CriticalCVE-2021-21985

    VMware vCenter Server Improper Input Validation Vulnerability (CVE-2021-21985)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    This vulnerability allows for remote code execution (RCE) due to improper input validation in the Virtual SAN Health Check plug-in of VMware vCenter Server. Attackers with network access to port 443 can execute commands with unrestricted privileges on the underlying operating system hosting vCenter Server. The vulnerability is actively exploited in ransomware campaigns and is listed in the CISA KEV catalog since November 2021. All versions of VMware vCenter Server are affected. An immediate update to the latest version is urgently required to mitigate the risk.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update cloud_foundation to version 3.10.2.1. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • VMware vCenter Server and VMware Cloud Foundation eq VMware vCenter Server (7.x before 7.0 U2b, 6.7 before 6.7 U3n, 6.5 before 6.5 U3p) and VMware Cloud Foundation (4.x before 4.2.1..VMware vCenter Server (7.x before 7.0 U2b, 6.7 before 6.7 U3n, 6.5 before 6.5 U3p) and VMware Cloud Foundation (4.x before 4.2.1

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    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CVE.org

    —

    EPSS score

    No value is available for this vulnerability.

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    253.98 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: ANSSI / CERT-FR How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    9.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.99
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 26 May 2021
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-20, CWE-470, CWE-918

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This CVE sits on KEV because it is unauthenticated, network-reachable on TCP/443 with no user interaction, and yields root-level code execution on the vCenter appliance — enough to own the entire virtualisation stack and every ESXi host beneath it. CISA flags it as used in ransomware campaigns, and operator telemetry showed mass scanning and payload delivery against port 443 within hours of exposure. For KRITIS and NIS2-scope organisations running vCenter in the management network this is a top-tier priority, on par with perimeter edge appliances, especially if vCenter is ever reachable from user or partner networks. Downgrade the risk only once management-VLAN micro-segmentation is proven and the appliance is confirmed on a VMSA-2021-0010 fixed build.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Upgrade vCenter Server to a fixed build listed in VMware advisory VMSA-2021-0010 (vCenter 6.5 U3p, 6.7 U3n, 7.0 U2b or later — pull exact build numbers from the vendor advisory). For VMware Cloud Foundation apply the matching BOM patch from the same advisory.
      • If a patch window within 24 h is not feasible, apply the workaround in VMware KB 83829: disable the Virtual SAN Health Check plug-in and the other plug-ins listed in the advisory via the compatibility-matrix.xml, then restart the vSphere Client service.
      • Restrict network reach to vCenter TCP/443 to dedicated admin jump hosts / PAWs only; firewall-deny any access from user VLANs, OT networks and the internet.
      • Treat the vcsa as potentially compromised if it was reachable from untrusted networks pre-patch: take forensic snapshots of the appliance and VMDKs, preserve /var/log/vmware/, /storage/log/vmware/ and audit.log.
      • Rotate credentials: SSO administrator (administrator@vsphere.local), vcsa local root, service accounts in connected AD/LDAP and all API tokens; invalidate active vSphere Client sessions.
      • Impose a change freeze on ESXi hosts and datastores until integrity checks complete, so lateral movement via vpxd/esxcli can be spotted.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: place vMotion, vSAN and vCenter management on a dedicated management VLAN behind a stateful firewall; permit ingress on TCP/443, 5480, 22 only from jump hosts.
      • WAF / reverse proxy in front of vCenter: block requests to /ui/h5-vsan/*, /ui/vropspluginui/* and the other plug-in paths named in the advisory while the appliance is unpatched.
      • IPS/IDS: deploy a Suricata/Snort rule for POST requests against /ui/h5-vsan/rest/* with suspicious Java deserialisation payloads or oversized bodies (> 10 KB); import available ET Open SIDs for CVE-2021-21985.
      • Endpoint hardening on vcsa: constrain appliancesh to named admins, enable SSH only on demand, log all shell access; file-integrity monitoring on /usr/lib/vmware-vsphere-ui/server/work/ and /tmp/.
      • IAM: back SSO with an external IdP enforcing MFA (ADFS, Azure AD, Okta); stop using the shared administrator@vsphere.local for automation — issue least-privilege API service accounts.
      • Configuration: permanently disable plug-ins you do not use (vROps, vSAN Health if not deployed); restart vsphere-ui and verify with service-control --status.

    3. Runbook step 3

      Detection rules

      • vcsa web access logs (/storage/log/vmware/vsphere-ui/logs/access.log): HTTP POST to URIs containing /h5-vsan/ or /vsan-health/ from external source IPs — Sigma shape: selection: cs-method: POST AND cs-uri-stem|contains: '/h5-vsan/'.
      • Process ancestry on vcsa (auditd / EDR): vsphere-ui Java process spawning /bin/bash, /bin/sh, curl, wget, python or nc — the classic RCE tell. KQL shape: DeviceProcessEvents | where InitiatingProcessFileName == "java" and InitiatingProcessCommandLine contains "vsphere-ui" and FileName in ("bash","sh","curl","wget","nc").
      • Filesystem: new files under /tmp/, /var/tmp/ or /usr/lib/vmware-vsphere-ui/server/work/ with mtime after the last boot and owner vsphere-ui; auditd watch -w /tmp/ -p wa -k vcsa_rce.
      • Network telemetry (Zeek/Suricata): outbound connections from the vcsa IP to non-VMware destinations on TCP/80, 443, 4444, 1337 — a healthy vcsa baseline only talks to VMware repos, NTP, DNS and AD.
      • Audit events: correlate vmware-sts-idmd.log for SSO logins (successful and failed) immediately before/after POSTs to plug-in URIs; alert on freshly created SSO accounts or role changes.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    The vSphere Client (HTML5) contains a remote code execution vulnerability due to lack of input validation in the Virtual SAN Health Check plug-in which is enabled by default in vCenter Server. A malicious actor with network access to port 443 may exploit this issue to execute commands with unrestricted privileges on the underlying operating system that hosts vCenter Server.

    Source: CISA KEV Catalog, cvelistv5, NVD (NIST)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    —VMware vCenter Server and VMware Cloud Foundation
    • VMware vCenter Server (7.x before 7.0 U2b, 6.7 before 6.7 U3n, 6.5 before 6.5 U3p) and VMware Cloud Foundation (4.x before 4.2.1
    • —
    vmwarecloud_foundation
    • 3.0 – < 3.10.2.1
    • 4.0 – < 4.2.1
    • 3.10.2.1
    • 4.2.1
    vmwarevcenter_server
    • 6.7
    • 7.0
    • 6.5
    • —
    • —
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-20Improper Input Validation

      Class

      The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

      Entry at MITRE CWE-20

    • CWE-470Use of Externally-Controlled Input to Select Classes or Code ('Unsafe Reflection')

      Base

      The product uses external input with reflection to select which classes or code to use, but it does not sufficiently prevent the input from selecting improper classes or code.

      Entry at MITRE CWE-470

    • CWE-918Server-Side Request Forgery (SSRF)

      Base

      The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.

      Entry at MITRE CWE-918

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2021-ALE-011 – Vulnérabilité dans VMware vCenter Server (clôturée 2021-10-19) (opens in a new tab)cert.cyber.gouv.fractively exploited
    Packet StormVMware Security Advisory 2021 0010 (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormVMware vCenter Server Virtual SAN Health Check Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • vmware.comsecurity/advisories/VMSA-2021-0010.htmlx_refsource_MISC
    • packetstormsecurity.comfiles/162812/VMware-Security-Advisory-2021-0010.htmlx_refsource_MISC
    • packetstormsecurity.comfiles/163487/VMware-vCenter-Server-Virtual-SAN-Health-Check-Rem…x_refsource_MISC
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-21985government-resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-011 – Vulnérabilité dans VMware vCenter Server (clôturée 2021-10-19)cert-fr
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