CVE-2026-27927 — Windows Projected File System Elevation of Privilege Vulnerability

Executive Summary

Concurrent execution using shared resource with improper synchronization ('race condition') in Windows Projected File System allows an authorized attacker to elevate privileges locally.

Overview

7

CVSS HIGH

Important

MS Severity

Not Exploited

MS Exploit Status

Less Likely

MS Exploit Likelihood

Category Elevation of Privilege

Weakness CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

Released Apr 14 2026

Last Updated Apr 14 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.00191 — 0.09026 percentile

NVD CVSS 7 HIGH — differs from MSRC

CVSS Vector

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

ATTACK VECTOR

Local

ATTACK COMPLEXITY

High

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

0.00191

probability of exploitation in the next 30 days

0.09026 percentile - updated 2026-08-14

View on FIRST.org

Affected Products

23 affected products

Product	KB Article	Severity	Impact	Restart Required
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 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

8 patches

Article	Type	Restart
`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 · prjflt.sys (KB5083769)

Race-condition use-after-free (CWE-362 + CWE-416) in the Windows Projected File System minifilter prjflt.sys PrjfCopyStreamData, local EoP. Running on the ProjFS expansion worker (PrjfExpansionWorker -> PrjfExpandFile -> PrjfCopyStreamData), it tracks the in-flight file in a per-union-context AVL handle table (UnionContext->AvlHandleTable) keyed by FileId (from StreamContext->Uuid), with entries _PRJ_HANDLE_AVL_TABLE_ENTRY{FileId, FileObject (from FltObjects->FileObject), FileHandle}. It RtlInsertElementGenericTableAvl's the entry, ObfReferenceObject's the FileObject if new, then calls PrjfSendGetFileStreamCommand (a user-mode round-trip that WAITS), and only afterwards RtlDeleteElementGenericTableAvl + ObDereferenceObjectDeferDelete. During the user-mode wait, another thread can operate on the same FileId/AVL entry without adequate synchronization, so the shared entry / referenced FILE_OBJECT can be freed or have its ref state changed concurrently -> this path then uses/dereferences a freed entry/object -> UAF. Runs in the SYSTEM expansion worker. IN-HOUSE ghidriff of prjflt.sys 10.0.26100.4202 -> .8246 confirms PrjfCopyStreamData is among the code-changed functions in the April fix (KB5083769); the precise synchronization instruction is heavily inlined and not cleanly isolable in the decompiled wide-span diff, so the mechanism follows the reporter PoC (serialize the AVL entry/FileObject lifetime across the user-mode command). Like the other April ProjFS fixes it is CFR-gated.

Pre-patch version 10.0.26100.4202 Download

Post-patch version 10.0.26100.8246 Download

Function	Address	Change	Note
`PrjfCopyStreamData`	`code change`	code (AVL handle-table entry lifetime synchronized across user-mode command)	Pre: RtlInsertElementGenericTableAvl(entry) -> ObfReferenceObject(FileObject if new) -> PrjfSendGetFileStreamCommand (user-mode wait) -> RtlDeleteElementGenericTableAvl -> ObDereferenceObjectDeferDelete; the AVL entry/FileObject lifetime is not synchronized across the wait, so a concurrent op on the same FileId frees/alters it -> UAF. Post (.8246): PrjfCopyStreamData reworked to synchronize the entry/object lifetime across the user-mode round-trip (confirmed changed in our diff; exact inlined instruction not isolable).

View full diff report View RCA report

Attack Path

A ProjFS stream-data expansion holds an AVL handle entry across a user-mode wait, letting a concurrent op free it mid-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

ChenJian with Sea Security Orca Team

References

MSRC Advisory MITRE NVD