Microsoft’s January security updates address two high-severity Windows NTFS driver vulnerabilities in ntfs.sys, the core component responsible for the NTFS file system in all modern versions of Windows. Both flaws, discovered by Positive Technologies researcher Sergey Tarasov, enable local privilege escalation to SYSTEM, effectively granting an attacker full control over a compromised Windows host after gaining an initial foothold.
Scope of the Windows NTFS Vulnerabilities and Affected Versions
The patches have been released for 37 different Windows editions, covering Windows 10, Windows 11, and server platforms including Windows Server 2019, 2022, and the upcoming Windows Server 2025. Because NTFS is the default file system on most Windows desktops and servers, and ntfs.sys runs in kernel mode with the highest level of privileges, such issues have a broad operational impact.
Both vulnerabilities are rated 7.8 on the CVSS scale, classed as high severity. They are formally categorized as local vulnerabilities, meaning the attacker must already have some level of access to the system. In real-world intrusions, however, local privilege escalation bugs are a critical step in multi-stage attacks, used after initial compromise to gain persistence and bypass defenses.
Technical Analysis of the NTFS Vulnerabilities in ntfs.sys
CVE-2026-20840: Heap-Based Buffer Overflow via Malicious VHD
The first flaw, CVE-2026-20840, is a heap-based buffer overflow in the processing of virtual hard disk (VHD) files by the ntfs.sys driver. When Windows parses a specially crafted VHD image, the driver can be tricked into writing data beyond the bounds of a dynamically allocated memory buffer on the heap.
Successful exploitation requires a local attacker—for example, malware running under a regular user account or an adversary with stolen credentials—to supply a malicious VHD and trigger its processing. By manipulating the overflow, an attacker can write controlled data into sensitive areas of kernel memory and ultimately execute arbitrary code with SYSTEM privileges.
CVE-2026-20922: Insufficient Validation of Partition Tables
The second vulnerability, CVE-2026-20922, also scored 7.8 CVSS, stems from insufficient validation of partition-related data structures inside ntfs.sys. The driver does not perform robust integrity checks on certain partition tables, allowing an attacker to craft a malicious partition layout that leads to unsafe behavior in kernel code.
As with CVE-2026-20840, this flaw enables privilege escalation to SYSTEM when successfully exploited. An attacker who can introduce or modify a disk or partition structure—directly or through malware—may leverage the bug to gain complete control over the system, install stealthy backdoors, tamper with security tooling, and access any data stored on the affected volumes.
How Attackers Can Abuse NTFS Vulnerabilities in Real-World Scenarios
Although both vulnerabilities are local only, they are highly valuable in advanced attack chains. In typical scenarios, attackers first obtain a foothold via phishing, exploitation of internet-facing services, or stolen credentials, and then use kernel-level privilege escalation bugs to move from standard user rights to full SYSTEM control.
In enterprise environments, successful exploitation of ntfs.sys issues can enable an attacker to:
- Bypass OS-level and endpoint security controls by disabling or tampering with antivirus, EDR, or logging agents running under lower privileges.
- Harvest additional credentials from memory or the local Security Account Manager (SAM), including administrator accounts.
- Use the compromised host as a lateral movement staging point to access other systems in the internal network.
- Hide persistence mechanisms and hinder incident response and forensic analysis by manipulating file system and system artifacts at kernel level.
Industry incident reports regularly show that post-exploitation privilege escalation and lateral movement are common techniques in both financially motivated attacks and state-aligned advanced persistent threat (APT) operations. Vulnerabilities in core components like the NTFS driver are therefore closely monitored by offensive and defensive teams alike.
Mitigation Strategies and Temporary Risk-Reduction Measures
The primary defense is to apply the January Microsoft security updates as soon as possible across all supported editions of Windows 10, Windows 11, and Windows Server. Organizations should ensure that these patches are integrated into centralized patch management processes and that deployment completeness is verified via configuration management or vulnerability scanning tools.
Where immediate patching is not feasible—for example, on tightly controlled production servers—additional hardening steps can reduce exposure:
- Limit usage of VHD and other virtual disk images, particularly those from external or untrusted sources.
- Restrict end users’ ability to mount disk images or modify storage configurations without administrative oversight.
- Enhance monitoring for suspicious disk operations, such as unusual VHD mounting, unexpected partition changes, or low-level disk access by unapproved processes.
- Perform regular malware scans and hunt for tools and frameworks commonly used to exploit local privilege-escalation bugs.
These NTFS driver vulnerabilities underline how weaknesses in foundational components of Windows can significantly amplify the impact of an initial compromise. Consistent Windows patching discipline, strict least-privilege access models, strong protection of user accounts, and ongoing security awareness training substantially reduce the likelihood that attackers can reach the point where kernel-level flaws like CVE-2026-20840 and CVE-2026-20922 become exploitable. Actively maintaining this security baseline is one of the most effective ways to contain modern threat actors and protect critical data and infrastructure.
