CVE-2021-31956: Windows NTFS Privilege Escalation Flaw

CVE-2021-31956 Overview

CVE-2021-31956 is a Windows NTFS Elevation of Privilege Vulnerability that affects the Windows NT File System (NTFS) driver. This vulnerability allows a local attacker with low privileges to escalate their access to SYSTEM-level privileges on affected Windows systems. The vulnerability exists due to an integer underflow (CWE-191) in the NTFS driver, which can be exploited to achieve arbitrary kernel memory manipulation.

Critical Impact

This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Organizations should treat patching as an urgent priority.

Affected Products

• Microsoft Windows 10 (versions 1507, 1607, 1809, 1909, 2004, 20H2, 21H1)
• Microsoft Windows 7 SP1, Windows 8.1, Windows RT 8.1
• Microsoft Windows Server 2008 SP2, 2008 R2 SP1, 2012, 2012 R2, 2016, 2019, 2004, 20H2

Discovery Timeline

• June 8, 2021 - CVE-2021-31956 published to NVD
• January 13, 2026 - Last updated in NVD database

Technical Details for CVE-2021-31956

Vulnerability Analysis

CVE-2021-31956 is an integer underflow vulnerability in the Windows NTFS kernel driver. The vulnerability occurs during the processing of NTFS extended attributes, where improper size calculations can lead to a heap-based buffer overflow condition in kernel memory. An attacker who successfully exploits this vulnerability can execute arbitrary code with elevated (SYSTEM) privileges, allowing complete compromise of the affected system.

The attack requires local access to the system and low-level privileges, meaning an attacker would first need to gain initial access through another vector (such as phishing or exploiting a different vulnerability) before leveraging CVE-2021-31956 for privilege escalation. No user interaction is required for exploitation once local access is obtained.

Root Cause

The root cause is an integer underflow vulnerability (CWE-191) in the NTFS driver's handling of extended attributes. When processing specially crafted NTFS extended attribute data, the driver performs arithmetic operations that can result in an integer underflow. This underflow causes incorrect buffer size calculations, leading to subsequent heap buffer overflow conditions in kernel pool memory.

Attack Vector

The attack vector is local, requiring the attacker to have authenticated access to the target system with low-level user privileges. The exploitation mechanism involves:

1. Creating or modifying NTFS extended attributes with malformed data designed to trigger the integer underflow
2. The NTFS driver processes the malicious data, causing incorrect memory allocation calculations
3. The resulting heap buffer overflow allows the attacker to corrupt adjacent kernel memory structures
4. By carefully controlling the memory corruption, an attacker can hijack execution flow and achieve arbitrary code execution in kernel context

This vulnerability has been observed being exploited in targeted attacks, often chained with browser vulnerabilities (such as CVE-2021-21224 in Chrome) to achieve full system compromise from a drive-by download scenario.

Detection Methods for CVE-2021-31956

Indicators of Compromise

• Unusual NTFS extended attribute operations on system files or directories
• Suspicious kernel crash dumps or Blue Screen of Death (BSOD) events related to ntfs.sys
• Evidence of privilege escalation attempts from low-privileged accounts to SYSTEM
• Abnormal memory allocation patterns in Windows kernel pool

Detection Strategies

• Monitor for suspicious NtfsQueryEaUserEaList and related NTFS extended attribute API calls
• Deploy endpoint detection and response (EDR) solutions with kernel-level visibility to detect exploitation attempts
• Implement behavioral analysis to identify privilege escalation patterns from user-mode to kernel-mode
• Use memory integrity checking tools to detect kernel pool corruption

Monitoring Recommendations

• Enable Windows Event Log auditing for file system operations, particularly extended attribute modifications
• Monitor for processes making unusual NTFS-related system calls, especially from non-administrative contexts
• Configure SentinelOne agents to alert on kernel exploit behaviors and privilege escalation attempts
• Review system stability logs for unexplained NTFS driver crashes that may indicate exploitation attempts

How to Mitigate CVE-2021-31956

Immediate Actions Required

• Apply the Microsoft security update released in June 2021 immediately to all affected systems
• Prioritize patching for systems accessible by multiple users or connected to untrusted networks
• Review CISA KEV catalog remediation guidance and comply with any applicable deadlines
• Implement network segmentation to limit lateral movement if exploitation occurs

Patch Information

Microsoft released security updates addressing CVE-2021-31956 as part of the June 2021 Patch Tuesday release. The patch corrects the integer arithmetic in the NTFS driver to prevent the underflow condition. Organizations should apply the appropriate update for their Windows version via Windows Update, Windows Server Update Services (WSUS), or Microsoft Update Catalog. For detailed patch information, refer to the Microsoft Security Advisory for CVE-2021-31956.

Workarounds

• Limit local access to systems to only trusted and necessary users while patches are deployed
• Implement application whitelisting to prevent unauthorized executables from running
• Use security tools with kernel exploit protection capabilities to detect and block exploitation attempts
• Consider temporarily restricting NTFS extended attribute functionality through Group Policy if operationally feasible

# Verify patch status by checking the installed Windows updates
# PowerShell command to check for June 2021 security updates
Get-HotFix | Where-Object {$_.InstalledOn -ge "2021-06-08"} | Format-Table -AutoSize

# Check NTFS driver version to confirm patching
Get-ItemProperty "C:\Windows\System32\drivers\ntfs.sys" | Select-Object VersionInfo