CVE-2026-62815
Critical CVSS 9.8Published 2026-08-11 ยท last changed 2026-08-14
The fix
Microsoft fixes this in updatesKB5120240, KB5121003, KB5120994, KB5121000, KB5120242, KB5120229, KB5120233, KB5120228. 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 11 Version 23H2 for ARM64-based Systems | 10.0.22631.7517 | KB5120240 | 2026-09-20 |
| Windows 11 Version 23H2 for x64-based Systems | 10.0.22631.7517 | KB5120240 | 2026-09-20 |
| Windows 11 Version 24H2 for ARM64-based Systems | 10.0.26100.9168 | KB5121003, KB5120994 | 2026-09-20 |
| Windows 11 Version 24H2 for x64-based Systems | 10.0.26100.9168 | KB5121003, KB5120994 | 2026-09-20 |
| Windows 11 Version 25H2 for ARM64-based Systems | 10.0.26200.9168 | KB5121003, KB5120994 | 2026-09-20 |
| Windows 11 Version 25H2 for x64-based Systems | 10.0.26200.9168 | KB5121003, KB5120994 | 2026-09-20 |
| Windows 11 Version 26H1 for ARM64-based Systems | 10.0.28000.2704 | KB5121000 | 2026-09-20 |
| Windows 11 version 26H1 for x64-based Systems | 10.0.28000.2704 | KB5121000 | 2026-09-20 |
| Windows Server 2022 | 10.0.20348.5499 | KB5120242, KB5120229 | 2026-09-20 |
| Windows Server 2022 (Server Core installation) | 10.0.20348.5499 | KB5120242, KB5120229 | 2026-09-20 |
| Windows Server 2025 | 10.0.26100.33296 | KB5120233, KB5120228 | 2026-09-20 |
| Windows Server 2025 (Server Core installation) | 10.0.26100.33296 | KB5120233, KB5120228 | 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 Microsoft QUIC allows an unauthorized attacker to execute code over a network.
Sources
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H