CVE-2026-69347 — Windows Fast FAT Driver Remote Code Execution Vulnerability

Executive Summary

Heap-based buffer overflow in Windows Fast FAT Driver allows an unauthorized attacker to execute code locally.

Overview

7.4

CVSS HIGH

Important

MS Severity

Not Exploited

MS Exploit Status

Exploitation Unlikely

MS Exploit Likelihood

Category Remote Code Execution

Weakness CWE-122: Heap-based Buffer Overflow

Released Sep 8 2026

Last Updated Sep 8 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

CVSS Vector

ATTACK VECTOR

Local

ATTACK COMPLEXITY

High

PRIVILEGES REQUIRED

None

USER INTERACTION

None

SCOPE

Unchanged

CONFIDENTIALITY

High

INTEGRITY

High

AVAILABILITY

High

EXPLOIT CODE MATURITY

Unproven

REMEDIATION LEVEL

Official Fix

REPORT CONFIDENCE

Confirmed

Temporal Score: 6.4

EPSS Score

No EPSS score available for this CVE.

View on FIRST.org

Affected Products

30 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 1607 for 32-bit Systems	`5123099 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 1607 for x64-based Systems	`5123099 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 1809 for 32-bit Systems	`5122876 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 1809 for x64-based Systems	`5122876 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 21H2 for 32-bit Systems	`5122878 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 21H2 for ARM64-based Systems	`5122878 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 21H2 for x64-based Systems	`5122878 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 22H2 for 32-bit Systems	`5122878 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 22H2 for ARM64-based Systems	`5122878 (Security Update)`	Important	Remote Code Execution	Yes
Windows 10 Version 22H2 for x64-based Systems	`5122878 (Security Update)`	Important	Remote Code Execution	Yes
Windows 11 Version 23H2 for ARM64-based Systems	`5122880 (Security Update)`	Important	Remote Code Execution	Yes
Windows 11 Version 23H2 for x64-based Systems	`5122880 (Security Update)`	Important	Remote Code Execution	Yes
Windows 11 Version 24H2 for ARM64-based Systems	`5124008 (Security Update)`	Important	Remote Code Execution	Yes
Windows 11 Version 24H2 for x64-based Systems	`5124008 (Security Update)`	Important	Remote Code Execution	Yes
Windows 11 Version 25H2 for ARM64-based Systems	`5124008 (Security Update)`	Important	Remote Code Execution	Yes
Windows 11 Version 25H2 for x64-based Systems	`5124008 (Security Update)`	Important	Remote Code Execution	Yes
Windows 11 Version 26H1 for ARM64-based Systems	`5124012 (Security Update)`	Important	Remote Code Execution	Yes
Windows 11 version 26H1 for x64-based Systems	`5124012 (Security Update)`	Important	Remote Code Execution	Yes
Windows Server 2012	`5123065 (Monthly Rollup)`	Important	Remote Code Execution	Yes
Windows Server 2012 (Server Core installation)	`5123065 (Monthly Rollup)`	Important	Remote Code Execution	Yes
Windows Server 2012 R2	`5123066 (Monthly Rollup)`	Important	Remote Code Execution	Yes
Windows Server 2012 R2 (Server Core installation)	`5123066 (Monthly Rollup)`	Important	Remote Code Execution	Yes
Windows Server 2016	`5123099 (Security Update)`	Important	Remote Code Execution	Yes
Windows Server 2016 (Server Core installation)	`5123099 (Security Update)`	Important	Remote Code Execution	Yes
Windows Server 2019	`5122876 (Security Update)`	Important	Remote Code Execution	Yes
Windows Server 2019 (Server Core installation)	`5122876 (Security Update)`	Important	Remote Code Execution	Yes
Windows Server 2022	`5122882 (Security Update)`	Important	Remote Code Execution	Yes
Windows Server 2022 (Server Core installation)	`5122882 (Security Update)`	Important	Remote Code Execution	Yes
Windows Server 2025	`5122871 (Security Update)`	Important	Remote Code Execution	Yes
Windows Server 2025 (Server Core installation)	`5122871 (Security Update)`	Important	Remote Code Execution	Yes

Patches

10 patches

Article	Type	Restart
`5123099`	Security Update	Yes
`5122876`	Security Update	Yes
`5122878`	Security Update	Yes
`5122880`	Security Update	Yes
`5124008`	Security Update	Yes
`5124012`	Security Update	Yes
`5123065`	Monthly Rollup	Yes
`5123066`	Monthly Rollup	Yes
`5122882`	Security Update	Yes
`5122871`	Security Update	Yes

Patch Diff

ghidriff · fastfat.sys (KB5124008)

Heap-based buffer overflow (CWE-122) in the Windows Fast FAT Driver fastfat.sys volume-geometry setup, local code execution from a crafted FAT volume (Important, RCE, AV:L/AC:H/PR:N/UI:N, C:H/I:H/A:H, CVSS 7.4; Exploitation Unlikely). FatSetupAllocationSupport reads the FAT BIOS Parameter Block fields (bytes-per-sector +0x120, reserved +0x124, number-of-FATs +0x126, sectors-per-FAT +0x128, root-entries +0x12a/+0x12e, total-sectors +0x138/+0x13c) and computes the size and layout of the FAT / allocation area, storing the results at +0x158/+0x160/+0x168 which then drive buffer allocation and indexing for the volume. PRE: several of those size products were computed in 32-bit without overflow checks, so a crafted FAT volume with hostile BPB values makes the computed FAT/allocation size wrap to a small value while the driver later reads/writes the real (large) amount - a heap buffer overflow reachable simply by mounting or accessing the malicious volume (PR:N). Diff of fastfat.sys 10.0.26100.9278 -> .9444 (Sep 8 2026, KB5124008, 24H2/25H2) confirms the fix: gated behind CFR flag Feature_4205292859, FatSetupAllocationSupport adds explicit integer-overflow guards on each size product (rejects when 0xffffffff < product, and when sum < addend, e.g. uVar15 + (int)uVar17 < uVar15) and bails to the error path (LAB_0) before the size is used, so an overflowing BPB can no longer produce an undersized allocation. This is the CWE-122 heap-overflow / RCE of the pair; the sibling stack overflow is CVE-2026-68878 (FatSetFatRun, Feature_3174802747). Note: the same Sep-2026 fastfat build also hardens an unrelated EFS FSCTL parsing path (EfspValidateAndParseFsctl / EfspValidateStreamTransitionMetadata, gates Feature_709593401 / Feature_591104313); those changes are not attributed to this CVE.

Pre-patch version 10.0.26100.9278 Download

Post-patch version 10.0.26100.9444 Download

Function	Address	Change	Note
`FatSetupAllocationSupport`	`code change`	code (adds integer-overflow guards on BPB-derived FAT/allocation sizes, CFR-gated)	Computes FAT/allocation geometry from the volume BPB (offsets +0x120..+0x13c) into +0x158/+0x160/+0x168. Post (Feature_4205292859): each size product (bytes-per-sector x sectors, x number-of-FATs, x root/dir entries) is checked for 32-bit overflow (0xffffffff < product) and for additive wrap (sum < addend); on overflow it jumps to the error path before the value sizes any allocation. Pre: the products were used unchecked, so a crafted BPB could wrap the size and undersize the heap buffer.
`Feature_4205292859`	`gate`	added (CFR gate)	CFR flag gating the new integer-overflow checks in FatSetupAllocationSupport; the original unchecked size computation ships when disabled.

View full diff report View RCA report

Attack Path

A crafted FAT volume's BPB overflows the computed FAT/allocation size, undersizing a heap buffer the driver then overruns

Derived from the patch delta: the checks added by the vendor identify which fields crossed a trust boundary unvalidated. Reachability and privilege are taken from the call chain in the RCA report.

Exploits & PoC

No public exploit or PoC has been linked for this CVE yet. When available, references are sourced from public repositories and may be unverified or non-functional — review carefully before use.

Detection Rules

No public Sigma or Nuclei detection rule has been mapped to this CVE yet. Coverage is concentrated on exploited / high-profile vulnerabilities; check SigmaHQ for updates.

Acknowledgments

renyidudumen with minsight-ai.com

References

MSRC Advisory MITRE NVD