CVE-2020-3992: VMware ESXi OpenSLP Use-After-Free Vulnerability

CVE-2020-3992 is a use-after-free vulnerability in the OpenSLP service on VMware ESXi. An attacker who already has access to the management network and can reach port 427 may achieve remote code execution on the hypervisor. Because ESXi underpins virtualized workloads, successful exploitation can give an adversary control over guest systems and storage. Public reporting links this issue to ransomware activity, so timely response matters for any environment still running the affected component.

How it works

The weakness is classified as CWE-416 (use-after-free). In this class of flaw, a program continues to use a region of memory after it has been freed. An attacker who can influence the allocation and freeing pattern may corrupt program state and redirect execution. On VMware ESXi the vulnerable code resides in OpenSLP. According to the CISA summary, an attacker positioned on the management network with reachability to TCP/UDP port 427 can trigger the condition and obtain remote code execution. Exact packet sequences or heap-grooming details are not required for defenders; the practical takeaway is that unauthenticated or lightly authenticated traffic to the SLP service is sufficient once network access exists. Confirm any deeper technical description against the vendor advisory.

Am I affected? How to find it in your systems

VMware ESXi is commonly deployed as bare-metal hypervisors in data centers, remote offices, and lab environments. Inventory every ESXi host—standalone, clustered under vCenter, or nested. Check whether the OpenSLP service is present and listening on port 427. Because precise affected version ranges are not restated here, compare each host’s build number directly with the fixed releases listed in VMware’s advisory for CVE-2020-3992.

• Use vCenter inventory views, PowerCLI, or ESXi shell commands to enumerate hosts and their build numbers.
• Confirm whether SLP is enabled; many environments leave it running by default.
• Review firewall and network diagrams to determine which segments can reach management interfaces on port 427.
• For exploitation signs, examine ESXi hostd and vmkernel logs for unexpected SLP-related crashes, service restarts, or anomalous connections to port 427 originating from outside normal management jump hosts. Correlate with any subsequent unusual process creation or file modifications on the hypervisor.

How to remediate

The primary action is to apply the updates VMware released for this CVE, following the vendor’s instructions exactly. CISA likewise directs organizations to apply updates per vendor guidance. After patching, verify the new build number and confirm that the OpenSLP service either no longer exposes the vulnerable code path or has been updated.

• Schedule maintenance windows for rolling upgrades of ESXi hosts, respecting cluster admission-control and storage requirements.
• Where SLP is not required for discovery, disable the service entirely as a permanent hardening step.
• Restrict management-network access with host firewalls, NSX distributed firewalls, or external ACLs so that only authorized jump hosts and vCenter can reach port 427 (and other management ports).
• Re-validate that backup, monitoring, and security-tooling accounts still function after the change.

If you can't patch immediately

Until the vendor update can be installed, reduce exposure with compensating controls focused on the attack path.

• Segment the management network so that port 427 is unreachable from general user or server VLANs; place ESXi management interfaces on isolated VLANs or private management fabrics.
• Disable the OpenSLP service if operationally feasible—many environments do not rely on it.
• Apply virtual patching or IDS/IPS signatures that detect anomalous SLP traffic if your network security tools provide them; treat these as temporary measures only.
• Increase monitoring: alert on any connection attempts to port 427 from unexpected sources, and forward ESXi logs to a SIEM for crash or privilege-escalation indicators.
• Enforce strict jump-host and multi-factor authentication for all administrative access so that an attacker must first compromise a controlled entry point.

If your data may have been exposed

Vulnerabilities that enable remote code execution on hypervisors and that have been used by ransomware operators frequently precede data theft or encryption. If you have reason to believe an ESXi host was compromised, treat connected datastores and guest workloads as potentially exposed. Rotate credentials, review backup integrity, and examine guest systems for follow-on activity. As a quick additional check, you can run a free exposure scan of your email addresses against known breach data sets to see whether associated accounts have appeared in prior incidents.