Windows unionfs.sys EUVD-2026-99836

Requires the attacker to already possess local, low-privileged code execution on the target Windows host (PR:L, AV:L) - this is a privilege-escalation flaw, not a remote entry point, so it cannot be triggered over the network. The affected code path is the Container Isolation FS Filter Driver (unionfs.sys), which is present by default on supported Windows 11 (23H2/24H2/25H2/26H1/26H2) and Windows Server 2022/2025 builds, so no special opt-in feature is strictly indicated by the description. The primary limiting factor is the prerequisite foothold: the attacker needs the ability to run code and interact with the driver interface locally; no authentication qualifier beyond low privileges and no user interaction is required. Apply the vendor-fixed builds (e.g., 10.0.26100.9711 for 24H2, 10.0.20348.5761 for Server 2022) to remediate.

This is a credible patch-priority issue for Windows environments, though not an emergency. The CVSS 3.1 vector (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H, base 7.8) describes a local privilege-escalation flaw: the attacker must already have low-privileged code execution on the host (PR:L) and attacks locally (AV:L), but complexity is low and no user interaction is needed, yielding full confidentiality/integrity/availability impact (likely SYSTEM). No EPSS score, KEV listing, or POC is present in the supplied data, so there is no evidence of active or imminent widespread exploitation - consistent with a Microsoft-internally-reported bug (reported by secure@microsoft.com) patched through the normal update cycle. The signals are internally consistent (local EoP with partial privileges), so there is no CVSS-vs-description conflict to flag. The practical risk is as a second-stage/post-compromise escalation primitive rather than an initial-access vector; it becomes materially more urgent if a POC emerges, since heap-overflow EoP bugs in filter drivers are attractive for weaponization. Treat as standard high-severity Patch Tuesday remediation, prioritized on hosts where untrusted users or low-trust workloads run.

A local attacker who has already gained low-privileged code execution on a Windows host (e.g., via phishing or a compromised service account) sends crafted input to the Container Isolation FS Filter Driver (unionfs.sys), triggering a heap-based buffer overflow in kernel mode. Successful exploitation corrupts kernel heap memory to escalate from the low-privileged context to SYSTEM, giving full control of the machine. No public exploit identified at time of analysis, and no user interaction is required.