Windows unionfs.sys EUVD-2026-100309

Exploitation requires the attacker to already possess authenticated low-privilege local access to a Windows host running the Container Isolation FS Filter Driver (unionfs.sys) - i.e. an existing foothold, not remote access. The core limiting factor is the race condition itself: per CVSS AC:H, the attacker must reliably win a narrow time-of-check-to-time-of-use window in the driver, typically requiring many repeated attempts and favorable scheduling/timing conditions, which makes exploitation probabilistic rather than deterministic. No user interaction is required (UI:N). Hosts that make heavy use of Windows container isolation present the most realistic attack surface; systems where the driver is never exercised have reduced exposure.

This is a locally-exploitable elevation-of-privilege flaw in the Windows Container Isolation FS Filter Driver (unionfs.sys), a kernel-mode component. The CVSS 3.1 vector (AV:L/AC:H/PR:L/UI:N/S:U) and the 7.0 'High' base score are internally consistent with the description of a local, authenticated race condition (CWE-362): an attacker must already have low-privilege code execution on the host (AV:L, PR:L), and the high attack complexity (AC:H) reflects the need to win a narrow timing window - a defining and risk-limiting characteristic of TOCTOU/race bugs. No EPSS score was provided and the vulnerability is not listed in CISA KEV; no proof-of-concept is referenced. Reliable exploitation of a kernel race typically requires repeated attempts and precise timing, so real-world weaponization is non-trivial despite the full C:H/I:H/A:H kernel impact. Net assessment: a credible local privilege-escalation primitive worth patching on multi-user and container hosts, but not an immediate network-facing emergency - it is a strong post-compromise escalation tool rather than an initial-access or wormable threat. Note a minor signal conflict: the input Tags include 'Information Disclosure,' whereas the description and CWE-362 characterize this as a privilege-escalation race; the C:H component supports some disclosure, but the primary impact is EoP.

A low-privileged local user (or malicious process already running on the host, e.g. within or alongside a container) repeatedly triggers filesystem operations handled by unionfs.sys to exploit a time-of-check-to-time-of-use race, winning the timing window to corrupt kernel state and execute code at SYSTEM. Because the vector is local with high attack complexity (AV:L/AC:H), the attacker needs existing foothold and multiple attempts to win the race. No public proof-of-concept exploit is referenced in the provided data.