CVE-2026-40409 — Windows Universal Disk Format File System Driver (UDFS) Elevation of Privilege Vulnerability

Executive Summary

None

Overview

7.8

CVSS HIGH

Important

MS Severity

Not Exploited

MS Exploit Status

Less Likely

MS Exploit Likelihood

Category Elevation of Privilege

Weakness CWE-197: Numeric Truncation Error

Released Jun 9 2026

Last Updated Jun 9 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.00298 — 0.22255 percentile

NVD CVSS 7.8 HIGH — matches MSRC

CVSS Vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

ATTACK VECTOR

Local

ATTACK COMPLEXITY

Low

PRIVILEGES REQUIRED

Low

USER INTERACTION

None

SCOPE

Unchanged

CONFIDENTIALITY

High

INTEGRITY

High

AVAILABILITY

High

EXPLOIT CODE MATURITY

Unproven

REMEDIATION LEVEL

Official Fix

REPORT CONFIDENCE

Confirmed

Temporal Score: 6.8

EPSS Score

0.00298

probability of exploitation in the next 30 days

0.22255 percentile - updated 2026-08-14

View on FIRST.org

Affected Products

30 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 1607 for 32-bit Systems	`5094122 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1607 for x64-based Systems	`5094122 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1809 for 32-bit Systems	`5094123 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1809 for x64-based Systems	`5094123 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for 32-bit Systems	`5094127 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for ARM64-based Systems	`5094127 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for x64-based Systems	`5094127 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for 32-bit Systems	`5094127 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for ARM64-based Systems	`5094127 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for x64-based Systems	`5094127 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 23H2 for ARM64-based Systems	`5093998 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 23H2 for x64-based Systems	`5093998 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for ARM64-based Systems	`5094126 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for x64-based Systems	`5094126 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 25H2 for ARM64-based Systems	`5094126 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 25H2 for x64-based Systems	`5094126 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 26H1 for ARM64-based Systems	`5095051 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 version 26H1 for x64-based Systems	`5095051 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2012	`5094042 (Monthly Rollup)`	Important	Elevation of Privilege	Yes
Windows Server 2012 (Server Core installation)	`5094042 (Monthly Rollup)`	Important	Elevation of Privilege	Yes
Windows Server 2012 R2	`5094041 (Monthly Rollup)`	Important	Elevation of Privilege	Yes
Windows Server 2012 R2 (Server Core installation)	`5094041 (Monthly Rollup)`	Important	Elevation of Privilege	Yes
Windows Server 2016	`5094122 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2016 (Server Core installation)	`5094122 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019	`5094123 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019 (Server Core installation)	`5094123 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022	`5094128 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022 (Server Core installation)	`5094128 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025	`5094125 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025 (Server Core installation)	`5094125 (Security Update)`	Important	Elevation of Privilege	Yes

Patches

10 patches

Article	Type	Restart
`5094122`	Security Update	Yes
`5094123`	Security Update	Yes
`5094127`	Security Update	Yes
`5093998`	Security Update	Yes
`5094126`	Security Update	Yes
`5095051`	Security Update	Yes
`5094042`	Monthly Rollup	Yes
`5094041`	Monthly Rollup	Yes
`5094128`	Security Update	Yes
`5094125`	Security Update	Yes

Patch Diff

ghidriff · udfs.sys (KB5094126)

Numeric truncation error (CWE-197) in udfs.sys UdfGetBlocksNeeded, local EoP via a crafted UDF image. The function converts a 64-bit byte offset/length into a logical block/sector number by right-shifting Vcb->LBSizeShift (Vcb+0x48). PRE: the converted result can exceed 32 bits but was used in a 32-bit slot with no upper-bound check, so a large 64-bit offset/length that does not fit in 32 bits even after the shift has its high bits silently dropped, producing a block/sector number smaller than the real value; that truncated value flows into a disk-I/O sector count or an MCB VBN, misaligning logical<->physical sectors or under-computing the extent. Diff of udfs.sys 10.0.26100.8521 -> .8655 (Jun 9 2026, KB5094126) confirms the fix: gated behind CFR flag Feature_3105867064, before the conversion if (*a3 < 0 || *a3 > 0xFFFFFFFF<<LBSizeShift) ExRaiseStatus(0xC000000D / STATUS_INVALID_PARAMETER). Note: our diff shows Feature_3105867064 (build/arch); the negative + 0xFFFFFFFF<<LBSizeShift bound is as described.

Pre-patch version 10.0.26100.8521 Download

Post-patch version 10.0.26100.8655 Download

Function	Address	Change	Note
`UdfGetBlocksNeeded`	`code change`	code (offset/length range guard, CFR-gated)	Post (Feature_3105867064): if ((longlong)param_3 < 0 \|\| 0xffffffffL<<(LBSizeShift&0x3f) < param_3) ExRaiseStatus(0xC000000D). Pre: 64-bit offset/length shifted by LBSizeShift and used as a 32-bit VBN/sector count with no bound -> truncation.
`Feature_3105867064`	`gate`	added (CFR gate)	CFR flag gating the negative / 0xFFFFFFFF<<LBSizeShift bound before the 64->32-bit block conversion.

View full diff report View RCA report

Attack Path

A 64-bit offset/length that overflows 32 bits after the block shift truncates to a small VBN/sector count

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

Thanatos Tian (PolyU) of Diffract
R4nger with Kunlun Lab
Zhiniang Peng with HUST
@2st___ of Diffract
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1nv0k3r

References

MSRC Advisory MITRE NVD