[Security Advisory] CVE-2023-2727: Bypassing policies imposed by the ImagePolicyWebhook admission plugin

[Rita Zhang]

Hello Kubernetes Community,

A security issue was discovered in Kubernetes where users may be able to launch containers using images that are restricted by ImagePolicyWebhook when using ephemeral containers. Kubernetes clusters are only affected if the ImagePolicyWebhook admission plugin is used together with ephemeral containers.

This issue has been rated Medium (CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:N), and assigned CVE-2023-2727

Am I vulnerable?

Clusters are impacted by this vulnerability if all of the following are true:

1. The ImagePolicyWebhook admission plugin is used to restrict use of certain images
2. Pods are using ephemeral containers.

Affected Versions

• kube-apiserver v1.27.0 - v1.27.2
• kube-apiserver v1.26.0 - v1.26.5
• kube-apiserver v1.25.0 - v1.25.10
• kube-apiserver <= v1.24.14

How do I mitigate this vulnerability?

This issue can be mitigated by applying the patch provided for the kube-apiserver component. This patch prevents ephemeral containers from using an image that is restricted by ImagePolicyWebhook.

Note: Validation webhooks (such as Gatekeeper and Kyverno) can also be used to enforce the same restrictions.

Fixed Versions

• kube-apiserver v1.27.3
• kube-apiserver v1.26.6
• kube-apiserver v1.25.11
• kube-apiserver v1.24.15

These releases have been published today, June 14th, 2023.

Detection
Pod update requests using an ephemeral container with an image that should have been restricted by an ImagePolicyWebhook will be captured in API audit logs. You can also use `kubectl get pods` to find active pods with ephemeral containers running an image that should have been restricted in your cluster with this issue.

Additional Details

See the GitHub issue for more details: https://github.com/kubernetes/kubernetes/issues/118640

Thank You,
Rita Zhang on behalf of the Kubernetes Security Response Committee