An improper access control flaw in the Windows Container Isolation file system filter driver (unionfs.sys) allows an attacker who already has local access to the system to elevate privileges. By bypassing the driver’s access restrictions, the attacker can gain higher privilege levels, compromising confidentiality, integrity, and availability of the affected machine, and potentially installing malware or executing further attacks.

The flaw affects Windows 11 23H2, 24H2, 25H2, 26H2 on both x64 and ARM64 architectures, and Windows Server 2022 and Windows Server 2025, including Server Core installations. Any system running these versions without the issued update is susceptible.

The CVSS score of 7.8 places this vulnerability in the high‑severity range. The EPSS score is not available and the vulnerability is not listed in the CISA KEV catalog, indicating that widescale exploitation has not been observed. The attack vector is local; an authorized user can trigger the flaw, so patching and limiting container usage are essential.