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

> Attackers can bypass Validating Admission Webhooks for Node updates if the webhook relies on previous object state, potentially allowing unauthorized node modifications.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 Sep 2021
- **Updated:** 16 Sep 2024
- **Also known as:** CLEANSTART-2025-YC51954, DEBIAN-CVE-2021-25735, GHSA-g42g-737j-qx6j, GO-2022-0907

## 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 (Incomplete Internal State Distinction)

## Which versions are affected by CVE-2021-25735?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `k8s.io/kubernetes` | Go | All versions | 1.18.18, 1.19.10, 1.20.6 |
| `k8s.io/kubernetes` | Go | All versions | 1.18.18 |
| `k8s.io/kubernetes` | Go | All versions | 1.18.18 |
| `k8s.io/kubernetes` | Go | From 1.19.0 | 1.19.10 |
| `k8s.io/kubernetes` | Go | From 1.19.0 | 1.19.10 |
| `k8s.io/kubernetes` | Go | From 1.20.0 | 1.20.6 |
| `k8s.io/kubernetes` | Go | From 1.20.0 | 1.20.6 |

Products: 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:

- Exploit CVE-2021-25735: Kubernetes Validating Admission Webhook Bypass (github_poc): https://github.com/darryk10/CVE-2021-25735

## Other vulnerabilities in Kubernetes Kubernetes

- [CVE-2018-1002105](https://insights.sbomify.com/CVE-2018-1002105/): Privilege Escalation in Kubernetes (critical 9.8)
- [CVE-2017-1000056](https://insights.sbomify.com/CVE-2017-1000056/): Kubernetes Privilege Escalation (critical 9.8)
- [CVE-2016-1906](https://insights.sbomify.com/CVE-2016-1906/): Authorization bypass in Openshift (critical 9.8)
- [CVE-2023-3676](https://insights.sbomify.com/CVE-2023-3676/): Kubernetes - Windows nodes - Insufficient input sanitization leads to privilege escalation (high 8.8)
- [CVE-2023-3955](https://insights.sbomify.com/CVE-2023-3955/): Kubernetes - Windows nodes - Insufficient input sanitization leads to privilege escalation (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-25735 and fix it if it is affected.

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

Affected packages and fixed versions:
- k8s.io/kubernetes (Go): affected all versions; fixed in 1.18.18, 1.19.10, 1.20.6
- k8s.io/kubernetes (Go): affected all versions; fixed in 1.18.18
- k8s.io/kubernetes (Go): affected all versions; fixed in 1.18.18
- k8s.io/kubernetes (Go): affected from 1.19.0; fixed in 1.19.10
- k8s.io/kubernetes (Go): affected from 1.19.0; fixed in 1.19.10
- k8s.io/kubernetes (Go): affected from 1.20.0; fixed in 1.20.6
- k8s.io/kubernetes (Go): affected from 1.20.0; fixed in 1.20.6

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-25735/ (Markdown: https://insights.sbomify.com/CVE-2021-25735/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-25735
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-25735
- OSV: https://osv.dev/vulnerability/CVE-2021-25735

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-25735/
Summaries are generated and may contain mistakes; the official description above is the record.