Security readout for executives and security teams
Plain-English summary
This is a VMware vCenter Server SSRF flaw in the HTML5 vSphere Client. An unauthenticated attacker with network access to vCenter port 443 could trigger a vulnerable plugin path and obtain limited information. Because vCenter is a high-value management plane and CISA lists the CVE in KEV, treat it as urgent despite a medium CVSS score.
Executive priority
Prioritize this above a normal medium-severity vulnerability because vCenter controls critical virtualization infrastructure and the CVE appears in CISA KEV. Remediation should be tracked as an infrastructure risk item until all affected versions are upgraded or vendor guidance is applied.
Technical view
CVE-2021-21973 is CWE-918 SSRF caused by improper URL validation in a vCenter Server plugin. A network attacker can send a POST request to the plugin over port 443, requiring no privileges or user interaction. Impact in the source is limited to confidentiality; integrity and availability impact are not indicated.
Likely exposure
Exposure is most likely where VMware vCenter Server 7.x, 6.7, or 6.5, or VMware Cloud Foundation 4.x or 3.x, remains below the fixed versions and port 443 is reachable from untrusted or broad internal networks.
Exploitation context
CISA KEV inclusion supports that this vulnerability has been exploited. The provided sources do not describe exploit campaigns, targets, tooling, or whether exploitation is currently ongoing. The attack requires network access to vCenter Server port 443 and does not require authentication.
Researcher notes
The source bundle supports SSRF, unauthenticated network attack vector, affected version ranges, confidentiality-only CVSS impact, and KEV status. It does not provide packet-level indicators, exploit mechanics, workarounds, or campaign attribution, so validation should focus on version, exposure, and vendor advisory alignment.
Mitigation direction
- Upgrade vCenter Server to 7.0 U1c, 6.7 U3l, 6.5 U3n, or later.
- Upgrade VMware Cloud Foundation to 4.2 or 3.10.1.2, or later.
- Review VMware VMSA-2021-0002 for vendor-specific remediation guidance.
- Prioritize remediation for internet-facing or broadly reachable vCenter management interfaces.
Validation and detection
- Inventory all vCenter Server and VMware Cloud Foundation deployments.
- Confirm affected versions are not below the fixed versions listed by VMware.
- Check whether vCenter port 443 is reachable from untrusted networks.
- Review security monitoring for suspicious access to vCenter plugin endpoints.
- Verify remediation status against VMware VMSA-2021-0002 and CISA KEV records.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-918: Information exposure and cloud metadata lookup
Information exposure and SSRF weaknesses can make discovery, cloud metadata, and credential material review relevant. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCloud metadata behavior lookup
The CVE wording references SSRF or metadata access, so cloud discovery and credential material review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-21973 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Medium
- CVSS
- 5.3 (3.1)
- Known Exploited
- Yes
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CISA KEV status
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N3.91.4Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.3MediumVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://www.vmware.com/security/advisories/VMSA-2021-0002.htmlCVE reference · x_refsource_CONFIRM
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-21973CVE reference · government-resource
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Server-Side Request Forgery (SSRF)
Server-Side Request Forgery (SSRF) represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.