Windows: Use-after-free in Program Compatibility Assistant Service
Summary
A use-after-free vulnerability in the Windows Program Compatibility Assistant Service allows an authorized local user to elevate privileges. Exploitation requires local access with low privileges and high attack complexity, and no user interaction. Successful exploitation can give the attacker high confidentiality, integrity and availability impact on the system (CVSS 3.1: 7.0, High). The flaw is not reachable remotely by itself.
Affected
- Windows 10 Version 1607 (builds before 10.0.14393.9612)
- Windows 10 Version 1809 (builds before 10.0.17763.9376)
- Windows 10 Version 21H2 (builds before 10.0.19044.7807)
- Windows 10 Version 22H2 (builds before 10.0.19045.7807)
- Windows 11 Version 23H2 (builds before 10.0.22631.7652)
- Windows 11 Version 24H2 (builds before 10.0.26100.9711)
- Windows 11 Version 25H2 (builds before 10.0.26200.9711)
- Windows 11 Version 26H1 (builds before 10.0.28000.3231)
- Windows 11 Version 26H2 (builds before 10.0.26300.9711)
- Windows Server 2016 incl. Server Core (builds before 10.0.14393.9612)
- Windows Server 2019 incl. Server Core (builds before 10.0.17763.9376)
- Windows Server 2022 (builds before 10.0.20348.5761)
Fixed in
- Windows 10 1607 and Server 2016: 10.0.14393.9612
- Windows 10 1809 and Server 2019: 10.0.17763.9376 (scanner metadata cites KB5126651, not verified against MSRC)
- Windows 10 21H2: 10.0.19044.7807
- Windows 10 22H2: 10.0.19045.7807
- Windows 11 23H2: 10.0.22631.7652
- Windows 11 24H2 and 25H2, 26H2: 10.0.26100.9711 / 10.0.26200.9711 / 10.0.26300.9711
- Windows 11 26H1: 10.0.28000.3231
- Windows Server 2022: 10.0.20348.5761
- KB number for Windows 10 1607 and Server 2016: Unknown. Check the MSRC update guide.
Workaround
Unknown. No workaround was identified beyond installing the vendor update.
Exploitation
- CISA SSVC assessment: exploitation none, automatable no, technical impact total.
- No evidence found of in-the-wild exploitation, a public proof of concept or a CISA KEV listing at the time of research.
- EPSS is very low (about 0.003, varies by snapshot).
Our recommendation
- Install the October 2026 Microsoft security update on all affected systems, prioritising legacy Windows 10 1607 and Server 2016 hosts and multi-user or terminal servers.
- Verify the fixed build numbers on the MSRC page before rollout, as the sources differ on some details.
- Confirm with Microsoft lifecycle pages whether Windows 10 1607 and Server 2016 still receive this fix.
- Restrict local logon to necessary accounts and monitor for unexpected privilege changes.
- Re-check CISA KEV and public exploit sources for changes.
Information from Microsoft (Patch Tuesday October 2026)
- Severity (Microsoft)
- Important
- Impact
- Elevation of privilege – Windows Program Compatibility Assistant 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)
- KB5126651, KB5126655, KB5126657, KB5126646, KB5127648, KB5126653
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide (MSRC) msrc.microsoft.com
- NVD mirror (Vulners) vulners.com
- CNA record mirror (Vulners CVEList) vulners.com
- Rapid7 mirror (Vulners) vulners.com
- Strix CVE summary www.strix.ai
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 4 Nov 2026, 08:52.
Original description (NVD)
Use after free in Windows Program Compatibility Assistant 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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