EUVD-2026-43907
GHSA-v4hp-j4fc-x9fp
Severity by source
AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
Local low-priv authenticated access (AV:L/PR:L/AC:L) with kernel memory disclosure gives C:H; description states only information disclosure so I:N and A:N, diverging from vendor's unexplained A:H.
Primary rating from Vendor (microsoft).
CVSS VectorNVD
Lifecycle Timeline
4DescriptionNVD
Out-of-bounds read in Windows Container Isolation FS Filter Driver (unionfs.sys) allows an authorized attacker to disclose information locally.
AnalysisAI
Local information disclosure in the Windows Container Isolation FS Filter Driver (unionfs.sys) on Windows 11 version 26H1 allows an authorized low-privileged user to read memory outside intended bounds and disclose sensitive kernel or process data. The flaw was reported by Microsoft, a vendor patch is available, and there is no public exploit identified at time of analysis. CVSS 7.1 (AV:L) reflects local access with low privileges but high confidentiality impact.
Technical ContextAI
The vulnerability lives in unionfs.sys, the file-system minifilter driver that implements Windows Container Isolation - the union/overlay filesystem layer used by Windows Server and client containers (and by isolation features such as MSIX/AppX and Windows Sandbox scratch layers) to merge a read-only base image with a writable scratch layer. As a kernel-mode filter driver it sits in the I/O stack and processes filter callback data on behalf of any local caller performing filesystem operations against isolated volumes. The root cause is CWE-125 (Out-of-bounds Read): the driver reads past the end of an allocated buffer, most likely due to insufficient validation of a length, offset, or reparse/placeholder structure supplied through an IRP or filter operation, causing adjacent kernel memory to be returned to the caller. Because the affected code runs in kernel context, leaked bytes can include pointers or secrets useful for defeating KASLR or staging further exploitation.
RemediationAI
Patch available per vendor advisory - apply the Microsoft security update for CVE-2026-50428 to Windows 11 version 26H1 as published at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-50428 (consult the MSRC page for the exact KB number and build, which were not included in the provided data). Because exploitation requires local, authenticated access, the primary compensating control until patching is to enforce least privilege and limit which users can run container/isolation workloads: restrict use of Windows Sandbox, MSIX app isolation, and container features on sensitive hosts, and monitor for untrusted local accounts. Disabling container isolation features where they are not needed reduces the attack surface but will break any application relying on the union filesystem, so validate workload dependencies before doing so. Standard hardening (removing unnecessary local interactive logon rights, application allow-listing) further reduces the pool of attackers who could reach the driver.
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Same weakness CWE-125 – Out-of-bounds Read
View allSame technique Buffer Overflow
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External POC / Exploit Code
Leaving vuln.today