OnlyPatch
Windows Common Log File System Driver

CVE-2021-36955 — Windows Common Log File System Driver Elevation of Privilege Vulnerability

Important CVSS 7.8 EPSS 0.04037 🔬 Patch diffed 2021-09 archive

Executive Summary

None

Overview

7.8
CVSS HIGH
Important
MS Severity
Not Exploited
MS Exploit Status
More Likely
MS Exploit Likelihood
Category Elevation of Privilege
Released Sep 14 2021
Last Updated Sep 14 2021
Publicly Disclosed No
CISA KEV Not Listed
Known Exploits None Known
EPSS Score 0.04037 — 0.89708 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:F/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
Functional
REMEDIATION LEVEL
Official Fix
REPORT CONFIDENCE
Confirmed
Temporal Score: 7.2

EPSS Score

0.04037
probability of exploitation in the next 30 days
0.89708 percentile - updated 2026-08-14
View on FIRST.org

Affected Products

28 affected products
Product KB Article Severity Impact Restart Required
Windows 10 for 32-bit Systems 5005569 (Security Update) Important Elevation of Privilege Yes
Windows 10 for x64-based Systems 5005569 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1607 for 32-bit Systems 5005573 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1607 for x64-based Systems 5005573 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for 32-bit Systems 5005568 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for ARM64-based Systems 5005568 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for x64-based Systems 5005568 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1909 for 32-bit Systems 5005566 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1909 for ARM64-based Systems 5005566 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1909 for x64-based Systems 5005566 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 2004 for 32-bit Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 2004 for ARM64-based Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 2004 for x64-based Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 20H2 for 32-bit Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 20H2 for ARM64-based Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 20H2 for x64-based Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H1 for 32-bit Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H1 for ARM64-based Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H1 for x64-based Systems 5005565 (Security Update) Important Elevation of Privilege Yes
Windows 7 for 32-bit Systems Service Pack 1 5005633 (Monthly Rollup) 5005615 (Security Only) Important Elevation of Privilege 5005088 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.1.7601.25712 Yes 5005633 5005615 Windows 7 for x64-based Systems Service Pack 1 5005633 (Monthly Rollup) 5005615 (Security Only) Important Elevation of Privilege 5005088 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.1.7601.25712 Yes 5005633 5005615 Windows 8.1 for 32-bit systems 5005613 (Monthly Rollup) 5005627 (Security Only) Important Elevation of Privilege 5005076 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.3.9600.20120 Yes 5005613 5005627 Windows 8.1 for x64-based systems 5005613 (Monthly Rollup) 5005627 (Security Only) Important Elevation of Privilege 5005076 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.3.9600.20120 Yes 5005613 5005627 Windows RT 8.1 5005613 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2008 for 32-bit Systems Service Pack 2 5005606 (Monthly Rollup) 5005618 (Security Only) Important Elevation of Privilege 5005090 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.0.6003.21218 Yes 5005606 5005618 Windows Server 2008 for 32-bit Systems Service Pack 2 (Server Core installation) 5005606 (Monthly Rollup) 5005618 (Security Only) Important Elevation of Privilege 5005090 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.0.6003.21218 Yes 5005606 5005618 Windows Server 2008 for x64-based Systems Service Pack 2 5005606 (Monthly Rollup) 5005618 (Security Only) Important Elevation of Privilege 5005090 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.0.6003.21218 Yes 5005606 5005618 Windows Server 2008 for x64-based Systems Service Pack 2 (Server Core installation) 5005606 (Monthly Rollup) 5005618 (Security Only) Important Elevation of Privilege 5005090 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.0.6003.21218 Yes 5005606 5005618 Windows Server 2008 R2 for x64-based Systems Service Pack 1 5005633 (Monthly Rollup) 5005615 (Security Only) Important Elevation of Privilege 5005088 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.1.7601.25712 Yes 5005633 5005615 Windows Server 2008 R2 for x64-based Systems Service Pack 1 (Server Core installation) 5005633 (Monthly Rollup) 5005615 (Security Only) Important Elevation of Privilege 5005088 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.1.7601.25712 Yes 5005633 5005615 Windows Server 2012 5005623 (Monthly Rollup) 5005607 (Security Only) Important Elevation of Privilege 5005094 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.2.9200.23462 Yes 5005623 5005607 Windows Server 2012 (Server Core installation) 5005623 (Monthly Rollup) 5005607 (Security Only) Important Elevation of Privilege 5005094 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.2.9200.23462 Yes 5005623 5005607 Windows Server 2012 R2 5005613 (Monthly Rollup) 5005627 (Security Only) Important Elevation of Privilege 5005076 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.3.9600.20120 Yes 5005613 5005627 Windows Server 2012 R2 (Server Core installation) 5005613 (Monthly Rollup) 5005627 (Security Only) Important Elevation of Privilege 5005076 Base: 7.8 Temporal: 7.2 Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C 6.3.9600.20120 Yes 5005613 5005627 Windows Server 2016 5005573 (Security Update) Important Elevation of Privilege Yes
Windows Server 2016 (Server Core installation) 5005573 (Security Update) Important Elevation of Privilege Yes
Windows Server 2019 5005568 (Security Update) Important Elevation of Privilege Yes
Windows Server 2019 (Server Core installation) 5005568 (Security Update) Important Elevation of Privilege Yes
Windows Server 2022 5005575 (Security Update) Important Elevation of Privilege Yes
Windows Server 2022 (Server Core installation) 5005575 (Security Update) Important Elevation of Privilege Yes
Windows Server, version 2004 (Server Core installation) 5005565 (Security Update) Important Elevation of Privilege Yes
Windows Server, version 20H2 (Server Core Installation) 5005565 (Security Update) Important Elevation of Privilege Yes

Patches

7 patches
Article Type Restart
5005569 Security Update Yes
5005573 Security Update Yes
5005568 Security Update Yes
5005566 Security Update Yes
5005565 Security Update Yes
5005613 Monthly Rollup Yes
5005575 Security Update Yes

Patch Diff

ghidriff · clfs.sys (KB5005565)

Use-after-free in Windows Common Logging File System (clfs.sys). When opening a Base Log File with a crafted Extend Context in the Control Record, ExtendMetadataBlockDescriptor frees the pbImage of the iFlushBlock metadata block and then repopulates it from the shadow block entry. Because the general block and its shadow were previously set to point to the same memory, this leaves a dangling reference. Subsequent access to the General Metadata Block results in a use-after-free.

Pre-patch version 10.0.19041.1052
Post-patch version 10.0.19041.1237
Function Address Change Note
CClfsBaseFilePersisted::ExtendMetadataBlockDescriptor modified Added validation that general block and shadow block do not point to same memory before freeing
CClfsBaseFilePersisted::ReadMetadataBlock modified Added nullification of dangling shadow block references after free
View full diff report View RCA report Download PoC

Attack Path

Use-after-free in CLFS metadata block handling - a block and its shadow point at the same allocation, so freeing one leaves the other dangling

Attack path for CVE-2021-36955 Use-after-free in CLFS metadata block handling - a block and its shadow point at the same allocation, so freeing one leaves the other dangling 01 — ENTRY Standard user creates a Base Log File in a user-writable directory CreateLogFile. CLFS is a core kernel component present on every install; no elevation required. 02 — CONTROLLED INPUT The .blf carries a crafted Extend Context in the Control Record The Extend Context drives which metadata block is extended and which shadow entry it is repopulated from. 03 — PATH Opening the log walks the metadata block descriptors CClfsBaseFilePersisted::ExtendMetadataBlockDescriptor frees the pbImage of the iFlushBlock metadata block, then repopulates it from the shadow block entry. 04 — MISSING CHECK The general block and its shadow were previously set to the same memory Freeing via one leaves the other holding a dangling pointer. The patch adds validation that the general and shadow blocks do not alias before the free, and nullifies dangling shadow references in ReadMetadataBlock afterwards. 05 — PRIMITIVE Subsequent access through the shadow reference touches freed pool Kernel use-after-free reached from an unprivileged log open - the same CLFS metadata-parsing surface as CVE-2022-24521, CVE-2022-37969 and CVE-2023-28252.

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

None

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