CVE-2021-31969 — Windows Cloud Files Mini Filter Driver 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

Released Jun 8 2021

Last Updated Jun 8 2021

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.01208 — 0.65196 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

0.01208

probability of exploitation in the next 30 days

0.65196 percentile - updated 2026-07-25

View on FIRST.org

Affected Products

18 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 1809 for 32-bit Systems	`5003646 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1809 for ARM64-based Systems	`5003646 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1809 for x64-based Systems	`5003646 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1909 for 32-bit Systems	`5003635 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1909 for ARM64-based Systems	`5003635 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1909 for x64-based Systems	`5003635 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 2004 for 32-bit Systems	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 2004 for ARM64-based Systems	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 2004 for x64-based Systems	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 20H2 for 32-bit Systems	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 20H2 for ARM64-based Systems	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H1 for 32-bit Systems	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H1 for ARM64-based Systems	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H1 for x64-based Systems	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019	`5003646 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019 (Server Core installation)	`5003646 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server, version 2004 (Server Core installation)	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server, version 20H2 (Server Core Installation)	`5003637 (Security Update)`	Important	Elevation of Privilege	Yes

Patches

3 patches

Article	Type	Restart
`5003646`	Security Update	Yes
`5003635`	Security Update	Yes
`5003637`	Security Update	Yes

Patch Diff

ghidriff · cldflt.sys (KB5003646)

Integer underflow in cldflt.sys HsmpRpiDecompressBuffer leading to controlled paged pool overflow from 0x20-byte allocation. cstmDataSize (WORD) controls allocation size (cstmDataSize + 8). No minimum check allows value 0, causing allocatedSize - 12 to underflow to 0xFFFFFFF4 for RtlDecompressBuffer. LZNT1 header can be set to uncompressed, making the call behave as memcpy with attacker-controlled size and content. Patch adds if (cstmDataSize >= 4) guard. Related: CVE-2023-36036 (same function, missing max check, exploited ITW).

Pre-patch version 10.0.19041.906

Post-patch version 10.0.19041.1052

Function	Address	Change	Note
`HsmpRpiDecompressBuffer`		code (minimum size check added)	Added check that cstmDataSize >= 4 before computing allocatedSize = cstmDataSize + 8. Prevents unsigned integer underflow when (allocatedSize - 12) is passed as UncompressedBufferSize to RtlDecompressBuffer.

View full diff report View RCA report Download PoC

Known Exploits

No known exploits have been linked for this CVE yet. When available, exploit references will be sourced from public repositories and may be unverified, incomplete, or non-functional. Always review code carefully before use in any environment.

Acknowledgments

Anonymous working with Trend Micro Zero Day Initiative
Keqi Hu

References

MSRC Advisory MITRE NVD