CVE-2026-58637 — Windows Client-Side Caching Elevation of Privilege Vulnerability

Executive Summary

Use after free in Windows Client-Side Caching (CSC) Service allows an authorized attacker to elevate privileges locally.

Overview

7

CVSS HIGH

Important

MS Severity

Not Exploited

MS Exploit Status

Exploitation Unlikely

MS Exploit Likelihood

Category Elevation of Privilege

Weakness CWE-416: Use After Free

Released Jul 14 2026

Last Updated Jul 14 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.00229 — 0.13801 percentile

NVD CVSS 7 HIGH — matches MSRC

CVSS Vector

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

ATTACK VECTOR

Local

ATTACK COMPLEXITY

High

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: 6.1

EPSS Score

0.00229

probability of exploitation in the next 30 days

0.13801 percentile - updated 2026-08-14

View on FIRST.org

Affected Products

27 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 1607 for 32-bit Systems	`5099535 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1607 for x64-based Systems	`5099535 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1809 for 32-bit Systems	`5099538 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1809 for x64-based Systems	`5099538 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for 32-bit Systems	`5099539 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for ARM64-based Systems	`5099539 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for x64-based Systems	`5099539 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for 32-bit Systems	`5099539 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for ARM64-based Systems	`5099539 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for x64-based Systems	`5099539 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for ARM64-based Systems	`5101650 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for x64-based Systems	`5101650 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 25H2 for ARM64-based Systems	`5101650 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 25H2 for x64-based Systems	`5101650 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 26H1 for ARM64-based Systems	`5101649 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 version 26H1 for x64-based Systems	`5101649 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2012	`5099445 (Monthly Rollup)`	Important	Elevation of Privilege	Yes
Windows Server 2012 (Server Core installation)	`5099445 (Monthly Rollup)`	Important	Elevation of Privilege	Yes
Windows Server 2012 R2	`5099444 (Monthly Rollup)`	Important	Elevation of Privilege	Yes
Windows Server 2012 R2 (Server Core installation)	`5099444 (Monthly Rollup)`	Important	Elevation of Privilege	Yes
Windows Server 2016	`5099535 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2016 (Server Core installation)	`5099535 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019	`5099538 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019 (Server Core installation)	`5099538 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022	`5099540 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025	`5099536 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025 (Server Core installation)	`5099536 (Security Update)`	Important	Elevation of Privilege	Yes

Patches

9 patches

Article	Type	Restart
`5099535`	Security Update	Yes
`5099538`	Security Update	Yes
`5099539`	Security Update	Yes
`5101650`	Security Update	Yes
`5101649`	Security Update	Yes
`5099445`	Monthly Rollup	Yes
`5099444`	Monthly Rollup	Yes
`5099540`	Security Update	Yes
`5099536`	Security Update	Yes

Patch Diff

ghidriff · cscsvc.dll (KB5101650)

Use-after-free (CWE-416) via an unsynchronized Offline Files enumerator in the Windows Client-Side Caching (CSC) service cscsvc.dll, local EoP to SYSTEM (race). The CSC service exposes an offline-files data enumerator, CEnumOfflineFilesData_Flat, whose methods (Next, Clone, Skip, Reset) advance an internal cursor over the enumerator's backing item data. PRE: these methods had no serialization, so concurrent calls on the same enumerator object from a client could race - one operation freeing/advancing the cursor or backing item-filter data while another was still using it - a use-after-free. Because cscsvc runs as SYSTEM and the enumerator is reachable by a (domain) user through the Offline Files interface, and exploitation requires winning the race (AC:H), the UAF is a local EoP-to-SYSTEM primitive. Diff of cscsvc.dll 10.0.26100.8737 -> .8875 (Jul 14 2026, KB5101650) confirms the fix: gated behind CFR flag Feature_892568891, CEnumOfflineFilesData_Flat gains a CRITICAL_SECTION member (the object grows by ~0x30 bytes; existing members shift from +0x278 to +0x2a8) and Next/Clone/Skip/Reset now EnterCriticalSection/LeaveCriticalSection around the cursor iteration and clone, serializing concurrent enumerator operations so the backing data cannot be freed/advanced while in use; Clone additionally reads the path via CPath::_GetConstBuffer instead of CCscScope::Path.

Pre-patch version 10.0.26100.8737 Download

Post-patch version 10.0.26100.8875 Download

Function	Address	Change	Note
`CEnumOfflineFilesData_Flat::Next`	`code change`	code (cursor iteration now under a critical section, CFR-gated)	Post (Feature_892568891): EnterCriticalSection(this+0x278) before _Next(...) and LeaveCriticalSection after, serializing concurrent enumeration so the cursor/backing data is not freed/advanced by a racing operation.
`CEnumOfflineFilesData_Flat::Clone`	`code change`	code (clone under the critical section; safer buffer access)	Post: clone/CreateInstance guarded by the critical section; reads the path via CPath::_GetConstBuffer instead of CCscScope::Path (avoids a transient/dangling buffer).
`CEnumOfflineFilesData_Flat::CEnumOfflineFilesData_Flat / ~ / Skip / Reset / CreateInstance`	`code change`	code (object grows to hold a CRITICAL_SECTION; init/teardown updated)	Constructor initializes and destructor deletes the new CRITICAL_SECTION; item-filter block and other members shift from +0x278 to +0x2a8; Skip/Reset participate in the serialized access.
`Feature_892568891`	`gate`	added (CFR gate)	CFR flag gating the critical-section serialization; the original unsynchronized enumerator still ships when disabled.

View full diff report View RCA report

Attack Path

Concurrent operations on an unsynchronized Offline Files enumerator free the cursor's backing data while another call uses it

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

No public exploit or PoC has been linked for this CVE yet. When available, references are sourced from public repositories and may be unverified or non-functional — review carefully before use.

Detection Rules

No public Sigma or Nuclei detection rule has been mapped to this CVE yet. Coverage is concentrated on exploited / high-profile vulnerabilities; check SigmaHQ for updates.

Acknowledgments

z1r0
Byunghyun Kang

References

MSRC Advisory MITRE NVD