# CVE-2021-25736: Windows kube-proxy LoadBalancer contention

> Windows kube-proxy may forward LoadBalancer traffic to local processes if the controller omits the ingress IP, allowing local access to internal services.

- **Severity:** Medium 5.8 (`CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 30 Oct 2023
- **Updated:** 12 Jun 2025
- **Also known as:** GHSA-35c7-w35f-xwgh, GO-2023-2159

## What is CVE-2021-25736?

CVE-2021-25736 is a medium-severity server-side request forgery vulnerability in Kubernetes Kubernetes, published on 30 Oct 2023. An attacker needs administrator access to exploit it, with no action from the victim.

### Official description

Kube-proxy
 on Windows can unintentionally forward traffic to local processes 
listening on the same port (“spec.ports[*].port”) as a LoadBalancer 
Service when the LoadBalancer controller
 does not set the “status.loadBalancer.ingress[].ip” field. Clusters 
where the LoadBalancer controller sets the 
“status.loadBalancer.ingress[].ip” field are unaffected.

Weakness: CWE-114 (Process Control)

## Which versions are affected by CVE-2021-25736?

3 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 | 1.21.0 |
| `k8s.io/kubernetes` | Go | All versions | 1.21.0 |
| `k8s.io/kubernetes` | Go | All versions | 1.21.0 |

Products: Kubernetes Kubernetes.

## How do I fix CVE-2021-25736?

Upgrade k8s.io/kubernetes to 1.21.0 or later.

## Is CVE-2021-25736 being exploited?

There is no known exploitation of CVE-2021-25736 in the wild and no public exploit code.

## 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-2021-25736 and fix it if it is affected.

What it is: CVE-2021-25736 is a medium-severity server-side request forgery vulnerability in Kubernetes Kubernetes, published on 30 Oct 2023. An attacker needs administrator access to exploit it, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-25736 in the wild and no public exploit code.

Affected packages and fixed versions:
- k8s.io/kubernetes (Go): affected all versions; fixed in 1.21.0
- k8s.io/kubernetes (Go): affected all versions; fixed in 1.21.0
- k8s.io/kubernetes (Go): affected all versions; fixed in 1.21.0

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-2021-25736/ (Markdown: https://insights.sbomify.com/CVE-2021-25736/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-25736
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-25736
- OSV: https://osv.dev/vulnerability/CVE-2021-25736

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-25736/
Summaries are generated and may contain mistakes; the official description above is the record.