Windows: Heap-based buffer overflow in Client-Side Caching (CSC) Service

Summary

A heap-based buffer overflow in the Windows Client-Side Caching (CSC) Service allows an authorized local user to elevate privileges. An attacker needs a low-privileged logon or code already running on the device, so the flaw is not remotely exploitable by itself. The CVSS 3.1 base score is 7.8 (high), with high impact on confidentiality, integrity and availability.

Affected

• Windows 10 Version 1607: 10.0.14393.0 to < 10.0.14393.9612
• Windows 10 Version 1809: 10.0.17763.0 to < 10.0.17763.9376
• Windows 10 Version 21H2: 10.0.19044.0 to < 10.0.19044.7807
• Windows 10 Version 22H2: 10.0.19045.0 to < 10.0.19045.7807
• Windows 11 Version 23H2: 10.0.22631.0 to < 10.0.22631.7652
• Windows 11 Version 24H2: 10.0.26100.0 to < 10.0.26100.9711
• Windows 11 Version 25H2: 10.0.26200.0 to < 10.0.26200.9711
• Windows 11 Version 26H1: 10.0.28000.0 to < 10.0.28000.3231
• Windows 11 Version 26H2: 10.0.26300.0 to < 10.0.26300.9711
• Windows Server 2012 (including Server Core): 6.2.9200.0 to < 6.2.9200.26434
• Windows Server 2012 R2 (including Server Core): 6.3.9600.0 to < 6.3.9600.23470
• Windows Server 2016: 10.0.14393.0 to < 10.0.14393.9612

Fixed in

• Fixed builds per the upper bound of the CNA record; verify on the MSRC advisory:
• Windows 10 1607 and Server 2016: 10.0.14393.9612
• Windows 10 1809: 10.0.17763.9376
• Windows 10 21H2: 10.0.19044.7807
• Windows 10 22H2: 10.0.19045.7807
• Windows 11 23H2: 10.0.22631.7652
• Windows 11 24H2: 10.0.26100.9711
• Windows 11 25H2: 10.0.26200.9711
• Windows 11 26H1: 10.0.28000.3231
• Windows 11 26H2: 10.0.26300.9711
• Windows Server 2012: 6.2.9200.26434
• Windows Server 2012 R2: 6.3.9600.23470
• Security updates of October 2026. KB numbers: Unknown.

Workaround

Unknown. No Microsoft workaround found.

• Disabling the CSC/Offline Files feature where it is not needed is a common hardening option, but it is not verified to mitigate this CVE.

Exploitation

• No public proof of concept or in-the-wild exploitation is known at the time of writing. This means nothing is known, not that exploitation is ruled out.
• CISA SSVC: exploitation none, not automatable, total technical impact.
• EPSS below 1 percent. Not listed in CISA KEV.

Our recommendation

• Patch all affected Windows 10, Windows 11 and Windows Server systems with the October 2026 security updates. Prioritise multi-user systems such as RDS servers and shared PCs.
• Inventory all Windows versions, not only 1607, and check the support status of older builds.
• Deploy the related October 2026 CSC fixes as well (CVE-2026-83699, CVE-2026-70149).
• Monitor for public proof-of-concept code or a KEV listing and reassess the risk if the status changes.

Public exploits iAttack code is publicly available (PoC on GitHub, Metasploit or Exploit-DB).

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

• Microsoft Security Update Guide: CVE-2026-83698 msrc.microsoft.com
• OpenCVE: CVE-2026-83698 app.opencve.io
• Strix: CVE-2026-83698 www.strix.ai
• Vulners CVE list: CVE-2026-83698 vulners.com
• Microsoft Security Update Guide: CVE-2026-83580 (same release) msrc.microsoft.com
• Microsoft Security Update Guide: CVE-2026-83699 (related CSC EoP) msrc.microsoft.com
• Microsoft Security Update Guide: CVE-2026-70149 (related csc.sys issue) msrc.microsoft.com

Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 2 Nov 2026, 03:17.