CVE-2021-43240: Windows 10 Privilege Escalation Vulnerability

CVE-2021-43240 Overview

CVE-2021-43240 is an elevation of privilege vulnerability affecting the NTFS file system's "Set Short Name" functionality in Microsoft Windows operating systems. This vulnerability allows an authenticated local attacker to escalate their privileges on the affected system, potentially gaining complete control over the compromised machine.

The vulnerability exists in how the NTFS file system handles short name (8.3 filename format) operations. When exploited successfully, an attacker with low-privilege access to a system can elevate their permissions to gain high-level access to confidential data, modify system files, and execute arbitrary operations with elevated privileges.

Critical Impact

Local attackers with low privileges can exploit this NTFS vulnerability to escalate privileges, potentially achieving complete system compromise with full confidentiality, integrity, and availability impact.

Affected Products

• Microsoft Windows 10 (versions 1909, 2004, 20H2, 21H1, 21H2 - x86, x64, ARM64)
• Microsoft Windows 11 (x64, ARM64)
• Microsoft Windows Server (20H2, 2022)
• Microsoft Windows Server 2016 (2004)

Discovery Timeline

• 2021-12-15 - CVE-2021-43240 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-43240

Vulnerability Analysis

This elevation of privilege vulnerability resides within the NTFS file system component of Microsoft Windows. NTFS supports both long filenames and short names (also known as 8.3 filenames) for backward compatibility with older applications. The vulnerability occurs during the processing of operations that set or modify these short names.

An attacker operating with local access and low-level user privileges can exploit this flaw to bypass security controls and gain elevated privileges on the system. The attack does not require any user interaction beyond the attacker having authenticated access to the target system.

When successfully exploited, this vulnerability allows the attacker to achieve high impact across confidentiality, integrity, and availability dimensions. This means sensitive data can be accessed, system files can be modified, and system stability can be affected.

Root Cause

The root cause of CVE-2021-43240 relates to improper handling within the NTFS file system when processing short name operations. While specific technical details have not been publicly disclosed by Microsoft, the vulnerability class suggests issues with privilege checking or access control validation during the NTFS Set Short Name operation. This allows a low-privileged user to perform operations that should be restricted to higher privilege levels.

Attack Vector

The attack vector for CVE-2021-43240 is local, meaning an attacker must have authenticated access to the target system to exploit this vulnerability. The attack complexity is low, and no user interaction is required beyond the attacker's own actions.

The exploitation scenario typically involves:

1. An attacker gains initial access to a Windows system with low-privilege credentials
2. The attacker invokes NTFS operations related to setting short names on files
3. Due to improper privilege checking, the operation executes with elevated permissions
4. The attacker leverages this privilege escalation to gain higher-level system access

The vulnerability is confined to the local system scope and does not allow an attacker to extend their reach to other systems directly.

Detection Methods for CVE-2021-43240

Indicators of Compromise

• Unusual file system activity involving short name modifications on NTFS volumes
• Unexpected privilege escalation events in Windows Security Event Logs
• Processes running with elevated privileges that originated from low-privilege user accounts
• Anomalous access patterns to system files or protected directories

Detection Strategies

• Monitor Windows Security Event Logs for privilege escalation indicators (Event IDs 4624, 4672, 4688)
• Deploy endpoint detection and response (EDR) solutions to identify anomalous NTFS operations
• Implement behavioral analysis to detect unexpected privilege context changes
• Use SentinelOne Singularity Platform for real-time detection of privilege escalation attempts

Monitoring Recommendations

• Enable detailed file system auditing on critical NTFS volumes
• Configure alerts for suspicious process creation events with elevated privileges
• Implement centralized log collection and analysis for Windows Security Events
• Regularly review user account activities and privilege assignments

How to Mitigate CVE-2021-43240

Immediate Actions Required

• Apply the Microsoft security update released in the December 2021 Patch Tuesday cycle
• Audit systems to identify potentially compromised accounts or systems
• Restrict local access to only necessary users and services
• Implement the principle of least privilege for all user accounts

Patch Information

Microsoft has released security updates to address this vulnerability as part of the December 2021 security updates. Organizations should apply the appropriate patches for their Windows versions through Windows Update, Windows Server Update Services (WSUS), or by downloading updates directly from the Microsoft Security Advisory for CVE-2021-43240.

Affected systems should be updated to the latest cumulative updates for their respective Windows versions:

• Windows 10 versions 1909, 2004, 20H2, 21H1, 21H2
• Windows 11
• Windows Server 20H2, 2022
• Windows Server 2016 version 2004

Workarounds

• Restrict physical and remote access to systems to only authorized personnel
• Implement application control policies to limit execution of unauthorized programs
• Consider disabling 8.3 short name creation using fsutil 8dot3name set 1 if application compatibility allows
• Employ network segmentation to limit the impact of potential exploitation

# Disable 8.3 short name creation on NTFS volumes (requires admin privileges)
# Note: This may affect legacy application compatibility
fsutil 8dot3name set 1

# Query current 8.3 name creation settings
fsutil 8dot3name query

# Disable 8.3 names on a specific volume (e.g., C:)
fsutil 8dot3name set C: 1