CVE-2021-38662: Windows 10 Information Disclosure Flaw

CVE-2021-38662 Overview

CVE-2021-38662 is an information disclosure vulnerability in the Windows Fast FAT File System Driver. Microsoft disclosed the issue on October 13, 2021 as part of its monthly security update cycle. A locally authenticated attacker with low privileges can leverage the flaw to read sensitive memory contents from the affected driver. The vulnerability impacts a broad range of Windows desktop and server releases, including Windows 7, Windows 8.1, Windows 10, Windows 11, and Windows Server editions from 2008 through 2022. No user interaction is required to trigger the condition.

Critical Impact

A local, low-privileged attacker can disclose confidential kernel-mode memory contents from the Fast FAT file system driver, undermining confidentiality of data on the host.

Affected Products

• Microsoft Windows 10 and Windows 11 (x86, x64, ARM64)
• Microsoft Windows 7 SP1 and Windows 8.1 / Windows RT 8.1
• Microsoft Windows Server 2008, 2012, 2016, 2019, 2022, and Server 20H2

Discovery Timeline

• 2021-10-13 - CVE-2021-38662 assigned and published to NVD
• 2021-10-13 - Microsoft releases security patch through Patch Tuesday advisory
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-38662

Vulnerability Analysis

The vulnerability resides in the Windows Fast FAT File System Driver, the kernel-mode component responsible for handling File Allocation Table (FAT) volumes. The driver fails to properly handle specific file system operations, resulting in the exposure of memory contents that should remain isolated from user-mode callers. NVD classifies the weakness under NVD-CWE-noinfo because Microsoft did not publish detailed root-cause information. Successful exploitation grants the attacker read access to data that may include kernel addresses, pointers, or residual contents of kernel buffers. This information can support follow-on attacks such as local privilege escalation when combined with memory corruption primitives.

Root Cause

The root cause is improper handling of file system data structures in fastfat.sys, the driver that services FAT/FAT32/exFAT volume operations. Insufficient initialization or boundary checks within the driver leak uninitialized or out-of-bounds memory back to the caller. Because the driver runs in kernel context, leaked data may originate from privileged memory regions.

Attack Vector

Exploitation requires local access and low-privilege code execution on the target system. An attacker mounts or interacts with a crafted FAT-formatted volume, such as a USB device, virtual disk image, or attached storage, and issues specific file system requests to the driver. No user interaction is required beyond the attacker's own session. The advisory does not indicate active exploitation in the wild, and the CVE is not listed on the CISA Known Exploited Vulnerabilities catalog. Refer to the Microsoft Security Advisory CVE-2021-38662 for vendor-provided details.

Detection Methods for CVE-2021-38662

Indicators of Compromise

• Unexpected mounting of FAT, FAT32, or exFAT volumes from removable media or virtual disk images on servers and workstations.
• Local processes issuing unusual IOCTLs or raw file system requests against fastfat.sys.
• Low-privileged user accounts performing repeated file system metadata queries against attacker-controlled volumes.

Detection Strategies

• Inventory installed Windows patch levels and flag endpoints missing the October 2021 cumulative update referenced by Microsoft.
• Monitor kernel driver load events for fastfat.sys interactions originating from non-administrative user sessions.
• Correlate removable storage insertion events with subsequent process activity that touches FAT volume paths.

Monitoring Recommendations

• Enable Windows audit policies for removable storage and file system access, then forward events to a centralized SIEM for analysis.
• Alert on local accounts mounting disk images (.vhd, .vhdx, .iso) followed by access patterns targeting FAT-formatted partitions.
• Track Microsoft Defender or third-party EDR telemetry for anomalous user-to-kernel transitions involving file system drivers.

How to Mitigate CVE-2021-38662

Immediate Actions Required

• Apply the October 2021 Microsoft security update for all supported Windows desktop and server SKUs listed in the advisory.
• Prioritize patching multi-user systems, terminal servers, and shared workstations where local low-privileged access is common.
• Restrict the ability of standard users to mount arbitrary disk images or attach removable media on sensitive hosts.

Patch Information

Microsoft published fixes for CVE-2021-38662 on October 13, 2021. Administrators should install the cumulative or monthly rollup update applicable to each Windows version. Refer to the Microsoft Security Advisory CVE-2021-38662 for the full list of KB articles and download links.

Workarounds

• Disable or block use of removable FAT-formatted media through Group Policy where business requirements allow.
• Use device control policies to limit which users can attach USB storage or mount virtual disk images.
• Apply the principle of least privilege so that interactive users cannot run arbitrary code on servers that process untrusted file system content.

# Example Group Policy command to audit removable storage access on Windows hosts
auditpol /set /subcategory:"Removable Storage" /success:enable /failure:enable