CVE-2026-20817 — Windows Error Reporting Service Elevation of Privilege Vulnerability

Executive Summary

Improper handling of insufficient permissions or privileges in Windows Error Reporting allows an authorized attacker to elevate privileges locally.

Overview

7.8

CVSS HIGH

Important

MS Severity

Not Exploited

MS Exploit Status

More Likely

MS Exploit Likelihood

Category Elevation of Privilege

Weakness CWE-280: Improper Handling of Insufficient Permissions or Privileges

Released Jan 13 2026

Last Updated Jan 13 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.05333 — 0.91766 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

0.05333

probability of exploitation in the next 30 days

0.91766 percentile - updated 2026-07-25

View on FIRST.org

Affected Products

17 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 21H2 for 32-bit Systems	`5073724 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for ARM64-based Systems	`5073724 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for x64-based Systems	`5073724 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for 32-bit Systems	`5073724 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for ARM64-based Systems	`5073724 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for x64-based Systems	`5073724 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 23H2 for ARM64-based Systems	`5073455 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 23H2 for x64-based Systems	`5073455 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for ARM64-based Systems	`5074109 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for x64-based Systems	`5074109 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 25H2 for ARM64-based Systems	`5074109 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 25H2 for x64-based Systems	`5074109 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022	`5073457 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022 (Server Core installation)	`5073457 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022, 23H2 Edition (Server Core installation)	`5073450 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025	`5073379 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2025 (Server Core installation)	`5073379 (Security Update)`	Important	Elevation of Privilege	Yes

Patches

6 patches

Article	Type	Restart
`5073724`	Security Update	Yes
`5073455`	Security Update	Yes
`5074109`	Security Update	Yes
`5073457`	Security Update	Yes
`5073450`	Security Update	Yes
`5073379`	Security Update	Yes

Patch Diff

ghidriff · WerSvc.dll (KB5074109)

ALPC-to-SYSTEM primitive in Windows Error Reporting Service. CWerService::SvcElevatedLaunch handles MessageFlags=0x50000000 ALPC messages, reading a wide-char command line out of a client-supplied File Mapping and running WerFault.exe as SYSTEM with attacker-controlled arguments via CreateProcessAsUserW. Any low-priv user that can reach the ALPC port \WindowsErrorReportingServicePort gets SYSTEM WerFault.exe invocation. Patch gates the entire handler behind WIL CFR flag Feature_2473284922; when enabled, the function returns E_FAIL (0x80004005) immediately without touching the vulnerable code. Public PoC by itm4n (https://github.com/itm4n/CVEs/tree/master/CVE-2026-20817).

Pre-patch version 10.0.26100.7309 Download

Post-patch version 10.0.26100.7623 Download

Function	Address	Change	Note
`wil::details::SubscribeFeatureStateCacheToConfigurationChanges`	`18000d368`	refcount, calling	similarity 1.0
`CWerService::SvcElevatedLaunch`	`18001a7e0 -> 18001a980`	code, length, address, called	similarity 0.2
`wil::details::ReportUsageToService`	`18000cb50`	refcount, calling	similarity 1.0
`wil::details::EnsureSubscribedToFeatureConfigurationChanges`	`180009e04`	refcount, calling	similarity 1.0
`_guard_dispatch_icall$thunk$10345483385596137414`	`18003a010`	refcount, calling	similarity 0.89

View full diff report View RCA report Download PoC

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

Ruslan Sayfiev with GMO Cybersecurity by Ierae
Denis Faiustov with GMO Cybersecurity by Ierae

References

MSRC Advisory MITRE NVD