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

What is CVE-2021-25735?

CVE-2021-25735 is a medium-severity vulnerability in Kubernetes Kubernetes, published on 6 Sep 2021. An attacker needs administrator access to exploit it, with no action from the victim.

Official description

A security issue was discovered in kube-apiserver that could allow node updates to bypass a Validating Admission Webhook. Clusters are only affected by this vulnerability if they run a Validating Admission Webhook for Nodes that denies admission based at least partially on the old state of the Node object. Validating Admission Webhook does not observe some previous fields.

Weakness: CWE-372

Which versions are affected by CVE-2021-25735?

7 open source packages ship an affected version, including k8s.io/kubernetes. It affects 1 product.

Products include Kubernetes Kubernetes.

How do I fix CVE-2021-25735?

Upgrade k8s.io/kubernetes to a fixed version for your release line: 1.18.18, 1.19.10 or 1.20.6.

Is CVE-2021-25735 being exploited?

There is no known exploitation of CVE-2021-25735 in the wild and no public exploit code.

Public exploit code: 1 proofs of concept on GitHub.

Other vulnerabilities in Kubernetes Kubernetes

• CVE-2018-1002105 Privilege Escalation in Kubernetes Critical 9.8
• CVE-2017-1000056 Kubernetes Privilege Escalation Critical 9.8
• CVE-2016-1906 Authorization bypass in Openshift Critical 9.8
• CVE-2023-3676 Kubernetes - Windows nodes - Insufficient input sanitization leads to privilege escalation High 8.8
• CVE-2023-3955 Kubernetes - Windows nodes - Insufficient input sanitization leads to privilege escalation High 8.8

All Kubernetes Kubernetes vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as CLEANSTART-2025-YC51954, DEBIAN-CVE-2021-25735, GHSA-g42g-737j-qx6j, GO-2022-0907.