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

CVE-2020-3992 Overview

CVE-2020-3992 is a critical use-after-free vulnerability in the OpenSLP service as implemented in VMware ESXi. A malicious actor residing in the management network who has access to port 427 on an ESXi machine may be able to trigger a use-after-free condition in the OpenSLP service, resulting in remote code execution with the privileges of the SLP daemon. This vulnerability poses a severe threat to virtualized infrastructure, as successful exploitation could allow attackers to compromise the hypervisor layer and potentially gain control over all virtual machines hosted on the affected system.

Critical Impact

This vulnerability is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Attackers with network access to port 427 can achieve unauthenticated remote code execution on VMware ESXi hosts, potentially compromising entire virtualized environments.

Affected Products

• VMware ESXi 7.0 (before ESXi_7.0.1-0.0.16850804)
• VMware ESXi 6.7 (before ESXi670-202010401-SG)
• VMware ESXi 6.5 (before ESXi650-202010401-SG)
• VMware Cloud Foundation (versions utilizing affected ESXi builds)

Discovery Timeline

• October 20, 2020 - CVE-2020-3992 published to NVD
• October 30, 2025 - Last updated in NVD database

Technical Details for CVE-2020-3992

Vulnerability Analysis

This vulnerability exists in the OpenSLP (Service Location Protocol) implementation bundled with VMware ESXi. OpenSLP is a network service that runs on port 427 and is used for service discovery within virtualized environments. The use-after-free condition occurs when the SLP daemon improperly handles memory during service request processing.

Use-after-free vulnerabilities occur when a program continues to use a pointer after the memory it references has been freed. In the context of CVE-2020-3992, when certain SLP messages are processed, the daemon may free a memory object but subsequently attempt to access or manipulate that freed memory region. An attacker can craft malicious SLP packets that exploit this timing window, potentially allowing them to control the contents of the reallocated memory and redirect program execution.

The OpenSLP service runs with elevated privileges on ESXi hosts, making this vulnerability particularly dangerous. Successful exploitation grants attackers code execution at the hypervisor level, which could be leveraged to escape virtual machine isolation, intercept network traffic, access sensitive data from other VMs, or deploy persistent backdoors.

Root Cause

The root cause of CVE-2020-3992 is improper memory management in the OpenSLP service implementation (CWE-416: Use After Free). The vulnerability stems from a failure to properly track object lifetimes during SLP message processing, leading to a dangling pointer condition where freed memory is subsequently dereferenced.

Attack Vector

The attack vector for this vulnerability is network-based, requiring the attacker to have access to the management network where the ESXi host resides. The attack targets port 427/TCP and port 427/UDP, which are used by the SLP service. No authentication is required to exploit this vulnerability, and no user interaction is necessary.

An attacker would typically craft specially formatted SLP service request or registration messages designed to trigger the use-after-free condition. By carefully controlling the timing and content of these messages, an attacker can manipulate heap memory allocation to achieve arbitrary code execution.

The vulnerability was disclosed through the Zero Day Initiative (ZDI) under advisories ZDI-20-1377 and ZDI-20-1385, indicating coordinated responsible disclosure through their vulnerability research program.

Detection Methods for CVE-2020-3992

Indicators of Compromise

• Unusual network traffic to port 427 (TCP/UDP) from unexpected sources on the management network
• Abnormal SLP service crashes or restarts on ESXi hosts indicating potential exploitation attempts
• Unexpected processes spawned by the SLP daemon or unusual system calls originating from the slpd process
• Memory corruption artifacts or core dumps associated with the OpenSLP service

Detection Strategies

• Monitor network traffic for anomalous SLP protocol activity, including malformed packets or unusual connection patterns to port 427
• Implement intrusion detection rules to identify known exploitation patterns for CVE-2020-3992 and related OpenSLP vulnerabilities
• Enable ESXi shell and SSH logging to capture evidence of post-exploitation activity
• Deploy host-based detection to identify memory corruption attempts targeting the SLP daemon

Monitoring Recommendations

• Implement continuous monitoring of ESXi management network traffic with focus on SLP protocol anomalies
• Configure SIEM alerts for multiple SLP connection attempts from single sources or connection attempts from unauthorized network segments
• Regularly audit ESXi host logs for SLP service errors or unexpected terminations that may indicate exploitation attempts

How to Mitigate CVE-2020-3992

Immediate Actions Required

• Apply VMware security patches immediately: ESXi 7.0.1 (build 16850804), ESXi670-202010401-SG for 6.7, and ESXi650-202010401-SG for 6.5
• If patching is not immediately possible, disable the SLP service on ESXi hosts using the command esxcli network firewall ruleset set -r CIMSLP -e false
• Block port 427 (TCP/UDP) at network firewalls to prevent external exploitation attempts
• Restrict management network access to authorized administrators and systems only

Patch Information

VMware has released security patches to address this vulnerability as documented in VMware Security Advisory VMSA-2020-0023. Organizations should upgrade to the following minimum versions:

• ESXi 7.0: Update to ESXi_7.0.1-0.0.16850804 or later
• ESXi 6.7: Apply patch ESXi670-202010401-SG or later
• ESXi 6.5: Apply patch ESXi650-202010401-SG or later
• VMware Cloud Foundation: Apply applicable async patches as specified in the advisory

Additional technical details about the vulnerability are available in ZDI Advisory ZDI-20-1377 and ZDI Advisory ZDI-20-1385. This vulnerability is tracked in the CISA Known Exploited Vulnerabilities Catalog.

Workarounds

• Disable the SLP service on ESXi hosts if it is not required for your environment's operation
• Implement strict network segmentation to isolate ESXi management interfaces from general network traffic
• Use firewall rules to restrict access to port 427 to only authorized management systems
• Consider deploying a jump host or bastion server for ESXi management access rather than allowing direct network connectivity

# Disable SLP service on ESXi host
esxcli network firewall ruleset set -r CIMSLP -e false

# Verify SLP service is disabled
esxcli network firewall ruleset list | grep CIMSLP

# Stop the SLP daemon (may require maintenance mode)
/etc/init.d/slpd stop

# Prevent SLP from starting on reboot
chkconfig slpd off