CVE-2025-24985: Windows 10 1507 Fast FAT Driver RCE Flaw

CVE-2025-24985 Overview

CVE-2025-24985 is an integer overflow vulnerability in the Windows Fast FAT File System Driver. An unauthorized attacker can trigger the flaw to execute code locally on affected Windows systems. Exploitation requires user interaction, typically by mounting or opening a maliciously crafted Virtual Hard Disk (VHD) that triggers the integer overflow in the FAT driver. The vulnerability is tracked under CWE-190 (Integer Overflow or Wraparound) and CWE-122 (Heap-based Buffer Overflow). CISA has added CVE-2025-24985 to its Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild.

Critical Impact

Successful exploitation grants local code execution with full impact to confidentiality, integrity, and availability across nearly two decades of Windows client and Server releases.

Affected Products

• Microsoft Windows 10 (1507, 1607, 1809, 21H2, 22H2) and Windows 11 (22H2, 23H2, 24H2)
• Microsoft Windows Server 2008 (SP2 and R2 SP1), Server 2012, 2012 R2, 2016, 2019, 2022, and 2022 23H2
• Microsoft Windows Server 2025

Discovery Timeline

• 2025-03-11 - CVE-2025-24985 published to NVD
• 2025-10-27 - Last updated in NVD database
• Listed - CISA Known Exploited Vulnerabilities catalog entry confirmed

Technical Details for CVE-2025-24985

Vulnerability Analysis

The vulnerability resides in the Windows Fast FAT File System Driver (fastfat.sys), which parses File Allocation Table (FAT) volumes. The driver performs arithmetic on attacker-controlled values derived from on-disk FAT structures without sufficient bounds checking. When values are crafted to wrap around an integer boundary, the result is used to compute the size of a kernel heap allocation that becomes too small to hold the data that follows.

The subsequent copy operation writes past the end of the undersized buffer, producing a heap-based buffer overflow in kernel memory. With careful pool grooming, an attacker can corrupt adjacent kernel objects and redirect execution flow. Because the issue triggers in kernel-mode driver code, successful exploitation yields code execution at SYSTEM privileges.

Root Cause

The defect is an arithmetic miscalculation in the Fast FAT driver where size or offset fields read from a FAT volume are multiplied or added without validating that the result remains within range. The truncated value is then passed to an allocator, and the subsequent write loop uses the original, larger length. This pattern aligns with the CWE-190 to CWE-122 chain Microsoft assigned to the issue.

Attack Vector

The attack vector is local and requires user interaction. A typical exploitation path involves an attacker delivering a malicious VHD or VHDX file. When the user mounts the file, Windows automatically parses the embedded FAT volume through fastfat.sys, triggering the overflow without further user action. Refer to the Microsoft CVE-2025-24985 advisory and the Vicarius Analysis for additional technical context.

Detection Methods for CVE-2025-24985

Indicators of Compromise

• Unexpected mounting events for .vhd or .vhdx files originating from email attachments, browser downloads, or removable media.
• Bug check (BSOD) events referencing fastfat.sys shortly after a virtual disk is opened or attached.
• New or unsigned kernel-mode threads spawned in proximity to file system mount operations on FAT-formatted volumes.

Detection Strategies

• Hunt for processes invoking Mount-DiskImage, Mount-VHD, or diskpart against files delivered by untrusted sources.
• Correlate Microsoft-Windows-VHDMP operational log events with subsequent fastfat.sys errors in the System event log.
• Alert on the creation of .vhd or .vhdx files in user download or temp directories followed by an automatic mount within seconds.

Monitoring Recommendations

• Enable kernel-mode crash dump collection so any fastfat.sys faults can be triaged for exploitation attempts.
• Forward Sysmon FileCreate and ProcessCreate events for virtual disk extensions to a central SIEM for correlation.
• Review CISA KEV catalog entries and prioritize alerts on hosts that lack the March 2025 cumulative update.

How to Mitigate CVE-2025-24985

Immediate Actions Required

• Apply the March 2025 Microsoft security updates that remediate CVE-2025-24985 across all affected Windows client and Server SKUs.
• Prioritize patching internet-facing and high-value hosts, including Windows Server 2025 and Windows 11 24H2 systems.
• Restrict the ability of standard users to mount arbitrary VHD or VHDX files via Group Policy or AppLocker.

Patch Information

Microsoft has released fixed builds for every supported Windows version listed in the Microsoft CVE-2025-24985 Update guide. Administrators should consult the advisory for the specific KB article matching each operating system release and confirm deployment through Windows Update, WSUS, or Microsoft Intune. Out-of-support builds such as Windows Server 2008 require Extended Security Updates (ESU) to receive the fix.

Workarounds

• Block delivery of .vhd and .vhdx attachments at the email gateway and web proxy until patches are deployed.
• Disable automatic mounting of virtual disks for non-administrative users where business workflows allow.
• Apply the community detection and mitigation scripts published in the Vicarius Mitigation for CVE-2025-24985 as a temporary control.

# Verify the installed Windows build against Microsoft's fixed version
# Replace <KB_ID> with the KB listed in the MSRC advisory for your OS
Get-HotFix -Id KB<KB_ID>

# Restrict standard users from mounting virtual disks (PowerShell, run as admin)
Disable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V-Management-PowerShell -NoRestart