# CVE-2021-25737: Holes in EndpointSlice Validation Enable Host Network Hijack

> Attackers can redirect pod traffic to private node networks by bypassing EndpointSlice IP validation, potentially accessing internal services.

- **Severity:** Low 2.7 (`CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:L/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:** 6 Sep 2021
- **Updated:** 16 Sep 2024
- **Also known as:** CLEANSTART-2025-VV56547, CLEANSTART-2026-TF89729, DEBIAN-CVE-2021-25737, GHSA-mfv7-gq43-w965, GO-2022-0908

## What is CVE-2021-25737?

CVE-2021-25737 is a low-severity server-side request forgery 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 Kubernetes where a user may be able to redirect pod traffic to private networks on a Node. Kubernetes already prevents creation of Endpoint IPs in the localhost or link-local range, but the same validation was not performed on EndpointSlice IPs.

Weakness: CWE-601 (URL Redirection to Untrusted Site), CWE-184 (Incomplete List of Disallowed Inputs)

## Which versions are affected by CVE-2021-25737?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `k8s.io/kubernetes` | Go | From 1.16.0 | 1.18.19, 1.19.11, 1.20.7, 1.21.1 |
| `k8s.io/kubernetes` | Go | From 1.16.0 | 1.18.19 |
| `k8s.io/kubernetes` | Go | From 1.16.0 | 1.18.19 |
| `k8s.io/kubernetes` | Go | From 1.19.0 | 1.19.11 |
| `k8s.io/kubernetes` | Go | From 1.19.0 | 1.19.11 |
| `k8s.io/kubernetes` | Go | From 1.20.0 | 1.20.7 |
| `k8s.io/kubernetes` | Go | From 1.20.0 | 1.20.7 |
| `k8s.io/kubernetes` | Go | From 1.21.0 | 1.21.1 |
| `k8s.io/kubernetes` | Go | From 1.21.0 | 1.21.1 |

Products: Kubernetes Kubernetes.

## How do I fix CVE-2021-25737?

Upgrade k8s.io/kubernetes to a fixed version for your release line: 1.18.19, 1.19.11, 1.20.7 or 1.21.1.

## Is CVE-2021-25737 being exploited?

There is no known exploitation of CVE-2021-25737 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-2021-25737 and fix it if it is affected.

What it is: CVE-2021-25737 is a low-severity server-side request forgery 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-2021-25737 in the wild and no public exploit code.

Affected packages and fixed versions:
- k8s.io/kubernetes (Go): affected from 1.16.0; fixed in 1.18.19, 1.19.11, 1.20.7, 1.21.1
- k8s.io/kubernetes (Go): affected from 1.16.0; fixed in 1.18.19
- k8s.io/kubernetes (Go): affected from 1.16.0; fixed in 1.18.19
- k8s.io/kubernetes (Go): affected from 1.19.0; fixed in 1.19.11
- k8s.io/kubernetes (Go): affected from 1.19.0; fixed in 1.19.11
- k8s.io/kubernetes (Go): affected from 1.20.0; fixed in 1.20.7
- k8s.io/kubernetes (Go): affected from 1.20.0; fixed in 1.20.7
- k8s.io/kubernetes (Go): affected from 1.21.0; fixed in 1.21.1
- k8s.io/kubernetes (Go): affected from 1.21.0; fixed in 1.21.1

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-2021-25737/ (Markdown: https://insights.sbomify.com/CVE-2021-25737/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-25737
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-25737
- OSV: https://osv.dev/vulnerability/CVE-2021-25737

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-25737/
Summaries are generated and may contain mistakes; the official description above is the record.