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NVD / CVE Program· CNA kubernetes

CVE-2021-25735: Validating Admission Webhook does not observe some previous fields

MediumCVSS 6.5 · v3.1—Proof of concept Fix available

Noroxi assessment

Priority: Monitor (action score 28/100)

  • Public proof-of-concept code exists (GitHub); there is no record of exploitation in the wild.
  • Probability of exploitation in the next 30 days: 5.3% (FIRST EPSS); higher than 92.7% of all records.
  • Anyone who can reach it over the internet can trigger it. Administrator privileges are required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: k3s (Alpine:v3.14) 1.20.6.1-r0; kubernetes (Debian:12) 1.20.5+really1.20.2-1; k8s.io/kubernetes (Go) 1.19.10, 1.20.6, 1.18.18.

Low priority for now. Upgrade to k3s (Alpine:v3.14) 1.20.6.1-r0; kubernetes (Debian:12) 1.20.5+really1.20.2-1; k8s.io/kubernetes (Go) 1.19.10, 1.20.6, 1.18.18 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

Published
Sep 6, 2021
Updated
Jun 16, 2026
EPSS
5.3% · 92th percentile

Report tools

JSON

Action score

28

Monitor

Low priority for now.

CVSS
26 / 40 · 6.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
2 / 30 · 5.3%

CNA vs NVD score

NVD
6.5
CNA · kubernetes
6.5
agree

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • Proof of concept exists, no weaponized module
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-25735

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
kuberneteskubernetes

Using kubernetes? Add it to your stack and be the first to know about CVEs like this one. (Kubernetes)

Watch new CVEs for kubernetes

When you'll hear from us

  • when a new CVE affecting this product is published
  • when a record in your stack enters KEV or is weaponized

One daily email digest (06:00 UTC); connect a webhook for KEV listings and weaponized exploits every 10 minutes on paid plans, on the hour on the free plan.

Your free account also gets you

  • CVE alerts: add the products and versions you run, get the new CVEs that affect them
  • Uptime monitoring: 1 site, instant email when it goes down
  • Add your domain: subdomains and takeover risk scanned every week (1 domain)
  • 10 scans an hour on the security tools, at most 30 a day (3 a day without an account), an API key and MCP
  • All your scans in one panel: findings, what changed in the last scan, a report per domain or per client (PDF, share link)
All free tools

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Free account, no card. Unfollow any time.

Don't want an account? Get "Exploited this week" by email every Monday

Versions reported by the vendor

Affected version ranges reported by the assigning authority (kubernetes). Independent of NVD's CPE analysis and usually ahead of it.

  • Kubernetes Kubernetes

    • unspecified and later · up to and including 1.18.17affected
    • unspecified and later · up to and including 1.19.9affected
    • unspecified and later · up to and including 1.20.5affected

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • kubernetes kubernetesbefore 1.18.18
  • kubernetes kubernetes1.19.0 and later · before 1.19.10
  • kubernetes kubernetes1.20.0 and later · before 1.20.6

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Alpine:v3.14k3sbefore 1.20.6.1-r01.20.6.1-r0
Debian:12kubernetesbefore 1.20.5+really1.20.2-11.20.5+really1.20.2-1
Gok8s.io/kubernetesfrom 1.19.0 · before 1.19.101.19.10
Gok8s.io/kubernetesfrom 1.20.0 · before 1.20.61.20.6
Gok8s.io/kubernetesbefore 1.18.181.18.18

Same product

kubernetes: all records

Other highest-scoring records for the same primary product.

  • CVE-2018-1002105In all Kubernetes versions prior to v1.10.11, v1.11.5, and v1.12.3, incorrect handling of error responses to proxied upgrade requests in…
    65This week
  • CVE-2019-11248Kubernetes kubelet exposes /debug/pprof info on healthz port
    55Plan
  • CVE-2017-1002101In Kubernetes versions 1.3.x, 1.4.x, 1.5.x, 1.6.x and prior to versions 1.7.14, 1.8.9 and 1.9.4 containers using subpath volume mounts with…
    42Plan

Published before and after this one in the same product

  • ← EarlierCVE-2020-8566Dec 7, 2020 · Ceph RBD adminSecrets exposed in logs when loglevel >= 4
    22Monitor
  • ← EarlierCVE-2020-8554Jan 21, 2021 · Kubernetes man in the middle using LoadBalancer or ExternalIPs
    23Monitor
  • Later →CVE-2021-25737Sep 6, 2021 · Holes in EndpointSlice Validation Enable Host Network Hijack
    19Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
alpine:k3s1.20.6.1-r0 · Alpine:v3.14Package registry (OSV)
debian:kubernetes1.20.5+really1.20.2-1 · Debian:12Package registry (OSV)
Go:k8s.io/kubernetes1.18.18Package registry (OSV)
Go:k8s.io/kubernetes1.19.10Package registry (OSV)
Go:k8s.io/kubernetes1.20.6Package registry (OSV)

Exploit status

Proof of concept available

A public proof of concept or detection signature exists. Exploit knowledge is out there; don't delay patching.

Evidence

  • Proof-of-concept repository on GitHub ×1

Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.

Public exploit evidence
SourceKindDate
Proof-of-concept repository on GitHubstars:172021-04-22

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

  1. First PoC-137 d
  2. Published
  3. TodayNov 12, 2026+1893 d

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Bug bounty scope

The product’s vendor appears in a public program. Matched by name; verify the scope text in the program.

Not on record for this CVE: EPSS history · patch commit or PR link · variant or chain candidate.

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • Administrator privileges are required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
high · data or configuration can be modified
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
Low
Privileges required
High
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
High
Availability impact
High

Weakness class (CWE)

CWE-372 · Incomplete Internal State Distinction

CVSS vector

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

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

ATT&CK techniques

—

References

Show all references (1 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

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