# CVE-2021-25736: Kubernetes kube-proxy traffic forwarding error on Windows

Source: https://junglewise.ai/threats/cve-2021-25736-kubernetes-kube-proxy-traffic-forwarding-error-on-windows  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2023-10-30. Updated: 2026-07-06  
License: CC BY 4.0 with a link back

CVE-2021-25736 · Severity: medium · CVSS 5.8 · Published 2023-10-30

A vulnerability in the Kubernetes networking component for Windows could allow network traffic to be misdirected to unintended local processes. This occurs when specific load balancer settings are missing, potentially allowing an attacker with high privileges to intercept data intended for legitimate services. This could lead to unauthorized access to sensitive information within the cluster environment.

Technologies: [k8s.io/kubernetes (Go)](https://junglewise.ai/threats/technologies/k8s-io-kubernetes), [Kubernetes](https://junglewise.ai/threats/technologies/kubernetes). Vendors: [Go](https://junglewise.ai/threats/vendors/go), [Kubernetes](https://junglewise.ai/threats/vendors/kubernetes).

## Quick answers

**Is CVE-2021-25736 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2021-25736 being exploited in the wild.

**How severe is CVE-2021-25736?** Medium severity, with a CVSS score of 5.8, published 30 October 2023.

**Which products and versions are affected?** k8s.io/kubernetes (Go) < 1.21.0, SEMVER:introduced=0,fixed=1.21.0; Kubernetes < 1.21.0.

**Is there a fix for CVE-2021-25736?** Yes. k8s.io/kubernetes (Go): upgrade to 1.21.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2021-25736 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2021-25736?** Check whether you run k8s.io/kubernetes (Go) or Kubernetes at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- k8s.io/kubernetes (Go): upgrade to 1.21.0 or later

  ```sh
  go get k8s.io/kubernetes@v1.21.0
  ```


## Technical details

A traffic misdirection vulnerability exists in kube-proxy on Windows nodes. The issue occurs when a LoadBalancer Service is created but the LoadBalancer controller fails to populate the `status.loadBalancer.ingress[].ip` field. In this scenario, kube-proxy may unintentionally forward traffic to local processes listening on the same port defined in `spec.ports[*].port`. Exploitation requires high privileges and specific network configurations (high complexity), but can result in a scope change allowing an attacker to intercept traffic. The issue is resolved in Kubernetes version 1.21.0.

## Affected products

- Kubernetes kubernetes < 1.21.0

## Identifiers

- CVE-2021-25736
- GHSA-35c7-w35f-xwgh
- GO-2023-2159

## Timeline

- 2023-10-30: advisory
- 2023-10-30: disclosed
- 2021-04-08: patched: Based on Kubernetes 1.21.0 release date and commit history

## References

- [https://api.github.com/users/cookesan](https://api.github.com/users/cookesan)
- [https://github.com/cookesan](https://github.com/cookesan)
- [https://api.github.com/users/cookesan/gists%7B/gist_id%7D](https://api.github.com/users/cookesan/gists%7B/gist_id%7D)
- [https://api.github.com/users/cookesan/repos](https://api.github.com/users/cookesan/repos)
- [https://avatars.githubusercontent.com/u/6601329?v=4](https://avatars.githubusercontent.com/u/6601329?v=4)
- [https://api.github.com/users/cookesan/events%7B/privacy%7D](https://api.github.com/users/cookesan/events%7B/privacy%7D)

## Related threats

- [kube-controller-manager is vulnerable to half-blind Server Side Request Forgery through in-tree Portworx StorageClass](https://junglewise.ai/threats/cve-2025-13281-kube-controller-manager-is-vulnerable-to-half-blind-server-side)
- [A SQL Injection vulnerability exists in Esri ArcGIS Server versions 11.3, 11.4 and 11.5 on Windows, Linux and…](https://junglewise.ai/threats/cve-2025-57870-a-sql-injection-vulnerability-exists-in-esri-arcgis-server)
- [GO-2025-3915 - Kubernetes Nodes can delete themselves by adding an OwnerReference in k8s.io/kubernetes](https://junglewise.ai/threats/cve-2025-5187-kubernetes-nodes-can-delete-themselves-by-adding-an-ownerreference)
- [GO-2025-3774 - Kubernetes allows nodes to bypass dynamic resource allocation authorization checks in k8s.io/kubernetes](https://junglewise.ai/threats/cve-2025-4563-kubernetes-allows-nodes-to-bypass-dynamic-resource-allocation)
- [GO-2025-3645 - Kubernetes did not effectively clear service account credentials in k8s.io/kubernetes](https://junglewise.ai/threats/cve-2019-11243-kubernetes-did-not-effectively-clear-service-account-credentials)