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Actively exploitedKnown ransomware use

CVE-2019-5544 VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow Vulnerability

VMware ESXi and Horizon Desktop as a Service (DaaS) OpenSLP contains a heap-based buffer overflow vulnerability that allows an attacker with network access to port 427 to overwrite the heap of the OpenSLP service to perform remote code execution.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 3 Nov 2021.
  • CISA lists it as known to be used in ransomware campaigns.
  • Rated Critical severity (CVSS 9.8, scored by CISA).
  • This product type is often exposed to the internet, which is where attackers scan for it first.
  • It has been in the catalogue for 1,829 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

OpenSLP as used in ESXi and the Horizon DaaS appliances has a heap overwrite issue. VMware has evaluated the severity of this issue to be in the Critical severity range with a maximum CVSSv3 base score of 9.8.

Affected Versions

ProductVersions
ESXi and Horizon DaaSESXi 6.7 prior to patch release ESXi670-201912001, ESXi 6.5 prior to patch release ESXi650-201912001, ESXi 6.0 prior to patch release ESXi600-201912001 and Horizon DaaS 8.x prior to BZ-2467224-Disable_SLPD_service_permanently_801_Hotfix.

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for VMware ESXi and Horizon DaaS. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated critical, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 3 May 2022 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

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CVE-2025-22226VMware ESXi, Workstation, and Fusion Information Disclosure VulnerabilityAdded 4 Mar 2025CVE-2025-22225 · RansomwareVMware ESXi Arbitrary Write VulnerabilityAdded 4 Mar 2025CVE-2025-22224VMware ESXi and Workstation TOCTOU Race Condition VulnerabilityAdded 4 Mar 2025CVE-2024-38813VMware vCenter Server Privilege Escalation VulnerabilityAdded 20 Nov 2024CVE-2024-38812VMware vCenter Server Heap-Based Buffer Overflow VulnerabilityAdded 20 Nov 2024CVE-2024-37085 · RansomwareVMware ESXi Authentication Bypass VulnerabilityAdded 30 Jul 2024

All 26 exploited VMware vulnerabilities

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2019-5544
  3. CVE record: CVE-2019-5544
  4. FIRST EPSS
  5. vmware.com/security/advisories/VMSA-2019-0022.html
  6. openwall.com/lists/oss-security/2019/12/10/2
  7. openwall.com/lists/oss-security/2019/12/11/2
  8. access.redhat.com/errata/RHSA-2019:4240
  9. lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZPXXJZLPLAQ
  10. lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DA3LYAJ2NRK
  11. access.redhat.com/errata/RHSA-2020:0199
  12. security.gentoo.org/glsa/202005-12