CVE-2021-25736: K8s.io/kubernetes Kube-proxy May Unintentionally Forward Traffic Vulnerability

MODERATE
Advisory Identifier: GHSA-35c7-w35f-xwgh
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CVSS 5.8
Published
Advisory Summary

CVE-2021-25736 is a MODERATE (CVSS 5.8) kube-proxy may unintentionally forward traffic vulnerability (General Security Weakness) affecting K8s.io/kubernetes libraries in the Go ecosystem. The issue involves kube-proxy may unintentionally forward traffic. Remediation is available by upgrading affected packages to version 1.21.0.

CVE-2021-25736 at a Glance

CVE Identifier CVE-2021-25736
GHSA Identifier GHSA-35c7-w35f-xwgh
Severity MODERATE
CVSS v3.1 Base Score 5.8
CWE Classification General Security Weakness
Affected Product K8s.io/kubernetes
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 5.8

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:H/I:N/A:N).

Attack Vector Network (AV:N)
Attack Complexity High (AC:H)
Privileges Required High (PR:H)
User Interaction None (UI:N)
Scope Changed (S:C)
Confidentiality High (C:H)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2021-25736

To remediate this vulnerability, upgrade all affected K8s.io/kubernetes dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

k8s.io/kubernetes fixed: 1.21.0 
Upgrade k8s.io/kubernetes to 1.21.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for k8s.io/kubernetes

How to Check if You Are Affected

To determine whether your application is affected by CVE-2021-25736, inspect your project dependencies for K8s.io/kubernetes:

inspect dependencies for k8s.io/kubernetes

Analyze the terminal output to locate any version of K8s.io/kubernetes in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.21.0), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under General Security Weakness. In K8s.io/kubernetes, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

CVE-2021-25736 Affected K8s.io/kubernetes Versions

1 component(s)
Fixed Versions:
1.21.0
Product / Package Ecosystem Affected Versions Fixed Version
k8s.io/kubernetes
golang/k8s.io/kubernetes
Go
< 1.21.0
1.21.0

References

NVD — CVE-2021-25736 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-25736
K8s.io/kubernetes Source Repository REPOSITORY
https://github.com/kubernetes/kubernetes/pull/99958
Security Fix Commit FIX COMMIT
https://github.com/kubernetes/kubernetes/commit/b014610de3e5cf1bb0f7844b5758d29fc18b75e6
K8s.io/kubernetes Source Repository REPOSITORY
https://github.com/kubernetes/kubernetes
K8s.io/kubernetes Security Information SECURITY
https://groups.google.com/g/kubernetes-security-announce/c/lIoOPObO51Q/m/O15LOazPAgAJ
K8s.io/kubernetes Security Information SECURITY
https://security.netapp.com/advisory/ntap-20231221-0003

Frequently Asked Questions

What is CVE-2021-25736?
CVE-2021-25736 is a security vulnerability in K8s.io/kubernetes involving Kube-proxy may unintentionally forward traffic under General Security Weakness.
Which K8s.io/kubernetes versions are affected?
Affected versions include releases prior to 1.21.0 across affected packages: k8s.io/kubernetes.
What version fixes CVE-2021-25736?
Upgrading to 1.21.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is k8s.io/kubernetes affected?
Yes, k8s.io/kubernetes is affected in releases prior to 1.21.0. Upgrade your dependency to 1.21.0 or later.
What is the CVSS score of CVE-2021-25736?
CVE-2021-25736 has a CVSS v3.1 base score of 5.8 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:H/I:N/A:N.
What CWE is associated with CVE-2021-25736?
CVE-2021-25736 is classified under General Security Weakness.
How can I check Go dependencies for CVE-2021-25736?
Run the dependency tree command: 'inspect dependencies for k8s.io/kubernetes'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-35c7-w35f-xwgh",
  "modified" : "2026-07-06T16:26:45Z",
  "published" : "2023-10-30T03:30:15Z",
  "aliases" : [ "CVE-2021-25736" ],
  "summary" : "Kube-proxy may unintentionally forward traffic",
  "details" : "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 \nwhere the LoadBalancer controller sets the `status.loadBalancer.ingress[].ip` field are unaffected.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "k8s.io/kubernetes"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.21.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-25736"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/kubernetes/kubernetes/pull/99958"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/kubernetes/kubernetes/commit/b014610de3e5cf1bb0f7844b5758d29fc18b75e6"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/kubernetes/kubernetes"
  }, {
    "type" : "WEB",
    "url" : "https://groups.google.com/g/kubernetes-security-announce/c/lIoOPObO51Q/m/O15LOazPAgAJ"
  }, {
    "type" : "WEB",
    "url" : "https://security.netapp.com/advisory/ntap-20231221-0003"
  } ],
  "database_specific" : {
    "cwe_ids" : [ ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2023-10-31T22:23:04Z",
    "nvd_published_at" : "2023-10-30T03:15:07Z"
  }
}