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Windows Common Log File System Driver

CVE-2026-40407 — Windows Common Log File System Driver Elevation of Privilege Vulnerability

Important CVSS 7.8 EPSS 0.00304 🔬 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
Exploitation Unlikely
MS Exploit Likelihood
Category Elevation of Privilege
Weakness CWE-122: Heap-based Buffer Overflow
Released May 12 2026
Last Updated May 12 2026
Publicly Disclosed No
CISA KEV Not Listed
Known Exploits None Known
EPSS Score 0.00304 — 0.22948 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

0.00304
probability of exploitation in the next 30 days
0.22948 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)

Heap-based buffer overflow fix in CClfsLogFcbPhysical::ReadLogBlock. Adds a new staged-rollout flag, Feature_748929339, gating a bounds check on the caller-supplied LSN against the resolved owner-page boundary before falling into the CcCopyRead/memset block-copy loop. Attribution to this CVE is inferred from the check's bounds-check 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::ReadLogBlock 140002470 code, length, name, sig, calling, called patch flag added: Feature_748929339__private_IsEnabledDeviceUsageNoInline
View full diff report View RCA report Download PoC

Attack Path

Heap overflow in ReadLogBlock - the caller-supplied LSN is not checked against the resolved owner-page boundary

Attack path for CVE-2026-40407 Heap overflow in ReadLogBlock - the caller-supplied LSN is not checked against the resolved owner-page boundary 01 — ENTRY Standard user opens a CLFS log and requests a block read Unprivileged; CLFS is a core kernel component. 02 — CONTROLLED INPUT The caller chooses the LSN identifying which block to read The LSN resolves to an owner page and an offset within it. 03 — PATH CClfsLogFcbPhysical::ReadLogBlock falls into the block-copy loop The path reaches CcCopyRead and a memset over the destination block buffer. 04 — MISSING CHECK No bounds check on the LSN against the resolved owner-page boundary An LSN pointing past the page's valid extent lets the copy loop run beyond the buffer. The patch adds the bounds check before the copy, gated behind Feature_748929339. 05 — PRIMITIVE Heap-based buffer overflow in the block copy Note: this flag lives on the 10.0.28000.x Server branch and is absent from 24H2 - it is a distinct defect from the CVE-2026-62728 double fetch that also touches this function.

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

Microsoft

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