CVE-2020-3999 Overview
CVE-2020-3999 is a denial of service vulnerability affecting multiple VMware virtualization products including ESXi, Workstation, Fusion, and Cloud Foundation. The vulnerability stems from improper input validation in the GuestInfo component, which allows a malicious actor with normal user privilege access to a virtual machine to crash the virtual machine's vmx process, leading to a denial of service condition.
Critical Impact
Attackers with local user access to a guest VM can crash the vmx process, causing disruption to virtualized workloads and potentially affecting availability of critical systems running on VMware infrastructure.
Affected Products
- VMware ESXi 7.0 (prior to ESXi70U1c-17325551)
- VMware Workstation 16.x (prior to 16.0) and 15.x (prior to 15.5.7)
- VMware Fusion 12.x (prior to 12.0) and 11.x (prior to 11.5.7)
- VMware Cloud Foundation
Discovery Timeline
- 2020-12-21 - CVE-2020-3999 published to NVD
- 2025-08-08 - Last updated in NVD database
Technical Details for CVE-2020-3999
Vulnerability Analysis
This vulnerability is classified as CWE-20 (Improper Input Validation) and affects the GuestInfo component within VMware's virtualization products. The GuestInfo mechanism is used for communication between the guest operating system and the VMware hypervisor, allowing the guest to query and set various configuration parameters.
The flaw exists because the GuestInfo component fails to properly validate input data received from the guest operating system. When a user within the guest VM sends specially crafted input through the GuestInfo interface, the vmx process—which manages the virtual machine execution on the host—does not adequately sanitize or validate this data before processing.
The attack requires local access to a virtual machine with normal user privileges, meaning an attacker does not need administrative rights within the guest OS to trigger the condition. This makes the vulnerability particularly concerning in multi-tenant environments where untrusted users may have access to virtual machines.
Root Cause
The root cause of CVE-2020-3999 is improper input validation (CWE-20) in the GuestInfo handling code. The vmx process accepts data from the guest VM through the GuestInfo interface without performing adequate bounds checking or validation. When malformed or unexpected input is processed, it causes the vmx process to enter an error state and crash.
The GuestInfo mechanism is designed to facilitate guest-to-host communication for legitimate purposes such as passing configuration data, but the lack of proper input sanitization allows malicious input to propagate to the vmx process and trigger a denial of service condition.
Attack Vector
The attack vector for CVE-2020-3999 is local, requiring the attacker to have user-level access within a guest virtual machine. The attack scenario involves:
- An attacker gains normal user access to a virtual machine running on a vulnerable VMware platform
- The attacker crafts malicious input targeting the GuestInfo interface
- The crafted input is sent through the guest-to-host communication channel
- The vmx process on the host fails to properly validate the input
- The vmx process crashes, resulting in termination of the virtual machine
The vulnerability does not require user interaction beyond the attacker's own actions, and the scope is changed (affects resources beyond the vulnerable component) since the crash of the vmx process impacts the host's management of the virtual machine.
Since no verified code examples are available for this vulnerability, readers should refer to the VMware Security Advisory VMSA-2020-0029 for additional technical details on the GuestInfo component and affected configurations.
Detection Methods for CVE-2020-3999
Indicators of Compromise
- Unexpected vmx process crashes or terminations on ESXi, Workstation, or Fusion hosts
- Virtual machine instances experiencing sudden shutdowns without administrator action
- Repeated GuestInfo-related error messages in VMware log files
- Unusual activity from guest VMs attempting to interact with GuestInfo mechanisms
Detection Strategies
- Monitor VMware host logs (vmware.log, hostd.log) for vmx process crash events and GuestInfo-related errors
- Implement anomaly detection for virtual machine availability, flagging unexpected VM terminations
- Use SentinelOne Singularity Platform to monitor for suspicious process behavior on virtualization hosts
- Audit guest VM user activity for attempts to interact with GuestInfo interfaces
Monitoring Recommendations
- Configure centralized logging for all VMware ESXi hosts, Workstation, and Fusion installations
- Set up alerting for vmx process crashes to enable rapid incident response
- Review virtual machine event logs for patterns indicating exploitation attempts
- Implement host-based intrusion detection on systems running VMware products
How to Mitigate CVE-2020-3999
Immediate Actions Required
- Update VMware ESXi 7.0 to version ESXi70U1c-17325551 or later
- Upgrade VMware Workstation 16.x to version 16.0 or later, or 15.x to version 15.5.7 or later
- Upgrade VMware Fusion 12.x to version 12.0 or later, or 11.x to version 11.5.7 or later
- Apply patches to VMware Cloud Foundation as specified in the vendor advisory
- Restrict user access within guest virtual machines to trusted personnel only
Patch Information
VMware has released security patches addressing this vulnerability as documented in VMware Security Advisory VMSA-2020-0029. Organizations should apply the following minimum versions:
- ESXi 7.0: Update to ESXi70U1c-17325551
- Workstation 16.x: Update to 16.0
- Workstation 15.x: Update to 15.5.7
- Fusion 12.x: Update to 12.0
- Fusion 11.x: Update to 11.5.7
Workarounds
- Restrict access to guest virtual machines to only trusted users until patches can be applied
- Implement network segmentation to limit potential impact if a VM becomes unavailable
- Configure virtual machine restart policies to automatically recover from vmx process crashes
- Monitor guest VM activity closely in environments where patching is delayed
# Example: Check ESXi version to verify patch status
esxcli system version get
# Example: List installed VIBs to verify patch installation
esxcli software vib list | grep -i esx-base
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
