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Windows Common Log File System Driver

CVE-2026-40397 — Windows Common Log File System Driver Elevation of Privilege Vulnerability

Important CVSS 7.8 EPSS 0.00273 🔬 Patch diffed 2026-05 archive

Executive Summary

Heap-based buffer overflow in Windows Common Log File System Driver allows an authorized attacker to elevate privileges locally.

Overview

7.8
CVSS HIGH
Important
MS Severity
Not Exploited
MS Exploit Status
More Likely
MS Exploit Likelihood
Category Elevation of Privilege
Weakness CWE-191: Integer Underflow (Wrap or Wraparound)
Released May 12 2026
Last Updated May 12 2026
Publicly Disclosed No
CISA KEV Not Listed
Known Exploits None Known
EPSS Score 0.00273 — 0.19543 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

0.00273
probability of exploitation in the next 30 days
0.19543 percentile - updated 2026-08-14
View on FIRST.org

Affected Products

23 affected products
Product KB Article Severity Impact Restart Required
Windows 10 Version 1607 for 32-bit Systems 5087537 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1607 for x64-based Systems 5087537 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for 32-bit Systems 5087538 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for x64-based Systems 5087538 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H2 for 32-bit Systems 5087544 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H2 for ARM64-based Systems 5087544 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H2 for x64-based Systems 5087544 (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 5087420 (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 5089549 (Security Update) 5089466 (Security Hotpatch Update) Important Elevation of Privilege 5082063 Base: 7.8 Temporal: 6.8 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 10.0.26100.8457 10.0.26100.8390 Yes None Windows 11 Version 24H2 for x64-based Systems 5089549 (Security Update) 5089466 (Security Hotpatch Update) Important Elevation of Privilege 5082063 Base: 7.8 Temporal: 6.8 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 10.0.26100.8457 10.0.26100.8390 Yes None Windows 11 Version 25H2 for ARM64-based Systems 5089549 (Security Update) 5089466 (Security Hotpatch Update) Important Elevation of Privilege 5083769 Base: 7.8 Temporal: 6.8 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 10.0.26200.8457 10.0.26200.8390 Yes None Windows 11 Version 25H2 for x64-based Systems 5089549 (Security Update) 5089466 (Security Hotpatch Update) Important Elevation of Privilege 5083769 Base: 7.8 Temporal: 6.8 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 10.0.26200.8457 10.0.26200.8390 Yes None Windows 11 Version 26H1 for ARM64-based Systems 5089548 (Security Update) Important Elevation of Privilege Yes
Windows 11 version 26H1 for x64-based Systems 5089548 (Security Update) Important Elevation of Privilege Yes
Windows Server 2012 5087470 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 (Server Core installation) 5087470 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 R2 5087471 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 R2 (Server Core installation) 5087471 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2016 5087537 (Security Update) Important Elevation of Privilege Yes
Windows Server 2016 (Server Core installation) 5087537 (Security Update) Important Elevation of Privilege Yes
Windows Server 2019 5087538 (Security Update) Important Elevation of Privilege Yes
Windows Server 2019 (Server Core installation) 5087538 (Security Update) Important Elevation of Privilege Yes
Windows Server 2022 5087545 (Security Update) 5087424 (Security Hotpatch Update) Important Elevation of Privilege 5082142 Base: 7.8 Temporal: 6.8 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 10.0.20348.5139 10.0.20348.5074 Yes None Windows Server 2022 (Server Core installation) 5087545 (Security Update) 5087424 (Security Hotpatch Update) Important Elevation of Privilege 5082142 Base: 7.8 Temporal: 6.8 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 10.0.20348.5139 10.0.20348.5074 Yes None Windows Server 2022, 23H2 Edition (Server Core installation) 5087541 (Security Update) Important Elevation of Privilege Yes

Patches

10 patches
Article Type Restart
5087537 Security Update Yes
5087538 Security Update Yes
5087544 Security Update Yes
5094127 Security Update Yes
5087420 Security Update Yes
5093998 Security Update Yes
5089548 Security Update Yes
5087470 Monthly Rollup Yes
5087471 Monthly Rollup Yes
5087541 Security Update Yes

Patch Diff

ghidriff · clfs.sys (KB5089549)

Integer underflow fix in CClfsLogFcbPhysical::AdjustReservation (new function, refactored out of UnmarkLogFileContainers). Adds a new staged-rollout flag, Feature_1201914170, gating a sector-alignment check on the caller-supplied offset before an unsigned subtraction that computes the new reservation size. Attribution to this CVE is inferred from the check's alignment-before-subtraction shape, not confirmed against Microsoft's internal records.

Pre-patch version 10.0.28000.1896 Download
Post-patch version 10.0.28000.2113 Download
Function Address Change Note
CClfsLogFcbPhysical::AdjustReservation 14007feb0 added new function (refactored out of UnmarkLogFileContainers); patch flag added: Feature_1201914170__private_IsEnabledDeviceUsageNoInline
CClfsLogFcbPhysical::UnmarkLogFileContainers 14006ebc0 -> 14007bcb0 code, length, calling, called context only, no flag check; reservation arithmetic moved out to AdjustReservation
View full diff report View RCA report Download PoC

Attack Path

Integer underflow in the CLFS reservation adjustment - an unaligned offset subtracts past zero

Attack path for CVE-2026-40397 Integer underflow in the CLFS reservation adjustment - an unaligned offset subtracts past zero 01 — ENTRY Standard user creates a CLFS log and drives container reservation changes CreateLogFile plus reservation operations. CLFS is present on every install and needs no privilege. 02 — CONTROLLED INPUT The caller supplies the offset used in the reservation arithmetic The value reaches the subtraction without an alignment guarantee. 03 — PATH UnmarkLogFileContainers computes the new reservation size The patch refactors this arithmetic out into a new function, CClfsLogFcbPhysical::AdjustReservation. 04 — MISSING CHECK No sector-alignment check before an unsigned subtraction An unaligned or oversized offset makes the subtraction underflow to a very large unsigned value. The patch adds the alignment check before the subtract, gated behind Feature_1201914170. 05 — PRIMITIVE Reservation size wraps to a huge value, corrupting log accounting Downstream appends and flushes size their writes from the corrupted reservation - the standard CLFS metadata-to-pool-overflow route.

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.

Known Exploits

Acknowledgments

Puneeth Kumar Katikela
Microsoft

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