CVE-2026-20840: Windows 10 1607 Buffer Overflow Flaw

CVE-2026-20840 Overview

CVE-2026-20840 is a heap-based buffer overflow [CWE-122] in the Windows New Technology File System (NTFS) driver. An authorized local attacker can exploit the flaw to execute code in the context of the operating system. Microsoft published the advisory on January 13, 2026 and assigned the issue to multiple supported Windows client and server releases. The vulnerability requires local access and low privileges, with no user interaction, and impacts confidentiality, integrity, and availability.

Critical Impact

Successful exploitation allows a local low-privileged user to corrupt heap memory inside the NTFS driver and run arbitrary code, enabling privilege escalation to SYSTEM on affected Windows hosts.

Affected Products

• Microsoft Windows 10 (1607, 1809, 21H2, 22H2)
• Microsoft Windows 11 (23H2, 24H2, 25H2)
• Microsoft Windows Server 2008, 2012, 2016, 2019, 2022, 2022 23H2, and 2025

Discovery Timeline

• 2026-01-13 - CVE-2026-20840 published to NVD
• 2026-01-15 - Last updated in NVD database

Technical Details for CVE-2026-20840

Vulnerability Analysis

The flaw resides in the NTFS driver (ntfs.sys), which parses on-disk metadata structures such as Master File Table (MFT) records, attributes, and index entries. A heap-based buffer overflow occurs when the driver copies attacker-influenced data into a kernel pool allocation without correctly bounding the destination size. Overwriting adjacent pool memory enables corruption of kernel objects and ultimately code execution at kernel privilege.

Because NTFS parsing executes in kernel mode, successful exploitation provides complete control of the host. The vulnerability affects every supported Windows release listed in the Microsoft advisory, including legacy Windows Server 2008 and current Windows 11 25H2 and Windows Server 2025 builds.

Root Cause

The root cause is missing or incorrect length validation when the NTFS driver processes a crafted file system structure. A size field read from attacker-controlled metadata is trusted during a copy operation, producing an out-of-bounds write in the kernel pool. CWE-122 classifies this pattern as a heap-based buffer overflow.

Attack Vector

Exploitation requires local access with low privileges. A typical attack chain involves mounting or accessing a crafted NTFS volume — for example, an attacker-supplied VHD, VHDX, ISO, or USB image — that triggers the vulnerable parsing path. No user interaction beyond standard file system access is required. Refer to the Microsoft Security Update Guide CVE-2026-20840 for vendor-confirmed technical details.

No public proof-of-concept exploit and no CISA Known Exploited Vulnerabilities listing are available at the time of publication. The EPSS probability is 0.039%.

Detection Methods for CVE-2026-20840

Indicators of Compromise

• Unexpected mounting of VHD, VHDX, ISO, or removable NTFS volumes by non-administrative users
• Kernel bugchecks referencing ntfs.sys following file system access events
• New SYSTEM-level processes spawned from sessions belonging to standard users
• Creation or execution of unsigned drivers shortly after NTFS volume operations

Detection Strategies

• Hunt for Mount-DiskImage, Mount-VHD, or vhdmount activity originating from non-administrator accounts
• Correlate Windows Event ID 41 (kernel-power) and 1001 (bugcheck) with the loaded image ntfs.sys
• Monitor for token manipulation and process parent-child anomalies following local logon by low-privileged users

Monitoring Recommendations

• Enable Sysmon driver-load (Event ID 6) and image-load (Event ID 7) auditing across endpoints and servers
• Centralize kernel crash dumps and review stack traces involving NTFS attribute parsing
• Alert on attachment of disk images from user-writable paths such as %TEMP% and %USERPROFILE%\Downloads

How to Mitigate CVE-2026-20840

Immediate Actions Required

• Apply the January 2026 Microsoft security updates referenced in the Microsoft Security Update Guide CVE-2026-20840 to all affected Windows client and server systems
• Prioritize patching multi-user systems, terminal servers, and hosts where untrusted users can log on locally
• Restrict the ability of standard users to mount disk images and attach removable storage

Patch Information

Microsoft has released cumulative updates for all impacted Windows 10, Windows 11, and Windows Server versions. Administrators should deploy the patches through Windows Update, Windows Server Update Services (WSUS), or Microsoft Update Catalog. Validate that systems report the January 2026 build numbers after reboot.

Workarounds

• Block standard users from mounting VHD, VHDX, and ISO files using Group Policy and AppLocker rules
• Disable AutoPlay and AutoRun for removable and network volumes via NoDriveTypeAutoRun
• Enforce device control policies that prevent attachment of unapproved USB mass storage
• Limit interactive and Remote Desktop logon rights on sensitive servers to administrators only

# Configuration example
# Disable AutoPlay for all drive types (run elevated)
reg add "HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer" /v NoDriveTypeAutoRun /t REG_DWORD /d 0xFF /f

# Remove the Mount option from the shell context menu for non-admins via Group Policy:
# Computer Configuration > Administrative Templates > Windows Components > File Explorer
#   > "Remove CD Burning features" and related AutoPlay policies set to Enabled