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

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: 2021-09-06 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-25735 is rated Moderate Risk (53/100): CVSS Medium severity, with high exploitation likelihood (EPSS 5.23%, 91th percentile). EPSS ranks this CVE among the most likely to be exploited in the near term. High exploitation likelihood—assess exposure and prioritize remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-25735

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 16.30% 5.23% -11.08%
2 2026-05-27 14.39% 16.30% +1.91%
3 2026-03-25 — 14.39% —

Full EPSS history (29 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-25735

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
6.5 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:H)
They need powerful rights—admin, root, or similar—before this pays off.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 1.2 5.2 [email protected]
6.5 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:H)
They need powerful rights—admin, root, or similar—before this pays off.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 1.2 5.2 [email protected]
5.5 2.0 MEDIUM
AV:N/AC:L/Au:S/C:N/I:P/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:S)
A single authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:P)
Partial availability impact.
 8.0 4.9 [email protected]

Weakness enumeration for CVE-2021-25735

GitHub Security Advisory for CVE-2021-25735

GHSA-g42g-737j-qx6j · Severity: medium · Ecosystem: go — Access Restriction Bypass in kube-apiserver

OS Trackers for CVE-2021-25735

vendor priority summary link
alpine medium CVE-2021-25735: 1 source package rows (k3s); 7 state rows across 7 repos (3.17-community, 3.18-community, 3.19-community, 3.20-community, 3.21-community, 3.22-community, edge-community); fixed 7, open 0. https://security.alpinelinux.org/vuln/CVE-2021-25735
debian not yet assigned CVE-2021-25735 not yet assigned priority: Debian including 1 source packages (kubernetes), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-25735
redhat medium — https://access.redhat.com/security/cve/CVE-2021-25735
suse medium — https://www.suse.com/security/cve/CVE-2021-25735/
ubuntu medium CVE-2021-25735 medium priority: Ubuntu including 1 source packages (kubernetes), 15 status rows across 15 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, trusty, upstream, xenial): ignored 7, DNE 4, not-affected 3, needs-triage 1. https://ubuntu.com/security/CVE-2021-25735

Affected software / configurations for CVE-2021-25735

Vendor Product Version Raw CPE
kubernetes kubernetes < 1.18.18 cpe:2.3:a:kubernetes:kubernetes:*:*:*:*:*:*:*:*
kubernetes kubernetes >= 1.19.0, < 1.19.10 cpe:2.3:a:kubernetes:kubernetes:*:*:*:*:*:*:*:*
kubernetes kubernetes >= 1.20.0, < 1.20.6 cpe:2.3:a:kubernetes:kubernetes:*:*:*:*:*:*:*:*

References for CVE-2021-25735

cvelogic Threat Intelligence