CVE-2026-21242 — Windows Subsystem for Linux Elevation of Privilege Vulnerability

Executive Summary

Use after free in Windows Subsystem for Linux 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-416: Use After Free

Released Feb 10 2026

Last Updated Feb 10 2026

Publicly Disclosed No

CISA KEV Not Listed

Known Exploits None Known

EPSS Score 0.0034 — 0.26911 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

0.0034

probability of exploitation in the next 30 days

0.26911 percentile - updated 2026-08-14

View on FIRST.org

Affected Products

9 affected products

Product	KB Article	Severity	Impact	Restart Required
Windows 10 Version 21H2 for ARM64-based Systems	`5075912 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 21H2 for x64-based Systems	`5075912 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for ARM64-based Systems	`5075912 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 10 Version 22H2 for x64-based Systems	`5075912 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 23H2 for ARM64-based Systems	`5075941 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 23H2 for x64-based Systems	`5075941 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 Version 24H2 for ARM64-based Systems 5077181 (Security Update) 5077212 (Security Hotpatch Update) Important Elevation of Privilege 5073379 Base: 7.0 Temporal: 6.1 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 10.0.26100.7840 10.0.26100.7781 Yes None Windows 11 Version 24H2 for x64-based Systems 5077212 (Security Hotpatch Update) 5077181 (Security Update) Important Elevation of Privilege 5074109 Base: 7.0 Temporal: 6.1 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 10.0.26100.7781 10.0.26100.7840 Yes None Windows 11 Version 25H2 for ARM64-based Systems 5077181 (Security Update) 5077212 (Security Hotpatch Update) Important Elevation of Privilege 5074109 Base: 7.0 Temporal: 6.1 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 10.0.26200.7840 10.0.26200.7781 Yes None Windows 11 Version 25H2 for x64-based Systems 5077181 (Security Update) 5077212 (Security Hotpatch Update) Important Elevation of Privilege 5074109 Base: 7.0 Temporal: 6.1 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 10.0.26200.7840 10.0.26200.7781 Yes None Windows 11 Version 26H1 for ARM64-based Systems	`5077179 (Security Update)`	Important	Elevation of Privilege	Yes
Windows 11 version 26H1 for x64-based Systems	`5077179 (Security Update)`	Important	Elevation of Privilege	Yes
Windows Server 2022 5075906 (Security Update) 5075943 (Security Hotpatch Update) Important Elevation of Privilege 5073457 Base: 7.0 Temporal: 6.1 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 10.0.20348.4773 10.0.20348.4711 Yes None Windows Server 2022 (Server Core installation) 5075906 (Security Update) 5075943 (Security Hotpatch Update) Important Elevation of Privilege 5073457 Base: 7.0 Temporal: 6.1 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 10.0.20348.4773 10.0.20348.4711 Yes None Windows Server 2022, 23H2 Edition (Server Core installation)	`5075897 (Security Update)`	Important	Elevation of Privilege	Yes

Patches

4 patches

Article	Type	Restart
`5075912`	Security Update	Yes
`5075941`	Security Update	Yes
`5077179`	Security Update	Yes
`5075897`	Security Update	Yes

Patch Diff

ghidriff · p9rdr.sys (KB5077181)

Use-after-free (CWE-416) in the Windows 9P/WSL redirector p9rdr.sys UserCallback timeout path, local EoP. Opening \Device\P9Rdr\wsl.localhost\... drives p9fs::P9CreateVNetRoot -> Device::CreateNetRootAndVNetRoot -> UserCallbackManager::CallUserCallback -> NotifyUserProcess, which registers a new p9fs::UserCallback and calls p9fs::UserCallback::SetCallbackTimeout to arm a timer. PRE: SetCallbackTimeout calls ExAllocateTimer (which does ExAllocatePool2(0xA0,'mTxE')); if an attacker exhausts pool so ExAllocateTimer fails, SetCallbackTimeout returns STATUS_INSUFFICIENT_RESOURCES (0xC000009A) up through NotifyUserProcess/CallUserCallback/Device::CallUserCallback, and CreateNetRootAndVNetRoot then calls rdbss!RxFinishPhysicalNetRootConstruction to free the NetRootConstructionContext while the just-registered callback still references the create RxContext/NetRoot - a use-after-free. Diff of p9rdr.sys 10.0.26100.7824 -> .8328 (Feb 10 2026, KB5077181) confirms the fix: gated behind CFR flags Feature_1655910715 / Feature_3906416953, the UserCallback registration/timeout is reworked so the callback is reference-counted and released under the manager lock (UserCallback::ReleaseReference in FindOrInsertCallbackWithLockHeld) and SetCallbackTimeout no longer leaves a dangling callback on the timer-allocation failure path, keeping the callback and the create context lifetimes in sync.

Pre-patch version 10.0.26100.7824 Download

Post-patch version 10.0.26100.8328 Download

Function	Address	Change	Note
`p9fs::UserCallback::SetCallbackTimeout`	`code change`	code (timer-allocation failure path reworked, CFR-gated)	Pre: inline ExAllocateTimer(TimerCallback,this,0) under Feature_1655910715; on ExAllocateTimer failure returns 0xC000009A, and the caller CreateNetRootAndVNetRoot frees NetRootConstructionContext while the registered callback still references the create context -> UAF. Post: timer lifecycle reworked; the ExAllocateTimer/DeleteTimer inline block is removed and the callback is reference-released, so a timer-alloc failure no longer strands a callback over a freed context.
`p9fs::UserCallbackManager::FindOrInsertCallbackWithLockHeld`	`code change`	code (reference release under lock, CFR-gated)	Now calls p9fs::UserCallback::ReleaseReference on the callback under the manager lock so the callback cannot outlive its create context.
`p9fs::UserCallbackManager::RegisterUserCallback`	`code change`	code (reference management, CFR-gated)	Reworked registration path participating in the reference-counted callback lifetime.
`Feature_1655910715 / Feature_3906416953`	`gate`	added (CFR gate)	CFR flags gating the reference-counted callback/timeout lifetime; the original path still ships when disabled.

View full diff report View RCA report

Attack Path

A timer-allocation failure on the WSL create path frees the NetRoot construction context while a registered user callback still references it

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