CVE-2021-41357 Overview
CVE-2021-41357 is an elevation of privilege vulnerability in the Windows Win32k kernel-mode subsystem. Microsoft addressed the flaw in the October 2021 Patch Tuesday release. The vulnerability affects multiple supported Windows client and server versions, including Windows 10 (2004, 20H2, 21H1), Windows 11 21H2, and Windows Server 2022. A locally authenticated attacker can leverage the flaw to escalate to SYSTEM privileges. CISA added CVE-2021-41357 to the Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild.
Critical Impact
Successful exploitation grants kernel-level SYSTEM privileges on the affected host, enabling full compromise of confidentiality, integrity, and availability.
Affected Products
- Microsoft Windows 10 versions 2004, 20H2, and 21H1
- Microsoft Windows 11 21H2
- Microsoft Windows Server 2004, 20H2, and Windows Server 2022
Discovery Timeline
- 2021-10-13 - CVE-2021-41357 published to NVD and addressed in Microsoft's October 2021 security update
- 2025-10-30 - Last updated in NVD database
Technical Details for CVE-2021-41357
Vulnerability Analysis
The vulnerability resides in Win32k.sys, the kernel-mode driver responsible for window management, graphics rendering, and user-mode input handling. Win32k has been a recurring target for local privilege escalation because it exposes a large attack surface through system calls reachable from low-integrity processes. An attacker who can execute code on the target system can invoke specific Win32k routines to corrupt kernel memory or manipulate kernel objects, yielding arbitrary code execution in ring 0.
The MITRE classification for this CVE is NVD-CWE-noinfo, and Microsoft has not published detailed root-cause information beyond the advisory. The CVSS vector indicates a local attack vector with low complexity and low privileges required, with no user interaction. EPSS reporting places the exploitation likelihood in the upper percentile range, consistent with its inclusion in the CISA KEV catalog.
Root Cause
The root cause is an unspecified memory-safety or object-handling defect within Win32k. Microsoft's advisory does not enumerate the exact internal function, but the class of bug is consistent with prior Win32k elevation-of-privilege issues involving callbacks, type confusion, or use-after-free conditions on kernel-managed GUI objects.
Attack Vector
Exploitation requires local code execution on the target host with at least standard user privileges. An attacker typically chains this primitive after initial access, for example following a phishing payload or browser exploit, to break out of a sandbox or move from a non-privileged user context to SYSTEM. No user interaction is required during the privilege escalation stage. See the Microsoft Security Advisory for CVE-2021-41357 for vendor guidance.
Detection Methods for CVE-2021-41357
Indicators of Compromise
- Unexpected processes running as NT AUTHORITY\SYSTEM that were spawned from a standard user session.
- Crashes or bug checks referencing win32k.sys or win32kfull.sys shortly before suspicious activity.
- Creation of new local administrator accounts or scheduled tasks immediately after a low-privilege process executes an unsigned binary.
Detection Strategies
- Monitor for token manipulation patterns where a non-elevated process suddenly accesses resources requiring administrative or SYSTEM rights.
- Hunt for child processes inheriting SYSTEM integrity level from parents running at medium integrity.
- Correlate Win32k-related crashes recorded in Windows Error Reporting (WerFault) with subsequent privileged process activity.
Monitoring Recommendations
- Enable kernel-mode crash dump collection and ingest WER telemetry into a centralized SIEM for correlation.
- Enable Microsoft Defender Attack Surface Reduction rules and audit Sysmon event IDs 1, 10, and 25 for anomalous process and image-load behavior.
- Track patch-state of win32k.sys across the fleet and flag hosts that remain on pre-October 2021 builds.
How to Mitigate CVE-2021-41357
Immediate Actions Required
- Apply the October 2021 Microsoft security updates to all affected Windows 10, Windows 11, and Windows Server systems.
- Prioritize patching for systems where standard users have interactive logon rights or where untrusted code may execute.
- Audit endpoints against the CISA KEV catalog and confirm remediation status, as documented in the CISA KEV listing for CVE-2021-41357.
Patch Information
Microsoft released fixes for CVE-2021-41357 as part of the October 12, 2021 Patch Tuesday cumulative updates. Administrators should consult the Microsoft Security Update Guide for the specific KB article that maps to each affected Windows build and apply it through Windows Update, WSUS, or their preferred patch management tool.
Workarounds
- No official workaround is published; applying the vendor update is the supported remediation.
- Restrict interactive and remote logon rights for non-administrative users on sensitive hosts until patches are deployed.
- Enforce application allowlisting (for example, Windows Defender Application Control) to limit execution of untrusted binaries that could trigger the local exploit chain.
# Verify patch state on a Windows host by listing installed October 2021 updates
wmic qfe get HotFixID,InstalledOn | findstr /i "2021"
# Force Windows Update scan and install via PowerShell
USoClient.exe StartScan
USoClient.exe StartDownload
USoClient.exe StartInstall
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
