Access Restriction Bypass in kube-apiserver

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:H

EPSS

• CVE-2021-25735: 5.38% estimated exploitation activity in the next 30 days; percentile 92.67 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

A vulnerability in Kubernetes kube-apiserver could allow node updates to bypass a Validating Admission Webhook and allow unauthorized node updates. The information that is provided to the admission controller could contain old configurations that overwrite values used for validation. Since the overwriting takes place before the validation, this could lead the admission controller to accept requests that should be blocked. The vulnerability can be exploited when an update action on node resources is performed and an admission controller is in place and configured to validate the action. Users are only affected by this vulnerability if they are running a Validating Admission Webhook for Nodes that denies admission based partially on the old state of the Node object. It only impacts validating admission plugins that rely on old values in certain fields and does not impact calls from kubelets that go through the built-in NodeRestriction admission plugin.

Published by the source

28 May 2021, 19:49 UTC

Last modified by the source

31 Jul 2023, 20:31 UTC

Source last checked

26 Oct 2026, 14:58 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-g42g-737j-qx6j

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2021-25735

Advisory: Access Restriction Bypass in kube-apiserver
IDs: CVE-2021-25735, GHSA-g42g-737j-qx6j
Record: https://patchnext.com/advisories/cve-2021-25735 (revision 1, source checked 26 Oct 2026, 14:58 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: k8s.io/kubernetes 1.20.6, 1.19.10, 1.18.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go k8s.io/kubernetes: affected >= 1.20.0, <= 1.20.5; >= 1.19.0, <= 1.19.9; <= 1.18.17; fixed in 1.18.18, 1.19.10, 1.20.6

## Suggested inspection (read-only)
- Go k8s.io/kubernetes: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on k8s.io/kubernetes.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-g42g-737j-qx6j (GitHub-reviewed advisory GHSA-g42g-737j-qx6j lists affected ranges: Go k8s.io/kubernetes >=1.20.0 =1.19.0 =0 <=1.18.17)

Commands are suggestions: review each one before running it.