Improper access control in Microsoft products - CVE-2026-20929

Vulnerability identifier: #VU121231

CSH Severity: Medium

CVSS v4: 7.7 [CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N]

CVE-ID: CVE-2026-20929

CWE-ID: CWE-284

Exploitation vector: Remote access

Exploit availability: No public exploit available

The vulnerability allows a remote user to compromise the affected system.

The vulnerability exists due to improper access restrictions Windows HTTP.sys. A remote user can send specially crafted packets to the system and execute arbitrary code with SYSTEM privileges. 

Note, successful exploitation of the vulnerability requires an Service Principal Name (SPN) that is registered to an account that no longer exists or is not in use.

Microsoft Internet Information Services (IIS)
Microsoft Windows
Windows Server

Install updates from vendor's website.

Microsoft Windows - addressed in versions 10 21H2 10.0.19044.6809, 10 22H2 10.0.19045.6809, 10 1607 10.0.14393.8783, 10 1809 10.0.17763.8276, 11 23H2 10.0.22631.6491, 11 24H2 10.0.26100.3223
Windows Server - addressed in versions 2008 R2 6.1.7601.28117, 2008 6.0.6003.23717, 2012 R2 6.3.9600.22968, 2012 6.2.9200.25868, 2016 10.0.14393.8783, 2019 10.0.17763.8276, 2022 23H2 10.0.25398.2092, 2022 10.0.20348.4648, 2025 10.0.26100.7623

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20929

• Privilege escalation in Microsoft Windows HTTP.sys