CVE-2020-3941: VMware Tools Privilege Escalation Flaw

CVE-2020-3941 Overview

CVE-2020-3941 is a race condition vulnerability affecting the repair operation of VMware Tools for Windows 10.x.y. This vulnerability allows for privilege escalation within virtual machines where VMware Tools is installed. The flaw exists in the repair functionality of VMware Tools, and successful exploitation could allow an attacker with low privileges to elevate their access to higher privilege levels on the affected system.

Critical Impact

Local attackers can exploit this race condition during VMware Tools repair operations to escalate privileges and gain elevated access within the virtual machine environment.

Affected Products

• VMware Tools 10.x.y for Windows
• Microsoft Windows (as the host operating system)

Discovery Timeline

• 2020-01-15 - CVE-2020-3941 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-3941

Vulnerability Analysis

This vulnerability is classified as CWE-362 (Race Condition), a time-of-check to time-of-use (TOCTOU) class flaw that occurs during the repair operation of VMware Tools. The race condition creates a window of opportunity where an attacker can manipulate system state between security checks and subsequent operations.

The vulnerability requires local access to the virtual machine and low privileges to exploit. While it demands precise timing (high attack complexity), successful exploitation leads to significant impact on confidentiality, integrity, and availability of the system. The scope remains unchanged, meaning the impact is confined to the vulnerable component itself.

Importantly, this vulnerability is not present in VMware Tools 11.x.y, as the affected functionality was removed from this version branch entirely.

Root Cause

The root cause is a race condition in the repair operation workflow of VMware Tools for Windows. During the repair process, there exists a timing gap between when security permissions are checked and when privileged operations are executed. This TOCTOU vulnerability allows attackers to intercept and manipulate the execution flow during this brief window.

Attack Vector

The attack vector is local, requiring the attacker to have authenticated access to the virtual machine with at least low-level privileges. The attacker must initiate or wait for a VMware Tools repair operation and exploit the race condition during this process.

Exploitation involves manipulating file system objects, symbolic links, or other system resources during the narrow timing window when the repair operation performs privileged actions. The attacker essentially races against the legitimate repair process to insert malicious payloads or redirect operations to achieve privilege escalation.

The attack requires precise timing and may need multiple attempts to succeed, which is reflected in the high attack complexity rating. However, no user interaction is required beyond the attacker's own actions.

Detection Methods for CVE-2020-3941

Indicators of Compromise

• Unexpected VMware Tools repair operations triggered by non-administrative users
• Suspicious process execution with elevated privileges following VMware Tools service activity
• Unusual symbolic link creation in VMware Tools installation directories
• Anomalous file system access patterns during VMware Tools maintenance operations

Detection Strategies

• Monitor Windows Event Logs for VMware Tools service events, particularly repair and maintenance operations
• Implement file integrity monitoring on VMware Tools installation directories
• Track privilege changes for processes spawned by or interacting with VMware Tools services
• Deploy endpoint detection rules to identify race condition exploitation patterns

Monitoring Recommendations

• Enable verbose logging for VMware Tools service operations
• Configure alerts for unauthorized repair operation attempts on VMware Tools
• Monitor for rapid file system operations characteristic of race condition exploitation
• Implement behavioral analysis to detect privilege escalation attempts following VMware Tools activity

How to Mitigate CVE-2020-3941

Immediate Actions Required

• Upgrade VMware Tools to version 11.x.y or later, which removes the vulnerable functionality entirely
• Restrict repair operation privileges to authorized administrators only
• Implement least privilege principles for user accounts in virtual machines
• Monitor systems for exploitation attempts until patching is complete

Patch Information

VMware has addressed this vulnerability through the release of VMware Tools 11.x.y, which removes the affected functionality entirely. Organizations should upgrade to the latest version of VMware Tools to eliminate this vulnerability. For detailed patching guidance, refer to the VMware Security Advisory VMSA-2020-0002.

Workarounds

• Restrict local user access to virtual machines to trusted personnel only
• Disable or restrict VMware Tools repair functionality where possible until patching
• Implement application whitelisting to prevent unauthorized code execution
• Use endpoint protection solutions to detect and block privilege escalation attempts

# Verify VMware Tools version on Windows
# Run in PowerShell to check if vulnerable version is installed
Get-WmiObject -Class Win32_Product | Where-Object { $_.Name -like "*VMware Tools*" } | Select-Object Name, Version

# Alternative: Check registry for VMware Tools version
Get-ItemProperty "HKLM:\SOFTWARE\VMware, Inc.\VMware Tools" | Select-Object InstallPath, InstallerVersion