Windows Microsoft Account Sign-in Assistant: Heap overflow enables privilege escalation

Summary

A heap-based buffer overflow in the Windows Microsoft Account Sign-in Assistant component allows an authorized local attacker to elevate privileges. Exploitation requires a valid low-privilege account and a high attack complexity, and no user interaction is needed. If exploited, the impact on confidentiality, integrity and availability is rated high. No active exploitation or public exploit code has been found.

Affected

• Windows 10 Version 1607 and Windows Server 2016 (including Server Core): builds 10.0.14393.0 to before 10.0.14393.9688
• Windows 10 Version 1809 and Windows Server 2019 (including Server Core): builds 10.0.17763.0 to before 10.0.17763.9487
• Windows 10 Version 21H2: builds 10.0.19044.0 to before 10.0.19044.7846
• Windows 10 Version 22H2: builds 10.0.19045.0 to before 10.0.19045.7846
• Windows 11 Version 23H2: builds 10.0.22631.0 to before 10.0.22631.7711
• Windows 11 Version 24H2: builds 10.0.26100.0 to before 10.0.26100.9974
• Windows 11 Version 25H2: builds 10.0.26200.0 to before 10.0.26200.9974
• Windows 11 Version 26H1: builds 10.0.28000.0 to before 10.0.28000.3501
• Windows 11 Version 26H2: builds 10.0.26300.0 to before 10.0.26300.9974
• Windows Server 2022: builds 10.0.20348.0 to before 10.0.20348.5870

Fixed in

• Windows 10 Version 1607 and Windows Server 2016: 10.0.14393.9688 or later
• Windows 10 Version 1809 and Windows Server 2019: 10.0.17763.9487 or later
• Windows 10 Version 21H2: 10.0.19044.7846 or later
• Windows 10 Version 22H2: 10.0.19045.7846 or later
• Windows 11 Version 23H2: 10.0.22631.7711 or later
• Windows 11 Version 24H2: 10.0.26100.9974 or later
• Windows 11 Version 25H2: 10.0.26200.9974 or later
• Windows 11 Version 26H1: 10.0.28000.3501 or later
• Windows 11 Version 26H2: 10.0.26300.9974 or later
• Windows Server 2022: 10.0.20348.5870 or later

KB article numbers: Unknown.

Workaround

Unknown. No vendor-documented workaround was found; installing the security update is the expected fix.

Exploitation

• Exploited in the wild: no evidence found.
• Public exploit code: none found.
• CISA SSVC: exploitation none, automatable no, technical impact total.
• CISA KEV status: not verified.

Our recommendation

• Install the November 2026 security update on all affected Windows clients and servers, and confirm each machine's build number after patching.
• Prioritise shared workstations, jump hosts and any system running an unsupported build.
• Limit local accounts and restrict local logon on servers where possible, since exploitation requires a foothold with low privileges.
• Monitor for unexpected SYSTEM-level processes and privilege changes.
• Re-check the CISA KEV catalogue before relying on the exploitation status above.

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-2026-119921 msrc.microsoft.com
• CVE Record: CVE-2026-119921 www.cve.org
• Tenable: Microsoft November 2026 Patch Tuesday www.tenable.com
• BleepingComputer: Microsoft November 2026 Patch Tuesday www.bleepingcomputer.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 11 Nov 2026, 08:26.