CriticalCVE-2019-5544

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

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    A heap-based buffer overflow in VMware ESXi and Horizon DaaS allows an attacker with network access to port 427 to overwrite the heap of the OpenSLP service, enabling remote code execution (RCE). The vulnerability is actively exploited in ransomware campaigns and is listed in the CISA KEV catalog since 2021. Affected products include VMware ESXi and Horizon DaaS versions 8.0.0 to 9.0.0.0. Immediate patching to the latest version is crucial to mitigate this critical vulnerability.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update horizon_daas to version 9.0.0.0. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • ESXi and Horizon DaaS eq ESXi 6.7 prior to patch release ESXi670-201912001, ESXi 6.5 prior to patch release ESXi650-201912001, ESXi 6.0 prior to patch re..ESXi 6.7 prior to patch release ESXi670-201912001, ESXi 6.5 prior to patch release ESXi650-201912001, ESXi 6.0 prior to patch re

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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

    98.1 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.91

    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
    253.86 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
    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 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“.

    Read in glossary
    98.1 %
    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
    99.91
    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

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable without authentication or user interaction and yields full code execution on the hypervisor. It ranks among the CVEs most likely to be exploited of any. CISA flags it for known ransomware use: compromising a single ESXi host lets attackers encrypt an entire VM estate in one move, and hosting providers, mid-market firms and critical-infrastructure operators with internet-facing or flat-network management interfaces are the typical victims. Treat any ESXi host with port 427 reachable as a P1 emergency. Internally segmented hosts are high priority for the next maintenance window, with SLP disabled immediately in the meantime.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Enumerate every ESXi host, Horizon DaaS appliance and Linux system carrying the openslp package (RHEL Desktop, RHEL for IBM z Systems incl. EUS, Fedora). Check whether 427/TCP and 427/UDP is reachable from anywhere outside the management segment, e.g. nmap -sU -sT -p 427 <esxi-range>.
      • Patch: A vendor patch is available. Look up the fixed version in the VMware security advisory for CVE-2019-5544. On RHEL and Fedora, install the updated openslp package via the normal distribution update channel (dnf/yum update openslp) and restart the service.
      • Disable SLP where patching is not possible: SLP is rarely required beyond CIM discovery. On ESXi, run /etc/init.d/slpd stop, esxcli network firewall ruleset set -r CIM-SLP -e 0 and chkconfig slpd off. Confirm the setting survives reboots with chkconfig --list | grep slpd.
      • Isolate exposed hosts: Put any ESXi host whose port 427 was reachable from the internet or from client networks behind a perimeter ACL immediately. Then run a compromise assessment (see Detection rules).
      • Rotate credentials: If compromise is suspected, rotate the ESXi root password, vCenter service accounts and local admin accounts. Secure offline/immutable VM backups before starting any recovery work.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Confine ESXi management (443, 902, 22, 427) to a dedicated management VLAN. Allow access only via jump hosts or PAWs, and default-deny 427/TCP+UDP at every zone boundary.
      • Host firewall: Keep the ESXi CIM-SLP ruleset disabled. Where SLP is operationally required, restrict it with esxcli network firewall ruleset allowedip add to named management IPs only.
      • IPS/NIDS: Enable vendor Suricata or Snort signatures for OpenSLP heap overflows (search the ruleset for "OpenSLP" or "SLP"). Add a local rule that alerts on SLP packets to port 427 with abnormally large length fields or extension offsets.
      • ESXi hardening: Enable Lockdown Mode, keep SSH and the ESXi Shell disabled by default, and set execInstalledOnly to block execution of dropped binaries.
      • Linux systems: If openslp is not needed, remove it (dnf remove openslp). Otherwise, confine slpd through its systemd unit with ProtectSystem=strict and NoNewPrivileges=yes.
      • Resilience: Maintain immutable or offline backups of VM datastores and of the ESXi configuration (vim-cmd hostsvc/firmware/backup_config), and test restores.

    3. Runbook step 3

      Detection rules

      • Network telemetry (Zeek/Suricata): Treat connections to port 427 from sources outside the management network as suspicious. Zeek example: conn.log | id.resp_p == 427 and not id.orig_h in $mgmt_nets. Also flag outbound connections from ESXi hosts to unknown destinations (reverse shells).
      • ESXi logs (forwarded via syslog): Look for slpd crashes or restarts in /var/log/syslog.log and vmkernel.log (core dumps under /var/core). In hostd.log and shell.log, watch for unexpected enablement of SSH or the ESXi Shell. SPL: index=esxi sourcetype=syslog (slpd AND (crash OR "core dump" OR segfault)).
      • ESXi file integrity: New or modified files in /tmp, /etc/rc.local.d/local.sh and /etc/motd are warning signs, as are new .sh or .py files on datastores. Mass renames or new file extensions on .vmdk and .vmx files indicate ransomware.
      • Linux auditd/EDR: Alert on the process chain slpd -> sh|bash|python|curl|wget. Sigma shape: ParentImage|endswith: '/slpd' AND Image|endswith: ['/sh','/bash','/python*','/wget','/curl'].
      • vCenter events: Correlate unplanned bursts of VM power-offs, snapshot deletions or new local ESXi users. KQL example: Syslog | where ProcessName == "hostd" and SyslogMessage has_any ("PowerOff","RemoveSnapshot").

    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

    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.

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

    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
    —ESXi and Horizon DaaS
    • ESXi 6.7 prior to patch release ESXi670-201912001, ESXi 6.5 prior to patch release ESXi650-201912001, ESXi 6.0 prior to patch re
    • —
    openslpopenslp
    • ≤ 2.0.0
    • —
    vmwarehorizon_daas
    • 8.0.0 – < 9.0.0.0
    • 9.0.0.0

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxredhatenterprise_linux_desktop6.0
    Linuxredhatenterprise_linux_desktop7.0
    Linuxredhatenterprise_linux_for_ibm_z_systems6.0_s390x
    Linuxredhatenterprise_linux_for_ibm_z_systems7.0_s390x
    Linuxredhatenterprise_linux_for_ibm_z_systems_eus7.7_s390x
    Linuxredhatenterprise_linux_for_power_big_endian6.0_ppc64
    Linuxredhatenterprise_linux_for_power_big_endian7.0_ppc64
    Linuxredhatenterprise_linux_for_power_big_endian_eus7.7_ppc64
    Linuxredhatenterprise_linux_for_power_little_endian7.0_ppc64le
    Linuxredhatenterprise_linux_for_power_little_endian_eus7.7_ppc64le
    Linuxredhatenterprise_linux_server6.0
    Linuxredhatenterprise_linux_server7.0
    Linuxredhatenterprise_linux_server_aus7.7
    Linuxredhatenterprise_linux_server_eus7.7
    Linuxredhatenterprise_linux_server_tus7.7
    Linuxredhatenterprise_linux_workstation6.0
    Linuxredhatenterprise_linux_workstation7.0
    Othervmwareesxi6.0
    Othervmwareesxi6.5
    Othervmwareesxi6.7
    Otherfedoraprojectfedora30
    Otherfedoraprojectfedora31

    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-2019-0022.htmlx_refsource_CONFIRM, Patch, Vendor advisory
    • openwall.com[oss-security] 20191210 Re: CVE-2019-5544 openslp 1.2.1, 2.0.0 heap overflow vulnerabilityMailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20191211 Re: CVE-2019-5544 openslp 1.2.1, 2.0.0 heap overflow vulnerabilityMailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • access.redhat.comRHSA-2019:4240Vendor advisory, x_refsource_REDHAT, Third-party advisory
    • lists.fedoraproject.orgFEDORA-2019-1e5ae33e87Vendor advisory, x_refsource_FEDORA, Release notes
    • lists.fedoraproject.orgFEDORA-2019-86bceb61b3Vendor advisory, x_refsource_FEDORA, Release notes
    • access.redhat.comRHSA-2020:0199Vendor advisory, x_refsource_REDHAT, Third-party advisory
    • security.gentoo.orgGLSA-202005-12Vendor advisory, x_refsource_GENTOO, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2019-5544government-resource, US government resource
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