CVE-2026-33098 — Windows Container Isolation FS Filter Driver Elevation of Privilege Vulnerability

Executive Summary

Use after free in Windows Container Isolation FS Filter Driver allows an authorized attacker to elevate privileges locally.

Overview

7.8

CVSS HIGH

Important

MS Severity

Not Exploited

MS Exploit Status

Exploitation Unlikely

MS Exploit Likelihood

Category Elevation of Privilege

Weakness CWE-416: Use After Free

Released Apr 14 2026

Last Updated Apr 14 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.00307 — 0.23209 percentile

NVD CVSS 7.8 HIGH — matches MSRC

CVSS Vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

ATTACK VECTOR

Local

ATTACK COMPLEXITY

Low

PRIVILEGES REQUIRED

Low

USER INTERACTION

None

SCOPE

Unchanged

CONFIDENTIALITY

High

INTEGRITY

High

AVAILABILITY

High

EXPLOIT CODE MATURITY

Unproven

REMEDIATION LEVEL

Official Fix

REPORT CONFIDENCE

Confirmed

Temporal Score: 6.8

EPSS Score

0.00307

probability of exploitation in the next 30 days

0.23209 percentile - updated 2026-08-14

View on FIRST.org

Affected Products

27 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 1607 for 32-bit Systems	`5082198 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1607 for x64-based Systems	`5082198 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1809 for 32-bit Systems	`5082123 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 1809 for x64-based Systems	`5082123 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for 32-bit Systems	`5082200 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for ARM64-based Systems	`5082200 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for x64-based Systems	`5082200 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for 32-bit Systems	`5082200 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for ARM64-based Systems	`5082200 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for x64-based Systems	`5082200 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 23H2 for ARM64-based Systems	`5082052 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 23H2 for x64-based Systems	`5082052 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for ARM64-based Systems	`5083769 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for x64-based Systems	`5083769 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 25H2 for ARM64-based Systems	`5083769 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 25H2 for x64-based Systems	`5083769 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 26H1 for ARM64-based Systems	`5083768 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 version 26H1 for x64-based Systems	`5083768 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2016	`5082198 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2016 (Server Core installation)	`5082198 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019	`5082123 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2019 (Server Core installation)	`5082123 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022	`5082142 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022 (Server Core installation)	`5082142 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022, 23H2 Edition (Server Core installation)	`5082060 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025	`5082063 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025 (Server Core installation)	`5082063 (Security Update)`	Important	Elevation of Privilege	Yes

Patches

9 patches

Article	Type	Restart
`5082198`	Security Update	Yes
`5082123`	Security Update	Yes
`5082200`	Security Update	Yes
`5082052`	Security Update	Yes
`5083769`	Security Update	Yes
`5083768`	Security Update	Yes
`5082142`	Security Update	Yes
`5082060`	Security Update	Yes
`5082063`	Security Update	Yes

Patch Diff

ghidriff · wcifs.sys (KB5083769)

Reference-count underflow use-after-free (CWE-416) via cleanup asymmetry in the Windows Container Isolation FS minifilter wcifs.sys WcScheduleExpansionWorkItem. The function allocates a work item (FltAllocateGenericWorkItem), then ObfReferenceObject(target) +1, then FltQueueGenericWorkItem. Two failure paths shared ONE cleanup block (LABEL_10/LABEL_14) that unconditionally ObfDereferenceObject'd: on queue-insert failure the ref was already taken (symmetric), but on ALLOCATION failure the goto reaches the same cleanup before ObfReferenceObject ran -> a dereference with no matching reference, so the object's refcount ends 1 lower than actual. A container doing heavy file I/O can exhaust NonPagedPoolNx to make the allocation fail repeatedly, decrementing until the count hits 0 while other paths still hold pointers -> object freed -> kernel UAF -> EoP. Diff of wcifs.sys 10.0.26100.7920 -> .8246 (Apr 14 2026, KB5083769) confirms the fix: gated behind CFR flag Feature_1773654331, a boolean flag (init false) is set true only immediately after ObfReferenceObject, and the shared-cleanup ObfDereferenceObject is guarded by `!feature || flag` so it runs only when a ref was actually taken; control flow restructured from goto-merge to if/else with early return on success. Note: our diff shows Feature_1773654331, differing from the circulated screenshots' Feature_2042089787 (build/arch difference); the ref-tracking-flag guard is identical.

Pre-patch version 10.0.26100.7920 Download

Post-patch version 10.0.26100.8246 Download

Function	Address	Change	Note
`WcScheduleExpansionWorkItem`	`code change`	code (ref-tracking flag guards shared-cleanup deref, CFR-gated)	Pre: alloc-fail -> goto shared cleanup -> unconditional ObfDereferenceObject with no matching ObfReferenceObject -> refcount underflow. Post (Feature_1773654331 enabled): bVar13=false initially, set true right after ObfReferenceObject; cleanup does ObfDereferenceObject only if `!feature \|\| bVar13`; goto-merge restructured to if/else + early return on success.
`Feature_1773654331`	`gate`	added (CFR gate)	CFR flag gating the guarded dereference; the original unconditional deref still ships when disabled.

View full diff report View RCA report

Attack Path

An allocation-failure path dereferences a wcifs object it never referenced, underflowing the refcount to a free-while-in-use

Derived from the patch delta: the checks added by the vendor identify which fields crossed a trust boundary unvalidated. Reachability and privilege are taken from the call chain in the RCA report.

Known Exploits

No known exploits have been linked for this CVE yet. When available, exploit references will be sourced from public repositories and may be unverified, incomplete, or non-functional. Always review code carefully before use in any environment.

Acknowledgments

Anonymous

References

MSRC Advisory MITRE NVD