Cis K8S V1111 1.2.11

1.2.11 Ensure that the admission control plugin AlwaysPullImages is set (Manual)

Profile Applicability

• Level 1 - Master Node

Description

Always pull images.

Rationale

Setting admission control policy to AlwaysPullImages forces every new pod to pull the required images every time. In a multi-tenant cluster users can be assured that their private images can only be used by those who have the credentials to pull them. Without this admission control policy, once an image has been pulled to a node, any pod from any user can use it simply by knowing the image's name, without any authorization check against the image ownership. When this plug-in is enabled, images are always pulled prior to starting containers, which means valid credentials are required.

Impact

Credentials would be required to pull the private images every time. Also, in trusted environments, this might increases load on network, registry, and decreases speed.

This setting could impact offline or isolated clusters, which have images preloaded and do not have access to a registry to pull in-use images. This setting is not appropriate for clusters which use this configuration.

Audit

Run the following command on the Control Plane node:

ps -ef | grep kube-apiserver

Verify that the --enable-admission-plugins argument is set to a value that includes AlwaysPullImages.

Remediation

Edit the API server pod specification file /etc/kubernetes/manifests/kube-apiserver.yaml on the Control Plane node and set the --enable-admission-plugins parameter to include AlwaysPullImages.

--enable-admission-plugins=...,AlwaysPullImages,...

Default Value

By default, AlwaysPullImages is not set.

References

1. https://kubernetes.io/docs/reference/command-line-tools-reference/kube-apiserver/
2. https://kubernetes.io/docs/reference/access-authn-authz/admission-controllers/#alwayspullimages

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