Microsoft Windows Installer: Elevation of privilege via use-after-free

Summary

Windows Installer, the Windows component for MSI package installs and maintenance, contains a use-after-free flaw (CWE-416). An authorized local user with low privileges can exploit it to gain higher privileges, up to SYSTEM level, on affected Windows 10, Windows 11 and Windows Server systems (CVSS 7.8). Network access and user interaction are not required, so an attacker needs an existing local foothold. Microsoft reports no public disclosure and no known exploitation.

Affected

• Windows 10 Version 1607 and Windows Server 2016: builds 10.0.14393.0 to before 10.0.14393.9612
• Windows 10 Version 1809: builds 10.0.17763.0 to before 10.0.17763.9376
• Windows 10 21H2 and 22H2: builds 10.0.19044.0 to before 10.0.19044.7807 and 10.0.19045.0 to before 10.0.19045.7807
• Windows 11 23H2, 24H2, 25H2, 26H1 and 26H2 (various builds, see the MSRC table)
• Windows Server 2012 and 2012 R2 (including Server Core): builds 6.2.9200.0 to before 6.2.9200.26434 and 6.3.9600.0 to before 6.3.9600.23470
• The version ranges come from the CVE record and aggregators; MSRC is authoritative.

Fixed in

• Windows 10 1607 / Server 2016: build 10.0.14393.9612 or later, most likely via the October 13, 2026 update KB5126868 (not yet confirmed against the MSRC product table)
• Other products: the fixed builds listed on the MSRC advisory
• Note: Windows 10 1607 (LTSB 2016) reached end of updates with the October 2026 release; no further fixes are expected.

Workaround

• Unknown. Microsoft has not published a workaround for this CVE in the sources reviewed.
• For unpatched systems that cannot be updated: restrict local logon to trusted users, limit local administrator rights, and restrict installation of untrusted MSI packages (general hardening, not vendor guidance).

Exploitation

• Microsoft assessment: not publicly disclosed, not exploited, exploitability rated "Exploitation Unlikely".
• CISA SSVC: exploitation none, automatable no, technical impact total.
• No public proof-of-concept found. EPSS is about 0.002 (low).

Our recommendation

• Inventory all Windows 10 1607 and Windows Server 2016 devices, and check other Windows versions against the MSRC advisory.
• Install KB5126868 or a later update on supported systems. Review the known Exchange mail issue with this update first, as described on Microsoft's release health page.
• Plan migration away from Windows 10 1607 / LTSB 2016 for devices that will receive no further updates.
• Restrict local admin rights and untrusted MSI installations on all systems.
• Priority: moderate for patched systems; high for unpatched or unsupported 1607 systems.

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

• MSRC Security Update Guide: CVE-2026-87700 msrc.microsoft.com
• Microsoft KB5126868: Windows 10 1607 security update (October 2026) support.microsoft.com
• Microsoft KB5134123: Windows 10 1607 update support.microsoft.com
• Microsoft Release Health: Windows 10 1607 and Windows Server 2016 learn.microsoft.com
• OpenCVE: CVE-2026-87700 app.opencve.io
• VulDB: CVE-2026-87700 vuldb.com
• Vulners VulnRichment: CVE-2026-87700 (EPSS) 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, 01:38.