CVE-2026-21251
High CVSS 7.8Published 2026-02-10 ยท last changed 2026-06-17
The fix
Microsoft fixes this in updatesKB5075999, KB5075904, KB5075906, KB5075943, KB5075897, KB5075899, KB5075942. The fixed build differs per Windows version, so the row that matters is the one matching the machine you are looking at.
| Windows version | Fixed in build | Update | Released |
|---|---|---|---|
| Windows Server 2016 | 10.0.14393.8868 | KB5075999 | 2026-09-20 |
| Windows Server 2016 (Server Core installation) | 10.0.14393.8868 | KB5075999 | 2026-09-20 |
| Windows Server 2019 | 10.0.17763.8389 | KB5075904 | 2026-09-20 |
| Windows Server 2019 (Server Core installation) | 10.0.17763.8389 | KB5075904 | 2026-09-20 |
| Windows Server 2022 | 10.0.20348.4773 | KB5075906, KB5075943 | 2026-09-20 |
| Windows Server 2022, 23H2 Edition (Server Core installation) | 10.0.25398.2149 | KB5075897 | 2026-09-20 |
| Windows Server 2022 (Server Core installation) | 10.0.20348.4773 | KB5075906, KB5075943 | 2026-09-20 |
| Windows Server 2025 | 10.0.26100.32370 | KB5075899, KB5075942 | 2026-09-20 |
| Windows Server 2025 (Server Core installation) | 10.0.26100.32370 | KB5075899, KB5075942 | 2026-09-20 |
Having the KB is not the same as knowing whether a machine has it. Windows updates supersede each other, so the update you installed often covers a fix that shipped in an earlier one. Paste your build into the patch checker and it walks that chain for you.
What it is
Use after free in Windows Cluster Client Failover allows an authorized attacker to elevate privileges locally.
Sources
CVSS vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H