OnlyPatch
Windows Telephony Service

CVE-2026-20931 — Windows Telephony Service Elevation of Privilege Vulnerability

Important CVSS 8 EPSS 0.00786 🔬 Patch diffed 2026-01 archive

Executive Summary

External control of file name or path in Windows Telephony Service allows an authorized attacker to elevate privileges over an adjacent network.

Overview

8
CVSS HIGH
Important
MS Severity
Not Exploited
MS Exploit Status
Exploitation Unlikely
MS Exploit Likelihood
Category Elevation of Privilege
Weakness CWE-73: External Control of File Name or Path
Released Jan 13 2026
Last Updated Jan 13 2026
Publicly Disclosed No
CISA KEV Not Listed
Known Exploits None Known
EPSS Score 0.00786 — 0.52983 percentile
NVD CVSS 8 HIGH — matches MSRC

CVSS Vector

CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
ATTACK VECTOR
Adjacent_network
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: 7.0

EPSS Score

0.00786
probability of exploitation in the next 30 days
0.52983 percentile - updated 2026-08-14
View on FIRST.org

Affected Products

29 affected products
Product KB Article Severity Impact Restart Required
Windows 10 Version 1607 for 32-bit Systems 5073722 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1607 for x64-based Systems 5073722 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for 32-bit Systems 5073723 (Security Update) Important Elevation of Privilege Yes
Windows 10 Version 1809 for x64-based Systems 5073723 (Security Update) Important Elevation of Privilege Yes
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 2008 for 32-bit Systems Service Pack 2 5073697 (Monthly Rollup) 5073700 (Security Only) Important Elevation of Privilege 5071504 Base: 8.0 Temporal: 7.0 Vector: CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 6.0.6003.23717 Yes None Windows Server 2008 for 32-bit Systems Service Pack 2 (Server Core installation) 5073697 (Monthly Rollup) 5073700 (Security Only) Important Elevation of Privilege 5071504 Base: 8.0 Temporal: 7.0 Vector: CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 6.0.6003.23717 Yes None Windows Server 2008 for x64-based Systems Service Pack 2 5073697 (Monthly Rollup) 5073700 (Security Only) Important Elevation of Privilege 5071504 Base: 8.0 Temporal: 7.0 Vector: CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 6.0.6003.23717 Yes None Windows Server 2008 for x64-based Systems Service Pack 2 (Server Core installation) 5073697 (Monthly Rollup) 5073700 (Security Only) Important Elevation of Privilege 5071504 Base: 8.0 Temporal: 7.0 Vector: CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 6.0.6003.23717 Yes None Windows Server 2008 R2 for x64-based Systems Service Pack 1 5073695 (Monthly Rollup) 5073699 (Security Only) Important Elevation of Privilege 5071501 Base: 8.0 Temporal: 7.0 Vector: CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 6.1.7601.28117 Yes None Windows Server 2008 R2 for x64-based Systems Service Pack 1 (Server Core installation) 5073695 (Monthly Rollup) 5073699 (Security Only) Important Elevation of Privilege 5071501 Base: 8.0 Temporal: 7.0 Vector: CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C 6.1.7601.28117 Yes None Windows Server 2012 5073698 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 (Server Core installation) 5073698 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 R2 5073696 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2012 R2 (Server Core installation) 5073696 (Monthly Rollup) Important Elevation of Privilege Yes
Windows Server 2016 5073722 (Security Update) Important Elevation of Privilege Yes
Windows Server 2016 (Server Core installation) 5073722 (Security Update) Important Elevation of Privilege Yes
Windows Server 2019 5073723 (Security Update) Important Elevation of Privilege Yes
Windows Server 2019 (Server Core installation) 5073723 (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

10 patches
Article Type Restart
5073722 Security Update Yes
5073723 Security Update Yes
5073724 Security Update Yes
5073455 Security Update Yes
5074109 Security Update Yes
5073698 Monthly Rollup Yes
5073696 Monthly Rollup Yes
5073457 Security Update Yes
5073450 Security Update Yes
5073379 Security Update Yes

Patch Diff

ghidriff · tapisrv.dll (KB5074109)

External control of file name/path (CWE-73) in the Windows Telephony Service tapisrv.dll ClientAttach handler, adjacent-network EoP. ClientAttach services remote service-provider attach requests and opens a client-named mailslot. PRE: it called CreateFileW(lpFileName, 0x40000000, 1, 0, 3, 0x80, 0) with the client-supplied path and, if the handle != INVALID_HANDLE_VALUE, treated the attach as connected and used the handle assuming it is a mailslot - with no check of the handle type. Because CreateFileW will open any object (file, named pipe, device), a client can supply a non-mailslot path; as long as CreateFileW succeeds the SYSTEM service then performs operations on that attacker-chosen handle (arbitrary file access/write with service privileges). IN-HOUSE ghidriff of tapisrv.dll 10.0.26100.7309 -> .7623 (Jan 13 2026, KB5074109) confirms the fix: gated behind CFR flag Feature_2464883000, ClientAttach now calls GetMailslotInfo(handle,...) immediately after CreateFileW and only proceeds if it succeeds (handle is a real mailslot); otherwise it CloseHandle()s and rejects the attach (log 'ClientAttach: GetMailslotInfo(%ws) failed, err=%u'). Note: our x64 24H2 diff shows Feature_2464883000, differing from the circulated screenshots' Feature_2733318456 (build/arch difference); the GetMailslotInfo validation is identical.

Pre-patch version 10.0.26100.7309 Download
Post-patch version 10.0.26100.7623 Download
Function Address Change Note
ClientAttach code change code (mailslot handle validation added, CFR-gated) Pre: CreateFileW(lpFileName,0x40000000,1,0,3,0x80,0); if handle != -1 goto accept - no type check, so any file/pipe/device opens successfully and is used as a mailslot by the SYSTEM service. Post (Feature_2464883000 enabled): CreateFileW then GetMailslotInfo(handle,...); accept only if it succeeds, else CloseHandle + reject (GetMailslotInfo(%ws) failed log).
Feature_2464883000 gate added (CFR gate) CFR flag gating the GetMailslotInfo validation; the original unchecked CreateFileW path still ships in .7623.
View full diff report View RCA report

Attack Path

Telephony ClientAttach opens a client-named path and uses it as a mailslot without checking the handle type

Attack path for CVE-2026-20931 Telephony ClientAttach opens a client-named path and uses it as a mailslot without checking the handle type 01 — ENTRY Attacker sends a Telephony service-provider attach request to tapisrv (SYSTEM) tapisrv.dll ClientAttach handles remote service-provider connections. AV:A / PR:L: an adjacent, low-privileged caller supplies the connection parameters including lpFileName. 02 — CONTROLLED INPUT The request names a path that is NOT a mailslot (a file, named pipe, or device) lpFileName is client-controlled. CreateFileW will open many object types, not just mailslots. 03 — PATH ClientAttach calls CreateFileW(lpFileName,...) and treats success as 'connected' Pre-patch: if CreateFileW returns a valid handle the attach proceeds and the handle is used as a mailslot - with no verification of its type (CWE-73). 04 — MISSING CHECK No check that the opened handle is actually a mailslot The client controls which object the privileged service opens and then operates on. 05 — PRIMITIVE SYSTEM service operates on the attacker-chosen object -> arbitrary file/device access -> EoP The service reads/writes the attacker-named file or device with its own privileges. The Jan 2026 fix (Feature_2464883000) calls GetMailslotInfo after CreateFileW and rejects (CloseHandle) any handle that is not a mailslot.

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

Sergey Bliznyuk with Positive Technologies

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