CVE-2026-49793 Overview
CVE-2026-49793 is a heap-based buffer overflow [CWE-122] in the Windows Resilient File System (ReFS) driver. An authenticated local attacker can trigger the overflow to execute arbitrary code on the target host. Microsoft published the advisory on July 14, 2026 and rates the issue HIGH with a CVSS 3.1 base score of 7.8. The flaw affects a broad range of Windows client and server releases that ship the ReFS driver, including Windows 10, Windows 11, and Windows Server 2016 through 2025. Successful exploitation compromises confidentiality, integrity, and availability of the affected system.
Critical Impact
Local attackers with low privileges can escalate to code execution on Windows endpoints and servers running vulnerable ReFS volumes.
Affected Products
- Microsoft Windows 10 (versions 1607, 1809, 21H2, 22H2) across x86, x64, and ARM64 architectures
- Microsoft Windows 11 (versions 24H2, 25H2, 26H1) across x64 and ARM64 architectures
- Microsoft Windows Server 2016, 2019, 2022, and 2025
Discovery Timeline
- 2026-07-14 - CVE-2026-49793 published to NVD and Microsoft advisory released
- 2026-07-20 - Last updated in NVD database
Technical Details for CVE-2026-49793
Vulnerability Analysis
The vulnerability resides in the Windows Resilient File System (ReFS) driver, which manages allocations for on-disk metadata structures on ReFS-formatted volumes. A heap-based buffer overflow occurs when the driver writes attacker-influenced data past the bounds of a heap allocation while processing filesystem operations. Because ReFS parsing runs in kernel context, corruption of adjacent pool memory can be steered into arbitrary code execution or elevation of privilege.
Exploitation requires local access and low privileges but no user interaction. The attack complexity is low, and the impact spans full confidentiality, integrity, and availability loss on the affected host. EPSS data as of July 20, 2026 indicates a probability of 0.263% for near-term exploitation, though authenticated local attackers with a foothold present a realistic threat model.
Root Cause
The root cause is classified as [CWE-122] Heap-based Buffer Overflow. The ReFS driver fails to correctly validate the size of a heap buffer relative to attacker-controlled input parsed from filesystem structures. When the driver writes beyond the allocated region, it corrupts adjacent kernel pool memory, enabling control of downstream kernel objects.
Attack Vector
An authorized local attacker mounts or interacts with a maliciously crafted ReFS volume, or triggers filesystem operations that reach the vulnerable code path. Common delivery scenarios include specially crafted VHD or VHDX images, removable media formatted as ReFS, or attacker-controlled file operations on an existing ReFS volume. Microsoft has not released public technical details or proof-of-concept code as of publication. Refer to the Microsoft CVE-2026-49793 Advisory for vendor guidance.
Detection Methods for CVE-2026-49793
Indicators of Compromise
- Unexpected mounting of ReFS-formatted VHD, VHDX, or removable media by non-administrative users
- Kernel bugchecks or crash dumps referencing refs.sys or refsv1.sys immediately following user-initiated filesystem operations
- New or unsigned kernel drivers loaded shortly after suspicious filesystem activity on ReFS volumes
Detection Strategies
- Monitor for process activity that mounts virtual disk images (diskpart, PowerShell Mount-DiskImage, Mount-VHD) originating from standard user contexts
- Correlate Windows Error Reporting events and kernel crashes tagged to the ReFS driver with preceding low-privilege user sessions
- Alert on privilege escalation patterns where a low-integrity process spawns SYSTEM-level children following filesystem I/O to ReFS volumes
Monitoring Recommendations
- Ingest kernel event logs, Sysmon driver-load events (Event ID 6), and disk-mount telemetry into a centralized analytics platform
- Enforce alerting on the loading of ReFS drivers on hosts where ReFS is not an operational requirement
- Review endpoint telemetry for anomalous VHD or VHDX file creation and mount sequences performed by standard users
How to Mitigate CVE-2026-49793
Immediate Actions Required
- Apply the Microsoft security update for CVE-2026-49793 to all affected Windows 10, Windows 11, and Windows Server systems
- Restrict the ability of non-administrative users to mount arbitrary virtual disk images or attach removable ReFS media
- Inventory hosts with ReFS volumes and prioritize patching for systems exposed to untrusted local users or workloads
Patch Information
Microsoft has released security updates addressing CVE-2026-49793 across all supported Windows client and server versions. Deploy the updates listed in the Microsoft CVE-2026-49793 Advisory through Windows Update, WSUS, or your preferred patch management platform. Servers hosting ReFS storage pools and Storage Spaces Direct deployments should be prioritized.
Workarounds
- Where patching is delayed, block standard users from mounting VHD and VHDX files via Group Policy and application control
- Disable attachment of removable ReFS-formatted media on endpoints that do not require it
- Enforce least privilege on interactive sessions and remove local administrator rights from standard user accounts
# Example: restrict non-admin VHD mounting via PowerShell Constrained Language and application control
# Block Mount-DiskImage / Mount-VHD for non-administrators using AppLocker or WDAC policy
New-AppLockerPolicy -RuleType Publisher -User "Everyone" -Action Deny \
-FilePath "C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\Modules\\Hyper-V\\Hyper-V.psd1"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
