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Windows Cloud Files Mini Filter Driver

CVE-2026-83991 — Windows Cloud Files Mini Filter Driver Tampering Vulnerability

Important CVSS 5.5 🔬 Patch diffed 2026-09 archive

Executive Summary

Missing authentication for critical function in Windows Cloud Files Mini Filter Driver allows an authorized attacker to perform tampering locally.

Overview

5.5
CVSS MEDIUM
Important
MS Severity
Not Exploited
MS Exploit Status
Less Likely
MS Exploit Likelihood
Category Tampering
Weakness CWE-306: Missing Authentication for Critical Function
Released Sep 8 2026
Last Updated Sep 8 2026
Publicly Disclosed No
CISA KEV Not Listed
Known Exploits None Known

CVSS Vector

ATTACK VECTOR
Local
ATTACK COMPLEXITY
Low
PRIVILEGES REQUIRED
Low
USER INTERACTION
None
SCOPE
Unchanged
CONFIDENTIALITY
None
INTEGRITY
High
AVAILABILITY
None
EXPLOIT CODE MATURITY
Unproven
REMEDIATION LEVEL
Official Fix
REPORT CONFIDENCE
Confirmed
Temporal Score: 4.8

EPSS Score

No EPSS score available for this CVE.

View on FIRST.org

Affected Products

22 affected products
Product KB Article Severity Impact Restart Required
Windows 10 Version 1809 for 32-bit Systems 5122876 (Security Update) Important Tampering Yes
Windows 10 Version 1809 for x64-based Systems 5122876 (Security Update) Important Tampering Yes
Windows 10 Version 21H2 for 32-bit Systems 5122878 (Security Update) Important Tampering Yes
Windows 10 Version 21H2 for ARM64-based Systems 5122878 (Security Update) Important Tampering Yes
Windows 10 Version 21H2 for x64-based Systems 5122878 (Security Update) Important Tampering Yes
Windows 10 Version 22H2 for 32-bit Systems 5122878 (Security Update) Important Tampering Yes
Windows 10 Version 22H2 for ARM64-based Systems 5122878 (Security Update) Important Tampering Yes
Windows 10 Version 22H2 for x64-based Systems 5122878 (Security Update) Important Tampering Yes
Windows 11 Version 23H2 for ARM64-based Systems 5122880 (Security Update) Important Tampering Yes
Windows 11 Version 23H2 for x64-based Systems 5122880 (Security Update) Important Tampering Yes
Windows 11 Version 24H2 for ARM64-based Systems 5124008 (Security Update) Important Tampering Yes
Windows 11 Version 24H2 for x64-based Systems 5124008 (Security Update) Important Tampering Yes
Windows 11 Version 25H2 for ARM64-based Systems 5124008 (Security Update) Important Tampering Yes
Windows 11 Version 25H2 for x64-based Systems 5124008 (Security Update) Important Tampering Yes
Windows 11 Version 26H1 for ARM64-based Systems 5124012 (Security Update) Important Tampering Yes
Windows 11 version 26H1 for x64-based Systems 5124012 (Security Update) Important Tampering Yes
Windows Server 2019 5122876 (Security Update) Important Tampering Yes
Windows Server 2019 (Server Core installation) 5122876 (Security Update) Important Tampering Yes
Windows Server 2022 5122882 (Security Update) Important Tampering Yes
Windows Server 2022 (Server Core installation) 5122882 (Security Update) Important Tampering Yes
Windows Server 2025 5122871 (Security Update) Important Tampering Yes
Windows Server 2025 (Server Core installation) 5122871 (Security Update) Important Tampering Yes

Patches

7 patches
Article Type Restart
5122876 Security Update Yes
5122878 Security Update Yes
5122880 Security Update Yes
5124008 Security Update Yes
5124012 Security Update Yes
5122882 Security Update Yes
5122871 Security Update Yes

Patch Diff

ghidriff · cldflt.sys (KB5124008)

Missing authentication for a critical function (CWE-306) in the Windows Cloud Files Mini Filter Driver cldflt.sys placeholder-create path, local tampering of protected file state (Important, Tampering, AV:L/AC:L/PR:L/UI:N, C:N/I:H/A:N, CVSS 5.5). CfCreatePlaceholders (kernel handler HsmpOpCreatePlaceholders) creates cloud placeholders under a registered sync root; the per-entry CF_PLACEHOLDER_CREATE_FLAG_SUPERSEDE (0x4) flag supplies overwrite semantics. PRE: the supersede path accepted an ordinary existing leaf file and converted it into a cloud placeholder based only on the caller's authority over the base directory, without access-checking the existing leaf's own DACL. A low-privileged caller who can create children in a directory (FILE_ADD_FILE) but is denied write/delete on a specific protected leaf could therefore supersede that leaf - turning a read-only file (attributes 0x20) into a Cloud Files placeholder (IO_REPARSE_TAG_CLOUD 0x9000001A, attributes 0x00401600) with the same NTFS file ID, i.e. tamper with protected system data beyond the caller's rights. Diff of cldflt.sys 10.0.26100.9278 -> .9444 (Sep 8 2026, KB5124008, 24H2/25H2) confirms the fix: gated behind CFR flag Feature_1245077819, HsmpOpCreatePlaceholders now calls a NEW HsmpAccessCheck helper against the existing leaf on the supersede path and rejects the operation when the caller lacks the required access to that leaf, closing the parent-authority-is-not-leaf-authority gap. Confirmed against the public PoC/writeup (karollooool/CVE-2026-83991-writeup-and-poc), which demonstrates GENERIC_WRITE and DeleteFileW failing with ERROR_ACCESS_DENIED on the leaf while CfCreatePlaceholders(supersede) returns S_OK and rewrites it as a placeholder under one restricted medium-integrity thread token. No memory corruption is involved; the flaw is a missing authorization check on a state-changing operation.

Pre-patch version 10.0.26100.9278 Download
Post-patch version 10.0.26100.9444 Download
Function Address Change Note
HsmpOpCreatePlaceholders code change code (adds leaf access check on the supersede path, CFR-gated) Kernel handler for CfCreatePlaceholders. Post (Feature_1245077819): invokes the new HsmpAccessCheck against the pre-existing leaf before applying CF_PLACEHOLDER_CREATE_FLAG_SUPERSEDE, rejecting the conversion when the caller lacks access to that leaf. Pre: superseded an existing file based only on base-directory authority.
HsmpAccessCheck new function added (authorization helper) New helper that performs the missing access check on the target object; called from the create-placeholders/supersede path so parent-directory rights no longer authorize a state-changing supersede against a more restrictive existing child.
Feature_1245077819 gate added (CFR gate) CFR flag gating the new access check in HsmpOpCreatePlaceholders; the original unchecked supersede path ships when disabled.
View full diff report View RCA report Download PoC

Attack Path

Base-directory create rights are abused to supersede a protected read-only leaf into a cloud placeholder without a leaf access check

Attack path for CVE-2026-83991 Base-directory create rights are abused to supersede a protected read-only leaf into a cloud placeholder without a leaf access check 01 — ENTRY Low-privileged caller controls a directory but not a protected leaf inside it The caller has FILE_ADD_FILE on the parent (enough to register a sync root and create placeholders) but the existing leaf's protected DACL grants only read - write/delete are denied. AV:L / PR:L / UI:N. 02 — CONTROLLED INPUT Register the parent as a sync root and call CfCreatePlaceholders with SUPERSEDE CF_PLACEHOLDER_CREATE_FLAG_SUPERSEDE (0x4) targets the existing leaf; the request reaches HsmpOpCreatePlaceholders in cldflt.sys. 03 — MISSING CHECK No access check against the existing leaf (CWE-306) Pre-patch the supersede path authorized the conversion from base-directory rights alone, never checking the leaf's own DACL - GENERIC_WRITE/DeleteFileW on the same leaf return ERROR_ACCESS_DENIED, but supersede returns S_OK. 04 — IMPACT Protected leaf tampered into a cloud placeholder The read-only file becomes an IO_REPARSE_TAG_CLOUD placeholder (attributes 0x20 -> 0x00401600) with the same file ID - unauthorized modification of protected file state (Important, 5.5, I:H).

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

Detection Rules

Acknowledgments

Karol Nieckarz

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