CVE-2026-83978

CVE-2026-83978 is a heap-based buffer overflow (CWE-122) in the Windows Biometric Service, the built-in component that handles fingerprint, face, and other biometric authentication on Windows. An attacker who already has a standard (low-privileged) account on the machine can trigger the flaw locally, with no user interaction required, causing memory corruption in the service. Successful exploitation allows the attacker to elevate privileges on the local host, gaining high confidentiality, integrity, and availability impact — a classic local privilege escalation to administrative/SYSTEM-level access. Anyone running the affected Windows 10 (1607, 1809, 21H2, 22H2) and Windows 11 (23H2, 24H2, 25H2, 26H1) client builds, or Windows Server 2016, 2019, 2022, or 2025, is in scope, and because the Biometric Service ships with Windows by default the exposure spans essentially the entire installed base on those branches. As of the September 2026 Patch Tuesday release (which fixed 973 vulnerabilities and two other actively exploited zero-days), no public proof-of-concept is known, the flaw is not in CISA's KEV catalog, and its EPSS of 0.3% (25th percentile) points to low near-term exploitation risk.

What to do: Apply Microsoft's September 2026 security updates (cumulative updates) for every affected Windows 10/11 client version and Windows Server version listed above, as no public PoC or workaround is known. Prioritize hosts where multiple low-privileged users log on locally — shared workstations, kiosks, RDS/VDI servers, and admin jump boxes — and verify the update is installed on all Windows Server builds in the estate.