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Fencer Intel
medium 5.8

CVE-2021-25736 json

Windows kube-proxy LoadBalancer contention

published 3y ago modified last mo. also GHSA-35c7-w35f-xwgh , GO-2023-2159 , EUVD-2023-2624

cvss
5.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.9%
EPSS 0.9% chance of exploitation in 30 days, percentile 60%
public exploits
0
none known

Summary

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.

Patch / fix

  • go k8s.io/kubernetes 1.21.0
  • kubernetes kubernetes 1.18.18, 1.19.10, 1.20.6
  • govulncheck-vulndb openSUSE Tumbleweed 0.0.20250807T150727-1.1

opensuse advisories openSUSE-SU-2025:15424-1

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-25736 updated cvss 5.8

kubernetes: LoadBalancer Service type don't create a HNS policy for empty or invalid external loadbalancer IP, what could lead to MITM

  • Red Hat OpenShift Container Platform 4.7 openshift4-wincw/windows-machine-config-rhel8-operator 2.0.1-6, openshift4-wincw/windows-machine-config-operator-bundle v2.0.1-8
    fixed: vendor fix RHSA-2021:2130

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
last yr.

Exploit prediction

0.92% chance of exploitation in the next 30 days, higher than 60% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
CNA 3.1 5.8 CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:H/I:N/A:N
NVD 3.1 6.3 CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N
GitHub 3.1 5.8 CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:H/I:N/A:N

Affected packages

ecosystem package affected fixed in
go k8s.io/kubernetes
<1.21.0
1.21.0

Affected products

  • Kubernetes Kubernetes

Classification

Weakness
Technology feeds

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.