BlackTreeCVE Intelligence
← Back to the CVE catalogue
Full vulnerability report · 2021
CVE-2021-25740High confidence

Holes in EndpointSlice Validation Enable Host Network Hijack

Kubernetes · Kubernetes

3.1LowCVSS 3.1
Recommended action
Patch only the product branches with a verified fix

Low technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority. Verified remediation exists for at least one product or source, but 14 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityLowOperational priority:Low, unchanged from published severity.unchanged

Evidence used

  • No CISA KEV confirmation is currently recorded.
  • EPSS is 1.95% for the current model date.

Compensating controls

  • Apply the linked authoritative mitigation while planning the permanent fix.
  • Restrict the affected network interface to trusted sources where business-safe.
  • Monitor vendor guidance and exploitation sources for a material change.

Verification

  1. Confirm that the asset runs Kubernetes Kubernetes and falls inside the recorded affected range.
  2. Recheck the vendor advisory before scheduling a change because no verified fixed version is currently retained.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: No default targetRemediation target: Normal maintenance

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Cross-source reconciliation

Remediation availability differs by product scope

Verified remediation exists for at least one product or source, but 14 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Distribution package intelligence

Release-specific package status

Debian findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.

4 package states
Repository candidate not checked

A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.

Distribution releaseSource packageVendor stateFixed versionEvidence
Debian trixietrixie · sourcekubernetesVendor fix publishedDebian records a fixed source-package version for this release.1.20.5+really1.20.2-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian bookwormbookworm · sourcekubernetesVendor fix publishedDebian records a fixed source-package version for this release.1.20.5+really1.20.2-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian forkyforky · sourcekubernetesVendor fix publishedDebian records a fixed source-package version for this release.1.20.5+really1.20.2-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian sidsid · sourcekubernetesVendor fix publishedDebian records a fixed source-package version for this release.1.20.5+really1.20.2-1Debian Security Tracker ↗Source updated 6 Oct 2026
Open-source package ranges1 source-attributed range

These OSV and GitHub advisory ranges apply only to the named package and ecosystem. A listed fixed version is not a universal product patch or proof that an update is installed.

Ecosystem and packageAffected rangeFirst fixed versionEvidence
gok8s.io/kubernetes<= 1.22.2Not statedGitHub advisory ↗github reviewed aggregator · 9 Jun 2026
Direct vendor intelligence

Authoritative vendor CSAF and VEX advisories

Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.

1 current
CVE-2021-25740 · CSAF 2.0 · revision 3 · status not statedRed Hat Product Securitykubernetes: Endpoint & EndpointSlice permissions allow cross-Namespace forwarding
14 known affected

The vendor explicitly identifies these products as affected by this CVE.

  • atomic-openshift as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-clients as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-clients-redistributable as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-docker-excluder as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-excluder as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-hyperkube as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-hypershift as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-master as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-node as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-pod as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-sdn-ovs as a component of Red Hat OpenShift Container Platform 3.11
  • atomic-openshift-template-service-broker as a component of Red Hat OpenShift Container Platform 3.11
Summary
A flaw was found in Kubernetes. This issue enables users to send network traffic to locations they would otherwise not have access to via a confused deputy attack.
Remediation
Out of support scope
Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2021-2120

No EUVD known-exploited evidence

ENISA has published the identifier mapping but no EUVD description has been stored yet.

EUVD state
Present in the current official mapping
Known exploitation
Not present in the current ENISA EUVD known-exploited dataset. This is not proof of no exploitation.
ENISA score
Not supplied in the stored EUVD record
Advisory evidence
No linked advisory details stored yet
Recommended actionPatch only the product branches with a verified fix

Low technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority. Verified remediation exists for at least one product or source, but 14 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
01

What, why and how

A security issue was discovered with Kubernetes that could enable users to send network traffic to locations they would otherwise not have access to via a confused deputy attack.

What

A security issue was discovered with Kubernetes that could enable users to send network traffic to locations they would otherwise not have access to via a confused deputy attack.

Why

The product receives a request, message, or directive from an upstream component, but the product does not sufficiently preserve the original source of the request before forwarding the request to an external actor that is outside of the product's control sphere. This causes the product to appear to be the source of the request, leading it to act as a proxy or other intermediary between the upstream component and the external actor.

How

An attacker operating through a network path may attempt exploitation with low privileges. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

What

A security issue was discovered with Kubernetes that could enable users to send network traffic to locations they would otherwise not have access to via a confused deputy attack.

Why

The product receives a request, message, or directive from an upstream component, but the product does not sufficiently preserve the original source of the request before forwarding the request to an external actor that is outside of the product's control sphere. This causes the product to appear to be the source of the request, leading it to act as a proxy or other intermediary between the upstream component and the external actor.

How

An attacker operating through a network path may attempt exploitation with low privileges. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
No confirmed evidence

No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
a network path → Unintended Proxy or Intermediary ('Confused Deputy') → cause the confidentiality, integrity or availability impact described by the vendor
Attack surface
Network
Privileges required
Low: a basic authenticated account is required
User interaction
None
Attack complexity
High: exploitation depends on specific conditions
Security boundary
Unchanged: impact remains within the vulnerable component's security authority
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-441 ↗

CWE-441: Unintended Proxy or Intermediary ('Confused Deputy'). The product receives a request, message, or directive from an upstream component, but the product does not sufficiently preserve the original source of the request before forwarding the request to an external actor that is outside of the product's control sphere. This causes the product to appear to be the source of the request, leading it to act as a proxy or other intermediary between the upstream component and the external actor.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACHighAttack complexity: Successful exploitation depends on specific conditions outside the attacker's direct control.PRLowPrivileges required: The attacker needs basic user-level privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CLowConfidentiality impact: A successful attack can cause a limited loss.INoneIntegrity impact: No direct loss is represented by this metric.ANoneAvailability impact: No direct loss is represented by this metric.
Post-exploitation / living off the land
No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
NetworkCWE-441
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

BSI · German · WID-SEC-2023-0019Kubernetes: Schwachstelle ermöglicht Umgehen von Sicherheitsvorkehrungen

Ein entfernter, authentisierter Angreifer kann eine Schwachstelle in Kubernetes ausnutzen, um Sicherheitsvorkehrungen zu umgehen.

Official advisory ↗
Cyber Security Agency of Singapore · English · CSA-SB-20210922Security Bulletin 22 Sep 2021

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 22 Sep 2021, published on 22 September 2021. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-012395Kubernetes における別領域リソースに対する外部からの制御可能な参照に関する脆弱性

Kubernetes には、別領域リソースに対する外部からの制御可能な参照に関する脆弱性が存在します。

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Fix availability varies by product
Affected
Fixed
No fixed version is explicitly recorded in the structured CVE data.
Action
Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
Workaround
A mitigation or workaround reference is available from the source linked below; validate it against the affected product and version.
04

Evidence and provenance

Published 20 Sept 2021 · Last source change 1 Jun 2026, 21:45 UTC · CWE-441 · Unintended Proxy or Intermediary ('Confused Deputy')

CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2021-2120
Product sourceVendor CSAF · Red Hat Product Security
Remediation sourceVendor CSAF · Red Hat Product Security
CWE sourceCNA
NVD statusNVD modified after enrichment

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗

No material field changes have been recorded since change tracking began. Routine source refreshes and cosmetic edits are intentionally excluded.

Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Mitigation reference recorded
ENISA EUVD · EUVD-2021-2120↗Vendor CSAF · Red Hat Product Security↗Mitigation reference↗Mitigation reference↗CVE authority or vendor reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2021-25740 · cve.blacktree.nl