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

[Tim Allclair‏]

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), 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 secu...@kubernetes.io

Additional Details

See Kubernetes Issue #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