Home/CVE/A security issue was discovered in kube-apiserver that could allow node updates to bypass a Validating Admission Webhook
CVE

CVE-2021-25735

A security issue was discovered in kube-apiserver that could allow node updates to bypass a Validating Admission Webhook

A security issue was discovered in kube-apiserver that could allow node updates to bypass a Validating Admission Webhook. Clusters are only affected by this vulnerability if they run a Validating Admission Webhook for Nodes that denies admission based at least partially on the old state of the Node object. Validating Admission Webhook does not observe some previous fields.

MEDIUM · CVSS 6.5 EPSS 0.16302
Schedule remediation
  • EPSS ≥ 0.10 - elevated exploitation probability
  • EPSS percentile: top 5% of all CVEs by exploitation likelihood
Sigma rules8 YARA rules0
⬡

Weakness Classification

CWE-372Incomplete Internal State Distinction
▤

Affected Products & Versions

3
kubernetes< 1.18.18
kubernetes>= 1.19.0 and < 1.19.10
kubernetes>= 1.20.0 and < 1.20.6
▤

Affected Packages

1
Language-ecosystem packages (from OSV) tied to this CVE, with the version that fixes it - the dependency-level detail NVD doesn’t carry.
Go k8s.io/kubernetes MODERATE fixed in 1.20.6
◈

Sigma Hunt Rules

8
Exact rules name this CVE ID. Product rules name an affected product in their title. Related rules cover techniques used by actors who exploited this CVE. Showing the most relevant matches; the complete related set is on the full drill-down.
productmediumKubernetes Events Deleted
productmediumKubernetes Admission Controller Modification
productmediumKubernetes CronJob/Job Modification
productmediumKubernetes Potential Enumeration Activity
productmediumKubernetes Secrets Modified or Deleted
productmediumKubernetes Rolebinding Modification
▣

Scoring & Timeline

6.5
MEDIUM · CVSS v3.1 · jordan@liggitt.net
View on NVD
Attack Vector
Network Adjacent Local Physical
Attack Complexity
Low High
Privileges Required
None Low High
User Interaction
None Required
Scope
Unchanged Changed
Confidentiality
None Low High
Integrity
None Low High
Availability
None Low High
Published to NVD06 Sep 2021 · 12:15 PM
CVSS VectorCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:H
⚑

Vendor Advisories

2
suse-csafopenSUSE-SU-2025:15424-1
rhsaRHSA-2021:2437Moderate
🔗

References & Sources

2
Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://github.com/kubernetes/kubernetes/issues/100096PatchThird Party Advisory
https://groups.google.com/g/kubernetes-security-announce/c/FKAGqT4jx9YMailing ListThird Party Advisory
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