow containers to write to the filesystem in a manner that allows them to traverse the host filesystem outside the pathPrefix, so readOnly: true must be used
• Putting limits on host filesystem use
• Enforcing read only for root file system via the ReadOnlyRootFilesystem
• Preventing privilege escalation to root privileges
• Rejecting containers with root privileges
• Restricting Linux capabilities to bare minimum in adherence with least privilege principles

Some of these attributes can also be controlled via securityContext. You can learn more about security context here. However, it’s generally recommended that you shouldn’t customize the pod-level security context but should instead use Pod Security Policies (see Recommendation #6 on how to apply these controls).

You can learn more about Pod Security Policies here.
You can learn more about admission controllers here and here.

3. Use Kubernetes namespaces to properly isolate your Kubernetes resources

Namespaces give you the ability to create logical partitions and enforce separation of your resources as well as limit the scope of user permissions. You can learn more about namespaces here.

4. Use Network Policies to segment and limit container and pod communication

Network Policies are used to determine how pods are allowed to communicate. Check out our blog post that takes a deep dive into building secure Kubernetes Network Policies.

5. Create policies to govern image provenance using the ImagePolicyWebhook

Prevent unapproved images from being used with the admission controller ImagePolicyWebhook to reject pods that use unapproved images including:

• Images that haven’t been scanned recently
• Images that use a base image that’s not whitelisted
• Images from insecure registries

You can learn more about ImagePolicyWebhook here.

6. Securely configure the Kubernetes API server

The Kubernetes API server handles all the REST API calls between external users and Kubernetes components.

Run the below command on your master node:

ps -ef | grep kube-apiserver

In the output, check to ensure that the:

• --anonymous-auth argument shows as false. This setting ensures that requests not rejected by other authentication methods are not treated as anonymous and therefore allowed against policy.
• --basic-auth-file argument isn’t there. Basic auth uses plaintext credentials, instead of the preferred tokens or certificates, for authentication.
• --insecure-allow-any-token argument isn’t there. This setting will ensure that only secure tokens that are authenticated are allowed.
• –kubelet-https argument either isn’t there or shows as true. This configuration ensures that connections between the API server and the kubelets are protected in transit via Transport Layer Security (TLS).
• --insecure-bind-address argument isn’t there. This configuration will prevent the API Server from binding to an insecure address, preventing non-authenticated and unencrypted access to your master node, which minimizes your risk of attackers potentially reading sensitive data in transit.
• --insecure-port argument shows as 0. This setting will prevent the API Server from serving on an insecure port, which would prevent unauthenticated and unencrypted access to the master node and minimize the risk of an attacker taking control of the cluster.
• --secure-port argument either doesn’t exist or shows up as an integer between 1 and 65535. The goal here is to make sure all your traffic is served over https with authentication and authorization.
• --profiling argument shows as false. Unless you’re experiencing bottlenecks or need to troubleshoot something that needs investigation, there’s no need for the profiler, and having it there unnecessarily opens you to exposure of system and program details.
• --repair-malformed-updates argument shows as false. This setting will ensure that intentionally malformed requests from clients are rejected by the API Server.
• --enable-admission-plugins argument is set with a value that doesn’t contain AlwaysAdmit. If you configure this setting to always admit, then it will admit requests even if they’re not explicitly allowed by the admissions control plugin, which would decrease the plugin’s effectiveness.
• --enable-admission-plugins argument is set with a value that contains AlwaysPullImages. This configuration ensures that users aren’t allowed to pull images from the node to any pod by simply knowing the name of the image. With this control enabled, images will always be pulled prior to starting a container, which will require valid credentials.
• --enable-admission-plugins argument is set with a value that contains SecurityContextDeny. This control ensures that you can’t customize pod-level security context in a way not outlined in the Pod Security Policy. See the Pod Security Policy section (#2) for additional information on security context.
• --disable-admission-plugins argument is set with a value that does not contain NamespaceLifecycle. You don’t want to disable this control, because it ensures that objects aren’t created in non-existent namespaces or in those namespaces set to be terminated.
• --audit-log-path argument is set to an appropriate path where you want your audit logs to be stored. It’s always a good security practice to enable auditing for any Kubernetes components, when available, including the Kubernetes API server.
• –-audit-log-maxage argument is set to 30 or whatever number of days you must store your audit log files to comply with internal and external data retention policies.
• --audit-log-maxbackup argument is set to 10 or any number that helps you meet your compliance requirements for retaining the number of old log files.
• --audit-log-maxsize argument is set to 100 or whatever number that helps you meet your compliance requirements. Note that number 100 represents 100 MB.
• --authorization-mode argument is there and is not set to AlwaysAllow. This setting ensures that only authorized requests are allowed by the API Server, especially in production clusters.
• --token-auth-file argument is not there. This argument, when present, uses static token-based authentication, which have several security flaws; use alternate authentication methods instead, such as certificates.
• --kubelet-certificate-authority argument is there. This setting helps prevent a man-in-the-middle attack when there’s a connection between the API Server and the kubelet.
• --kubelet-client-certificate and --kubelet-client-key arguments are there. This configuration ensures that the API Server authenticates itself to the kubelet’s HTTPS endpoints. (By default, the API Server doesn’t take this step.)
• --service-account-lookup argument is there and set to true. This setting helps prevent an instance where the API Server verifies only the validity of the authentication token without ensuring that the service account token included in the request is present in etcd.
• --enable-admission-plugins argument is set to a value that contains PodSecurityPolicy. See above section on Pod Security Policies (#2) for more details.
• --service-account-key-file argument is there and is set to a separate public/private key pair for signing service account tokens. If you don’t specify public/private key pair, it will use the private key from the TLS serving certificate, which would inhibit your ability to rotate the keys for service account tokens.
• --etcd-certfile and --etcd-keyfile arguments are there so that the API server identifies itself to the etcd server using client cert and key. Note that etcd stores objects that are likely sensitive in nature, so any client connections must use TLS encryption.
• --disable-admission-plugins argument is set and doesn’t contain ServiceAccount. This configuration will make sure that when a new pod is created, it will not use a default service account within the same namespace.
• --tls-cert-file and --tls-private-key-file arguments are there such that the API Server serves only HTTPS traffic via TLS.
• --client-ca-file argument exists to ensure that TLS and client cert authentication is configured for Kube cluster deployments.
• --etcd-cafile argument exists and it is set such that the API Server must verify itself to the etcd server via SSL Certificate Authority file.
• --tls-cipher-suites argument is set in a way that uses strong crypto ciphers.
• --authorization-mode argument is there with a value containing Node. This configuration limits which objects kubelets can read associated with their nodes.
• --enable-admission-plugins argument is set and contains the value NodeRestriction. This plugin ensures that a kubelet is allowed to modify only its own Node API object and those Pod API objects associated to its node.
• --encryption-provider-config argument is set to a EncryptionConfig file and this file should have all the needed resources. This setting ensures that all the REST API objects stored in the etcd key-value store are encrypted at rest.
• Make sure aescbc encryption provider is utilized for all desired resources as this provider of encryption is considered the strongest.
• --enable-admission-plugins argument contains the value EventRateLimit to set a limit on the number of events accepted by the API Server for performance optimization of the cluster.
• --feature-gates argument is not set with a value containing AdvancedAuditing=false. In other words, make sure advanced auditing is not disabled for auditing and investigation purposes.
• --request-timeout argument is either not set or set to an appropriate value (neither too short, nor too long). Default value is 60 seconds.
• --authorization-mode argument exists and is set to a value that includes RBAC. This setting ensures that Role-based access control (RBAC) is turned on. Beyond simply turning it on, you should follow several other recommendations for how to best use Kubernetes RBAC, including:
  • Avoid giving users cluster-admin role because it gives very broad powers over the environment and should be used very sparingly, if at all.
  • Audit your role aggregation rules to ensure you’re using them properly
  • Don’t grant duplicated permissions to subjects because it can make access revocation more difficult
  • Regularly remove unused Roles

7. Securely configure the kube-scheduler

As the default scheduler for Kubernetes, kube-scheduler selects the node that a newly created Pod should run on. You can learn more about kube-scheduler here.

Run the below command on your master node:

ps -ef | grep kube-scheduler

In the output, check to ensure that the:

• --profiling argument is set to false so that you have a reduced attack surface. While profiling can be useful when you have a performance bottleneck by identifying the bottleneck, it can also be exploited to reveal details about your system.
• --address argument is set to 127.0.0.1 so that the scheduler is not bound to a non-loopback insecure address, since the scheduler API service is available without authentication or encryption.

8. Securely configure the kube-controller-manager

Run the below command on your master node:

ps -ef | grep kube-controller-manager

In the output, check to ensure that the:

• --terminated-pod-gc-threshold argument is set to a value that ensures you have enough resources available and performance isn’t degraded.
• --profilingargument is set to false.
• --use-service-account-credentials argument is set to true. When combined with RBAC, this setting ensures that control loops run with minimum permissions required to adherence to least privilege design principles.
• --service-account-private-key-file argument is set such that a separate public/private key pair is used for signing service account tokens.
• --root-ca-file argument exists and is set to a cert file containing the root cert for the API Server’s serving cert, which will allow pods to verify the API Server’s serving cert before making a connection.
• RotateKubeletServerCertificate argument is there and set as true, and applies only when kubelets get their certs from the API Server.
• --address argument is set to 127.0.0.1, so that the controller manager service is not bound to non-loopback insecure addresses.

Secure the configuration files on the master node

Secure the API server pod specification file permissions.

Run the following command on the master node (specifying your file location on your system):

stat -c %a /etc/kubernetes/manifests/kube-apiserver.yaml

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the API Server pod specification file ownership.

Run the following command on the master node (specifying your file location on your system):

stat -c %U:%G /etc/kubernetes/manifests/kube-apiserver.yaml

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the controller manager pod specification file permissions.

Run the following command on the master node (specifying your file location on your system):

stat -c %a /etc/kubernetes/manifests/kube-controller-manager.yaml

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the controller manager pod specification file ownership.

Run the following command on the master node (specifying your file location on your system):

stat -c %U:%G /etc/kubernetes/manifests/kube-controller-manager.yaml

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the scheduler pod specification file permissions.

Run the following command on the master node (specifying your file location on your system):

stat -c %a /etc/kubernetes/manifests/kube-scheduler.yaml

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the scheduler pod specification file ownership.

Run the following command on the master node (specifying your file location on your system):

stat -c %U:%G /etc/kubernetes/manifests/kube-scheduler.yaml

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the etcd pod specification file permissions.

Run the following command on the master node (specifying your file location on your system):

stat -c %a /etc/kubernetes/manifests/etcd.yaml

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file. As a reminder on a topic already discussed, etcd is a key-value store, and protecting it is of the utmost importance, since it contains your REST API objects.

Secure the etcd pod specification file ownership.

Run the following command on the master node (specifying your file location on your system):

stat -c %U:%G /etc/kubernetes/manifests/etcd.yaml

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the Container Network Interface file permissions.

Run the following command on the master node (specifying your file location on your system):

stat -c %a <path/to/cni/files>

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the Container Network Interface file ownership.

Run the following command on the master node (specifying your file location on your system):

stat -c %U:%G <path/to/cni/files>

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the etcd data directory permissions.

First run the following command to the get etcd data directory:

ps -ef | grep etcd

Now run the following command based on the etcd data directory you found from the previous command:

stat -c %a /var/lib/etcd

In the output, check to ensure that permissions are 700 or more restrictive to ensure your etcd data directory is protected against unauthorized reads/writes.

Secure the etcd data directory ownership.

First run the following command to the get etcd data directory:

ps -ef | grep etcd

Now run the following command based on the etcd data directory you found from the previous command:

stat -c %U:%G /var/lib/etcd

In the output, check to ensure that ownership is etcd:etcd to ensure your etcd data directory is protected against unauthorized reads/writes.

Secure the admins.conf file permissions.

Run the following command on the master node (specifying your file location on your system):

stat -c %a /etc/kubernetes/admin.conf

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the admins.conf file ownership.

Run the following command on the master node (specifying your file location on your system):

stat -c %U:%G /etc/kubernetes/admin.conf

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the scheduler.conf file permissions.

Run the following command on the master node (specifying your file location on your system):

stat -c %a /etc/kubernetes/scheduler.conf

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the scheduler.conf file ownership.

Run the following command on the master node (specifying your file location on your system):

stat -c %U:%G /etc/kubernetes/scheduler.conf

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the controller-manager.conf file permissions.

Run the following command on the master node (specifying your file location on your system):

stat -c %a /etc/kubernetes/controller-manager.conf

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the controller-manager.conf file ownership.

Run the following command on the master node (specifying your file location on your system):

stat -c %U:%G /etc/kubernetes/controller-manager.conf

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the Kubernetes PKI directory and file ownership.

Run the following command on the master node (specifying your file location on your system):

ls -laR /etc/kubernetes/pki/

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the Kubernetes PKI directory and file permissions.

Run the following command on the master node (specifying your file location on your system):

ls -laR /etc/kubernetes/pki/*.crt

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the Kubernetes PKI key file permissions.

Run the following command on the master node (specifying your file location on your system):

ls -laR /etc/kubernetes/pki/*.key

In the output, check to ensure that permissions are 600 to maintain the integrity of the file.

10. Securely configure etcd

As mentioned in previous sections, etcd (a CNCF project) is a key-value store (a CNCF project) used by distributed systems such as Kubernetes for data access. etcd is considered the source of truth for Kubernetes, and you can read data from and write into etcd as needed. Securely configuring etcd and communications to its servers are of utmost criticality.

Run the following command on the etcd server node:

ps -ef | grep etcd

In the output, check to ensure that the:

• --cert-file and the --key-file arguments are set as needed to ensure client connections are served only over TLS (in transit encryption).
• --client-cert-auth argument shows as true to ensure all access attempts from clients include a valid client cert.
• --auto-tls argument is there and is not true, or isn’t there at all, which will prohibit clients from using self-signed certs for TLS.
• If you’re using a etcd cluster (instead of a single etcd server), check to see that --peer-cert-file and --peer-key-file arguments are appropriately set to ensure etcd peer connections is encrypted within the etcd cluster. In addition, check that --peer-client-cert-auth argument is set to true, as this setting would ensure that only authenticated etcd peers can access the etcd cluster. Lastly verify that if --peer-auto-tls argument is there, it is not set to true.
• As a best practice, don’t use the same certificate authority for etcd as you do for Kubernetes. You can ensure this separation by verifying that the file referenced by the --client-ca-file for API Server is different from the --trusted-ca-file used by etcd.

11. Securely configure the Kubelet

The kubelet is the main “node agent” running on each node. Misconfiguring kubelet can expose you to a host of security risks, as this Medium article last year outlines. You can either use arguments on the running kubelet executable or a kubelet config file to set the configuration of your kubelet.

To find the kubelet config file, run the following command:

ps -ef | grep kubelet | grep config

Look for --config argument, which will give you the location of the kubelet config file.

Then run the following command on each node:

ps -ef | grep kubelet

In the output, make sure that the:

• --anonymous-auth argument is false. In the kubelet article previously referenced, one of the misconfigurations exploited was one where anonymous (and unauthenticated) requests were allowed to be served by the kubelet server.
• --authorization-mode argument shows as AlwaysAllow if it’s there. If it is not there, make sure there’s a kubelet config file specified by --config and that file has set authorization: mode to something besides AlwaysAllow.
• --client-ca-file argument is there and set to the location of the client certificate authority file. If it’s not there, make sure there’s a kubelet config file specified by --config and that file has set authentication: x509: clientCAFile to the location of the client certificate authority file.
• --read-only-port argument is there and set to 0. If it’s not there, make sure there’s a kubelet config file specified by --config, and readOnlyPort is set to 0 if it’s there.
• --protect-kernel-defaults shows as true. If it’s not there, make sure there’s a kubelet config file specified by --config, and that file has set protectKernelDefaults as true.
• --hostname-override argument is not there, to ensure that the TLS setup between the kubelet and the API Server doesn’t break.
• --event-qps argument is there and set to 0. If it’s not there, make sure there’s a kubelet config file specified by --config and eventRecordQPS shows as 0.
• --tls-cert-file and --tls-private-key-file arguments are set appropriately or the kubelet config specified by --config contains appropriate settings for tlsCertFile and tlsPrivateKeyFile. This configuration ensures that all connections happen over TLS on the kubelets.
• RotateKubeletServerCertificate and --rotate-certificates is set to true if your kubelets get their certs from the API Server, and make sure your kubelet uses only strong crypto ciphers.

12. Secure the worker node configuration files

Secure the kubelet service file permissions.

Run the following command on each worker node (specifying your file location on your system):

stat -c %a /etc/systemd/system/kubelet.service.d/10-kubeadm.conf

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the kubelet.conf file permissions.

Run the following command on each worker node (specifying your file location on your system):

stat -c %a /etc/kubernetes/kubelet.conf

In the output, check to ensure that permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the kubelet.conf file ownership.

Run the following command on each worker node (specifying your file location on your system):

stat -c %U:%G /etc/kubernetes/kubelet.conf

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the kublete service file ownership.

Run the following command on each worker node (specifying your file location on your system):

stat -c %U:%G /etc/systemd/system/kubelet.service.d/10-kubeadm.conf

In the output, check to ensure that ownership is set as root:root to maintain the integrity of the file.

Secure the proxy kubeconfig file permissions.

Run the following command to first find the kubeconfig file being used:

ps -ef | grep kube-proxy

Get the kube-proxy file location (if it’s running) from --kubeconfig, then run the following command on each worker node (specifying your file location on your system).

stat -c %a <proxy kubeconfig file>

In the output, check to make sure permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the proxy kubeconfig file ownership.

Run the following command first to find the kubeconfig file being used:

ps -ef | grep kube-proxy

Get the kube-proxy file location (if it’s running) from --kubeconfig, then run the following command on each worker node (specifying your file location on your system):

stat -c %U:%G <proxy kubeconfig file>

In the output, check to make sure ownership is set as root:root to maintain the integrity of the file.

Secure the certificate authorities file permissions.

Run the following command first:

ps -ef | grep kubelet

Look for the file name that’s identified by --client-ca-file argument. Then run the following command, specifying the previous file name:

stat -c %a <filename>

In the output, check to make sure permissions are 644 or more restrictive to maintain the integrity of the file.

Secure the client certificate authorities file ownership.

Run the following command first:

ps -ef | grep kubelet

Look for the file name that’s identified by –-client-ca-file argument. Then run the following command, specifying the previous file name:

stat -c %U:%G <filename>

In the output, check to make sure ownership is set asroot:root to maintain the integrity of the file.

Secure the kubelet configuration file permissions.

First locate the kubelet config file with following command:

ps -ef | grep kubelet | grep config

In the output, you may see the location of the config file if it exists. It would look something like /var/lib/kubelet/configuration.yaml.

Using the location of the file (we’ll use the file location from this previous example), run the following command to identify the file’s permissions:

stat -c %a /var/lib/kubelet/configuration.yaml

In the output, check to make sure permissions are set to 644 or more restrictive to ensure the integrity of the file.

Secure the kubelet configuration file ownership.

Run the following command:

ps -ef | grep kubelet | grep config

In the output, you may see the location of the config file if it exists – it would look something like /var/lib/kubelet/configuration.yaml.

Using the location of the file (we’ll use the file location from this previous example), run the following command to identify the file’s permissions:

stat -c %U:%G /var/lib/kubelet/configuration.yaml

In the output, check to make sure ownership is set to root:root to maintain the integrity of the file.

This cloud-native stack offers compelling capabilities for building the most secure applications we’ve ever created – we just need to make sure we’ve got all the knobs and dials set correctly. Leverage these configurations, code examples, and detailed recommendations to avoid the security risks associated with the most common Kubernetes misconfigurations.