Critical Microsoft Vulnerability (CVE-2026-20817) Allows Remote Code Execution

Microsoft confirmed CVE-2026-20817 as a critical security vulnerability impacting multiple Windows versions. This flaw permits remote code execution when exploited. Attackers could take full control of affected systems without user interaction.

Affected products include Windows 10 versions 21H2 through 22H2, Windows 11 versions 21H2 to 23H2, and Windows Server 2022. Unpatched systems face immediate risk. The vulnerability scored 8.8 on the CVSS v3.1 scale, classified as High severity. This rating reflects the attack's network-based vector and low complexity.

Technical analysis reveals improper handling of memory objects during DLL loading procedures. Malicious actors craft specially designed requests triggering memory corruption. Successful exploitation allows arbitrary code execution at SYSTEM privilege level. No authentication is required for attacks.

Microsoft released security updates addressing this vulnerability on September 12, 2026. Apply patches immediately through Windows Update or the Microsoft Update Catalog. Administrators should prioritize systems exposed to untrusted networks. For systems requiring delayed patching, restrict SMBv3 traffic as a temporary mitigation.

Monitor the official Microsoft Security Update Guide for latest information. The Microsoft Security Response Center confirmed active exploitation attempts detected in enterprise environments. Complete remediation requires installing KB5027859 or later security updates.

Security teams should audit all Windows endpoints for vulnerable versions. Verify patch installation using the winver command. Microsoft's advisory emphasizes this vulnerability as actively weaponized. Failure to patch creates significant organizational risk.