[kubernetes] CVE-2021-25735: Validating Admission Webhook does not observe some previous fields

[Tim Allclair <timallclair () gmail com>]

A security issue was discovered in kube-apiserver that could allow node
updates to bypass a Validating Admission Webhook. You are only affected by
this vulnerability if you run a Validating Admission Webhook for Nodes that
denies admission based at least partially on the old state of the Node
object.

This issue has been rated Medium (
CVSS:3.0/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:H
<https://www.first.org/cvss/calculator/3.0#CVSS:3.0/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:H>),
and assigned CVE-2021-25735.

Note: This only impacts validating admission plugins that rely on old
values in certain fields, and does not impact calls from kubelets that go
through the built-in NodeRestriction admission plugin.
Affected Versions

   -

   kube-apiserver v1.20.0 - v1.20.5
   -

   kube-apiserver v1.19.0 - v1.19.9
   -

   kube-apiserver <= v1.18.17

Fixed Versions

This issue is fixed in the following versions:

   -

   kube-apiserver v1.21.0
   -

   kube-apiserver v1.20.6
   -

   kube-apiserver v1.19.10
   -

   kube-apiserver v1.18.18

Detection

If you find evidence that this vulnerability has been exploited, please
contact security () kubernetes io
Additional Details

See Kubernetes Issue #100096
<https://github.com/kubernetes/kubernetes/issues/100096> for more details.
Acknowledgements

This vulnerability was reported by Rogerio Bastos & Ari Lima from RedHat

Thank You,

Tim Allclair on behalf of the Kubernetes Product Security Committee