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Windows Telephony Service

CVE-2026-42912 — Windows Telephony Service Elevation of Privilege Vulnerability

Important CVSS 7 EPSS 0.00205 🔬 Patch diffed 2026-06 archive

Executive Summary

Concurrent execution using shared resource with improper synchronization ('race condition') in Windows Telephony Service 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 Jun 9 2026
Last Updated Jun 9 2026
Publicly Disclosed No
CISA KEV Not Listed
Known Exploits None Known
EPSS Score 0.00205 — 0.10795 percentile
NVD CVSS 7 HIGH — matches 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: 6.1

EPSS Score

0.00205
probability of exploitation in the next 30 days
0.10795 percentile - updated 2026-08-14
View on FIRST.org

Affected Products

30 affected products
Product KB Article Severity Impact Restart Required
Windows 10 Version 1607 for 32-bit Systems 5094122 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1607 for x64-based Systems 5094122 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for 32-bit Systems 5094123 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for x64-based Systems 5094123 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H2 for 32-bit Systems 5094127 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H2 for ARM64-based Systems 5094127 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 21H2 for x64-based Systems 5094127 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 22H2 for 32-bit Systems 5094127 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 22H2 for ARM64-based Systems 5094127 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 22H2 for x64-based Systems 5094127 (Security Update) Important Elevation of Privilege Yes
Windows 11 Version 23H2 for ARM64-based Systems 5093998 (Security Update) Important Elevation of Privilege Yes
Windows 11 Version 23H2 for x64-based Systems 5093998 (Security Update) Important Elevation of Privilege Yes
Windows 11 Version 24H2 for ARM64-based Systems 5094126 (Security Update) Important Elevation of Privilege Yes
Windows 11 Version 24H2 for x64-based Systems 5094126 (Security Update) Important Elevation of Privilege Yes
Windows 11 Version 25H2 for ARM64-based Systems 5094126 (Security Update) Important Elevation of Privilege Yes
Windows 11 Version 25H2 for x64-based Systems 5094126 (Security Update) Important Elevation of Privilege Yes
Windows 11 Version 26H1 for ARM64-based Systems 5095051 (Security Update) Important Elevation of Privilege Yes
Windows 11 version 26H1 for x64-based Systems 5095051 (Security Update) Important Elevation of Privilege Yes
Windows Server 2012 5094042 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 (Server Core installation) 5094042 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 R2 5094041 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 R2 (Server Core installation) 5094041 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2016 5094122 (Security Update) Important Elevation of Privilege Yes
Windows Server 2016 (Server Core installation) 5094122 (Security Update) Important Elevation of Privilege Yes
Windows Server 2019 5094123 (Security Update) Important Elevation of Privilege Yes
Windows Server 2019 (Server Core installation) 5094123 (Security Update) Important Elevation of Privilege Yes
Windows Server 2022 5094128 (Security Update) Important Elevation of Privilege Yes
Windows Server 2022 (Server Core installation) 5094128 (Security Update) Important Elevation of Privilege Yes
Windows Server 2025 5094125 (Security Update) Important Elevation of Privilege Yes
Windows Server 2025 (Server Core installation) 5094125 (Security Update) Important Elevation of Privilege Yes

Patches

10 patches
Article Type Restart
5094122 Security Update Yes
5094123 Security Update Yes
5094127 Security Update Yes
5093998 Security Update Yes
5094126 Security Update Yes
5095051 Security Update Yes
5094042 Monthly Rollup Yes
5094041 Monthly Rollup Yes
5094128 Security Update Yes
5094125 Security Update Yes

Patch Diff

ghidriff · tapisrv.dll (KB5094126)

Two race conditions fixed in the Windows Telephony Service RPC server. (1) Sub-mask index OOB: GetSubMaskIndex() return value was used without bounds validation to index into the per-object event mask array; patch adds a bounds check (index > 0x1e → error) via new GetEventMasksOrSubMasks helper, gated behind Feature_500158777. TGetEventMasksOrSubMasks was refactored from 1431 to 938 instructions by consolidating five per-object-type code-duplicated paths into a single call to this helper. (2) Conference participant UAF in DestroytCall: participant removed from conf slot array without holding exclusive access to the conf call object; patch acquires ReferenceObject + WaitForExclusivetCallAccess before modifying the conf participant list, gated behind Feature_784066872. Both fixes are feature-flagged (WIL DeviceUsage CFR) and inert until Microsoft enables the respective flags per cohort.

Pre-patch version 10.0.26100.8521 Download
Post-patch version 10.0.26100.8655 Download
Function Address Change Note
SetEventMasksOrSubMasks 18003787c -> 180037ad8 code, length, address, called similarity 0.51
TGetEventMasksOrSubMasks 180037f10 -> 1800381a0 code, length, address, called similarity 0.31
DestroytCall 180004c88 code, length, sig, called similarity 0.37
View full diff report View RCA report Download PoC

Attack Path

Two races in the Telephony Service RPC server - an unvalidated sub-mask index and a conference-participant use-after-free

Attack path for CVE-2026-42912 Two races in the Telephony Service RPC server - an unvalidated sub-mask index and a conference-participant use-after-free 01 — ENTRY Standard user calls the Telephony Service over RPC The tapisrv RPC surface is reachable by unprivileged local callers; the service runs inside svchost as SYSTEM. 02 — CONTROLLED INPUT Attacker controls the sub-mask index and the conference-call teardown ordering GetSubMaskIndex() derives an index from request fields; DestroytCall is driven by the client closing a call that is part of a conference. 03 — PATH Event-mask handling and call destruction both walk per-object arrays TGetEventMasksOrSubMasks reached five per-object-type paths that each indexed the event mask array; DestroytCall removes a participant from the conference slot array. 04 — MISSING CHECK No bounds check on the sub-mask index, and no exclusive access held during participant removal The patch consolidates the five duplicated paths into a new GetEventMasksOrSubMasks helper that rejects index > 0x1e (gated behind Feature_500158777), and makes DestroytCall take ReferenceObject plus WaitForExclusivetCallAccess before touching the conference slot array. 05 — PRIMITIVE Out-of-bounds access on the event mask array, and a UAF on the conference call object Two independent primitives in one patch, both inside a SYSTEM service. The refactor shrank TGetEventMasksOrSubMasks from 1431 to 938 instructions.

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

Acknowledgments

DongJun Kim

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