CriticalCVE-2021-21973

    VMware vCenter Server and Cloud Foundation Server Side Request Forgery (SSRF) Vulnerability (CVE-2021-21973)

    Published In CISA KEV since Sources: CVE.org

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    What is affected, what to do

    The vSphere Client (HTML5) contains an SSRF (Server Side Request Forgery) vulnerability due to improper validation of URLs in a vCenter Server plugin. A malicious actor with network access to port 443 may exploit this issue by sending a POST request to vCenter Server plugin leading to information disclosure.

    The vulnerability is being actively exploited; treat it as a priority. Update cloud_foundation to version 3.10.1.2. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • VMware vCenter Server eq 7.x before 7.0 U1c..7.x before 7.0 U1c
    • VMware vCenter Server eq 6.7 before 6.7 U3l..6.7 before 6.7 U3l
    • VMware vCenter Server eq 6.5 before 6.5 U3n..6.5 before 6.5 U3n
    • VMware Cloud Foundation eq 4.x before 4.2..4.x before 4.2
    • VMware Cloud Foundation eq 3.x before 3.10.1.2..3.x before 3.10.1.2

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    5.3

    Priority

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

    Source: NVD, CVSS v3

    5.3

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: CVE.org

    88.0 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.77

    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
    206.13 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 exploited
    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
    5.3 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
    5.3
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

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    88.0 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    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
    99.77
    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: partial
    Source: CISA Vulnrichment, as of 16 February 2024
    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-918

    Response and mitigation

    Why act now?

    Prioritisation rationale

    On its own the flaw has limited severity because it "only" discloses information. However, it is exploitable over the network without authentication, and its exploitation likelihood is among the highest of any known CVE. Attackers have mass-scanned for it since 2021 alongside the critical RCE in the same vCenter plugin area, using the SSRF for reconnaissance and for pivoting into internal management networks. CISA does not flag ransomware use, but vCenter remains a prime target because it grants control over the entire virtualisation layer. Treat any vCenter reachable from the internet or from user networks as critical and patch it within 24 hours. Tightly segmented instances with egress filtering can go into the regular patch cycle, but should still be patched promptly.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Patch: Upgrade vCenter Server to 7.0 U1c, 6.7 U3l or 6.5 U3n (or later). Upgrade VMware Cloud Foundation to 4.2 or 3.10.1.2 (or later). Take a snapshot or file-based backup of the VCSA beforehand.
      • Check exposure: Find out whether any vCenter instance (port 443/tcp, vSphere Client HTML5) is reachable from the internet or from non-admin network segments. Cross-check your public IP ranges against Shodan/Censys and review your NAT rules. Block any external reachability at the perimeter firewall immediately.
      • If you cannot patch, disable the plugin: Apply the vendor's documented workaround, which disables the affected vCenter plugin (the vRealize Operations plugin in the vSphere Client). Look up the exact procedure in the vendor advisory. Then restart the vsphere-ui service and verify that the plugin is disabled.
      • Compromise assessment: Attackers have typically scanned for and exploited this bug together with the RCE in the same plugin context. Search the vSphere UI logs for POST requests to /ui/vropspluginui/ going back to early 2021. Also hunt for unknown .jsp files under /usr/lib/vmware-vsphere-ui/.
      • Credentials: If you find hits, rotate the service accounts that have vCenter access (SSO admins, backup and monitoring integration accounts). Assess the event against your NIS2 incident-reporting obligations.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Run vCenter only in a dedicated management VLAN. Allow inbound 443/tcp solely from admin jump hosts or a PAW network, and deny all other sources by default.
      • Egress filtering (SSRF-specific): Restrict outbound connections from the VCSA to the destinations it actually needs: ESXi hosts, NTP, DNS, identity source and update repository. Drop outbound HTTP(S) to arbitrary internal services or to the internet. This directly limits how far an SSRF can reach.
      • WAF/reverse proxy/IPS: If a proxy sits in front of the vSphere Client, deny requests to /ui/vropspluginui/* from unauthorised sources. On the IPS, enable the existing signatures for VMware vCenter vROps plugin exploits (Suricata ET / Snort community, search "vCenter vropspluginui") and set them to block mode.
      • Configuration hardening: Permanently disable vCenter plugins you don't use, and audit the list of active plugins regularly. Enable VCSA shell and SSH access only when needed.
      • IAM/least privilege: Cut integration-account privileges to the minimum. Enforce MFA for vCenter access through your IdP so that leaked information is harder to exploit.

    3. Runbook step 3

      Detection rules

      • vSphere UI/rhttpproxy logs: Treat POST requests to /ui/vropspluginui/rest/services/ from non-admin source networks as suspicious, especially when no authenticated session precedes them. The logs live in /var/log/vmware/rhttpproxy/ and /var/log/vmware/vsphere-ui/logs/. Example SPL: index=vmware sourcetype=vcenter* "POST" "/ui/vropspluginui/" | stats count by src_ip, uri
      • Network telemetry (Zeek/Suricata): Unexpected outbound connections from the vCenter IP to internal hosts outside the ESXi/infrastructure allowlist, or to the internet, are SSRF indicators. Example KQL: DeviceNetworkEvents | where DeviceName has "vcenter" and RemoteIP !in (allowlist) and RemotePort in (80,443,8080)
      • IDS alerts: Correlate hits on the ET rules for the VMware vCenter vROps plugin (SSRF/RCE). Pay particular attention to bursts from a single source IP, which suggest mass scanning.
      • Follow-on activity on the VCSA: Use auditd or file-integrity monitoring to catch new .jsp or .war files in the vSphere UI work directory. Also watch for sh, bash, curl or wget spawned by the vsphere-ui Java process. Sigma shape: ParentImage|endswith: '/java' AND CommandLine|contains: 'vsphere-ui' AND Image|endswith: ['/sh','/bash','/curl','/wget']
      • Authentication anomalies: After any SSRF hit, scrutinise unusual SSO logins (vCenter events com.vmware.sso.LoginSuccess from new source IPs). They may indicate that disclosed information is being reused.

    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 an SSRF (Server Side Request Forgery) vulnerability due to improper validation of URLs in a vCenter Server plugin. A malicious actor with network access to port 443 may exploit this issue by sending a POST request to vCenter Server plugin leading to information disclosure. This affects: VMware vCenter Server (7.x before 7.0 U1c, 6.7 before 6.7 U3l and 6.5 before 6.5 U3n) and VMware Cloud Foundation (4.x before 4.2 and 3.x before 3.10.1.2).

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

    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 Cloud Foundation
    • 3.x before 3.10.1.2
    • 4.x before 4.2
    • —
    • —
    —VMware vCenter Server
    • 6.7 before 6.7 U3l
    • 7.x before 7.0 U1c
    • 6.5 before 6.5 U3n
    • —
    • —
    • —
    vmwarecloud_foundation
    • 3.0 – < 3.10.1.2
    • 4.0 – < 4.2
    • 3.10.1.2
    • 4.2
    vmwarevcenter_server
    • 6.7
    • 7.0
    • 6.5
    • —
    • —
    • —

    Vendors and products

    Weakness classes (CWE)

    • 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

    References & sources

    • vmware.comsecurity/advisories/VMSA-2021-0002.htmlx_refsource_CONFIRM, Vendor advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-21973government-resource, US government resource
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