CVE-2026-49790 Overview
CVE-2026-49790 is an elevation of privilege vulnerability in the Windows Universal Disk Format File System Driver (UDFS). The flaw is classified as a heap-based buffer overflow [CWE-122] within a kernel-mode driver. An authenticated local attacker can exploit the issue to elevate privileges to SYSTEM on affected Windows client and server versions. Microsoft assigned the vulnerability a CVSS 3.1 base score of 7.8, reflecting high impact to confidentiality, integrity, and availability. The vulnerability affects a wide range of Windows 10, Windows 11, and Windows Server releases still under support.
Critical Impact
Successful exploitation grants SYSTEM-level privileges on the affected host, enabling full compromise of the operating system.
Affected Products
- Microsoft Windows 10 (1607, 1809, 21H2, 22H2)
- Microsoft Windows 11 (24H2, 25H2, 26H1)
- Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, and 2025
Discovery Timeline
- 2026-07-14 - CVE-2026-49790 published to NVD
- 2026-07-20 - Last updated in NVD database
Technical Details for CVE-2026-49790
Vulnerability Analysis
The vulnerability resides in the Windows Universal Disk Format File System Driver (udfs.sys), which implements support for the UDF file system commonly used on optical media and disk images. The driver contains a heap-based buffer overflow [CWE-122] triggered while parsing attacker-controlled UDFS structures. Because the driver executes in kernel mode, an overflow in a paged or nonpaged pool buffer can be leveraged to corrupt adjacent kernel objects and hijack control flow. The attack is local and requires low privileges, with no user interaction needed once the malicious volume is mounted or accessed by the target driver code path.
Root Cause
The root cause is improper validation of size or length fields inside UDFS metadata parsed by the driver. When the driver allocates a pool buffer based on one field and then copies data using a different, attacker-controlled length, the write exceeds the allocation boundary. This heap corruption in kernel address space enables privilege escalation techniques such as pool grooming and kernel object overwrite.
Attack Vector
Exploitation requires local access to the system with standard user privileges. An attacker crafts a malicious UDF-formatted image (ISO, IMG, or similar) and induces the OS to parse it, for example by mounting the image, inserting removable media, or invoking a component that opens the file. The malformed structures reach udfs.sys and trigger the overflow, allowing the attacker to execute code in kernel context and gain SYSTEM privileges. No verified public proof-of-concept is available at the time of publication.
Detection Methods for CVE-2026-49790
Indicators of Compromise
- Unexpected mounting of UDF-formatted ISO or IMG files from user-writable locations such as %TEMP%, %APPDATA%, or Downloads folders.
- Kernel bug checks referencing udfs.sys, particularly SYSTEM_SERVICE_EXCEPTION, KERNEL_MODE_HEAP_CORRUPTION, or DRIVER_VERIFIER_DETECTED_VIOLATION.
- Standard user processes suddenly spawning children running as NT AUTHORITY\SYSTEM shortly after disk image access.
Detection Strategies
- Monitor filesystem and shell activity for programmatic mounting of ISO/IMG files using PowerShell Mount-DiskImage, IMAPI, or Explorer shell handlers by non-administrative users.
- Alert on the creation or download of UDF-formatted disk images followed by process token elevation events (Windows Event ID 4672 in unexpected contexts).
- Correlate udfs.sys load events with subsequent kernel crashes captured in Windows Error Reporting or minidumps.
Monitoring Recommendations
- Enable and forward Sysmon events 1 (process create), 11 (file create), and 25 (process tampering) to a central analytics platform for correlation with disk image activity.
- Track patch state of udfs.sys across Windows and Windows Server assets and flag hosts running unpatched builds.
- Baseline legitimate use of removable and virtual media to reduce noise when hunting for anomalous UDFS mounts.
How to Mitigate CVE-2026-49790
Immediate Actions Required
- Apply the Microsoft security update for CVE-2026-49790 to all affected Windows and Windows Server systems as soon as testing permits.
- Restrict standard user ability to mount arbitrary disk images through Group Policy and AppLocker rules covering ISO/IMG handling.
- Prioritize patching for multi-user systems, jump hosts, and shared workstations where local privilege escalation offers the highest attacker value.
Patch Information
Microsoft has published patch guidance and update packages through the Microsoft Security Response Center. Refer to the Microsoft Security Update CVE-2026-49790 advisory for the specific KB articles and build numbers for each affected Windows release.
Workarounds
- Block or remove the Windows shell association that auto-mounts ISO and IMG files where UDFS parsing is not a business requirement.
- Apply mail and web gateway policies that strip or quarantine ISO/IMG attachments delivered from untrusted sources.
- Enforce least privilege so that attackers who obtain low-privileged code execution have limited opportunity to stage malicious disk images.
# Disable Windows Explorer auto-mount handler for ISO files (per-user)
reg add "HKCU\Software\Classes\Windows.IsoFile\shell\mount" /v "LegacyDisable" /t REG_SZ /d "" /f
# Verify installed KB for udfs.sys after patching
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10
Get-Item C:\Windows\System32\drivers\udfs.sys | Select-Object VersionInfo
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
