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PatchSiren cyber security CVE debrief

CVE-2021-21985 VMware CVE debrief

CVE-2021-21985 is an improper input validation vulnerability in VMware vCenter Server. CISA lists it in the Known Exploited Vulnerabilities catalog and notes known ransomware campaign use, which raises the defensive priority for exposed or widely relied-on vCenter deployments. The practical response is to apply vendor updates per VMware guidance as soon as possible.

Vendor
VMware
Product
vCenter Server
CVSS
CRITICAL 9.8
CISA KEV
Listed
Original CVE published
2021-11-03
Original CVE updated
2026-09-27
Advisory published
2026-09-27
Advisory updated
2026-09-27

Who should care

Organizations running VMware vCenter Server, especially teams responsible for virtualization, infrastructure, identity, and patch management. Security teams should treat this as a high-priority remediation item because it is in CISA’s KEV catalog and flagged for known ransomware campaign use.

Technical summary

The available official sources identify CVE-2021-21985 as an improper input validation issue in VMware vCenter Server. CISA added it to the Known Exploited Vulnerabilities catalog on 2021-11-03 and set a remediation due date of 2021-11-17. The catalog entry instructs organizations to apply updates per vendor instructions.

Defensive priority

High. CISA KEV inclusion indicates confirmed exploitation in the wild, and the known ransomware campaign use note increases urgency for remediation.

Recommended defensive actions

  • Apply VMware updates or patches according to vendor instructions.
  • Prioritize remediation of any internet-facing or broadly accessible vCenter Server instances.
  • Verify whether vCenter Server is present across all environments, including lab, DR, and delegated management planes.
  • Track remediation against the CISA KEV due date and escalate if patching is delayed.
  • Review exposure paths and access controls around administrative virtualization management interfaces after updating.

Evidence notes

The debrief is based on the supplied CISA KEV source item and official resource links. The source item identifies the vulnerability as VMware vCenter Server Improper Input Validation Vulnerability, marks it as a KEV entry, states known ransomware campaign use, and instructs to apply updates per vendor instructions. Published/modified dates used here are 2021-11-03, matching the provided CVE and source timestamps.

Sources and references

Verified primary and authoritative sources

  • CVE-2021-21985 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2021-21985

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2021-21985 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2021-21985

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

  • CISA Known Exploited Vulnerabilities catalog

    Publisher, destination, and source semantics verified

    URL: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

    Cybersecurity and Infrastructure Security Agency - Official CISA catalog of vulnerabilities known to be exploited in the wild.

Supplemental references

  • Source item URL

    Unverified legacy reference

    URL: https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json

    cisa_kev

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.