Microsoft Windows Client License Service: Elevation of privilege via use-after-free
Summary
A use-after-free weakness (CWE-416) in the Windows Client License Service allows an authorized local user with low privileges to elevate their privileges. The affected component is part of Windows 10, Windows 11 and Windows Server. Exploitation requires a local foothold, such as a compromised low-privileged account or malware already running on the system; remote exploitation is not indicated.
Affected
- Windows 10 Version 1809: below 10.0.17763.9376
- Windows 10 Version 21H2: below 10.0.19044.7807
- Windows 10 Version 22H2: fixed build unconfirmed, verify with MSRC
- Windows 11 Version 23H2: below 10.0.22631.7652
- Windows 11 Version 24H2: below 10.0.26100.9711
- Windows 11 Version 25H2: below 10.0.26200.9711
- Windows 11 Version 26H1: below 10.0.28000.3231
- Windows 11 Version 26H2: below 10.0.26300.9711
- Windows Server 2019 (including Server Core): below 10.0.17763.9376
- Windows Server 2022: below 10.0.20348.5761
- Windows Server 2025 (including Server Core): below 10.0.26100.33650 (verify against MSRC)
Fixed in
- Security update from Microsoft released on 13 October 2026, per the builds listed above.
- Patch identifier: KB5126646 for Windows Server 2025 (third-party listing). KB number for other builds: Unknown.
Workaround
- Unknown. No vendor-documented interim mitigation found.
Exploitation
- CISA SSVC: Exploitation none, Automatable no, Technical Impact total.
- EPSS was 0.00318 at the time of the snapshot.
- No public proof-of-concept found at the time of research.
- Not listed in CISA KEV as of the search; not verified directly.
Our recommendation
- Apply the Windows security update from October 2026 on all listed systems, prioritising shared workstations, multi-user hosts and RDS/terminal servers.
- Verify the fixed build numbers in the MSRC Security Update Guide before rollout, especially for Windows 10 22H2.
- Check whether Windows 10 1809 is still supported; an unsupported build cannot be reliably patched.
- Monitor for exploitation indicators and recheck CISA KEV and public PoC sources.
Information from Microsoft (Patch Tuesday October 2026)
- Severity (Microsoft)
- Important
- Impact
- Elevation of privilege – Windows Client License Service
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Exploitability assessment
- Exploitation less likely
- Product family
- Windows, Windows ESU (Extended Security Updates)
- Updates (KB)
- KB5127648, KB5126646, KB5127646, KB5126651, KB5126657, KB5126655
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- MSRC advisory msrc.microsoft.com
- CNA data copy (Vulners) vulners.com
- OpenCVE app.opencve.io
- Strix.ai www.strix.ai
- VulDB vuldb.com
- Vulnrichment (Vulners) vulners.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, 00:43.
Original description (NVD)
Use after free in Windows Client License Service allows an authorized attacker to elevate privileges locally.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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