CVE-2021-25740Network attack vector

Cross-namespace backend exposure via crafted endpoints

Published Sep 20, 2021 · Updated Jun 1, 2026

Authorization weakness in all Kubernetes versions allows remote authenticated users to expose cross-namespace backends through crafted endpoint objects. The Endpoints and EndpointSlice APIs accept user-selected backend IP addresses, which LoadBalancer or Ingress implementations then trust as Service destinations. Exploitation requires endpoint create or edit permission and a selectorless Service, enabling access past NetworkPolicy or source-range controls.

CVSS severity3.1
Low
EPSS probability1.95%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Authorization weakness in all Kubernetes versions allows remote authenticated users to expose cross-namespace backends through crafted endpoint objects. The Endpoints and EndpointSlice APIs accept user-selected backend IP addresses, which LoadBalancer or Ingress implementations then trust as Service destinations. Exploitation requires endpoint create or edit permission and a selectorless Service, enabling access past NetworkPolicy or source-range controls.

The record

CVE
CVE-2021-25740
Published
Sep 20, 2021
Updated
Jun 1, 2026
Vendor
Kubernetes
Product
Kubernetes
Classifications
CWE-441, T1090.001
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Sep 20, 2021CVE publishedPublication date reported by the CVE source.
  2. Jun 1, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <*

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Kubernetes · Kubernetesversion=0 <*

Published CVSS scores

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

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-441Unintended Proxy or Intermediary ('Confused Deputy')
T1090.001Internal Proxy

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo