Win32k Elevation of Privilege Vulnerability (CVE-2021-41357)
Published In CISA KEV since Sources: CVE.org
What is affected, what to do
Win32k Elevation of Privilege Vulnerability
The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 21H1 to version 10.0.19043.1288, Microsoft Windows Server 2022 to version 10.0.20348.288, Microsoft Windows 10 Version 2004 to version 10.0.19041.1288. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Windows 10 Version 21H1
lt 10.0.0..10.0.19043.1288 - Microsoft Windows Server 2022
lt 10.0.0..10.0.20348.288 - Microsoft Windows 10 Version 2004
lt 10.0.0..10.0.19041.1288 - Microsoft Windows Server version 2004
lt 10.0.0..10.0.19041.1288 - Microsoft Windows 10 Version 20H2
lt 10.0.0..10.0.19042.1288 - Microsoft Windows Server version 20H2
lt 10.0.0..10.0.19042.1288 - Microsoft Windows 11 version 21H2
lt 10.0.0..10.0.22000.258
- Microsoft Windows 10 Version 21H1
- Fixed in
- Microsoft Windows 10 Version 21H1
10.0.19043.1288 - Microsoft Windows Server 2022
10.0.20348.288 - Microsoft Windows 10 Version 2004
10.0.19041.1288 - Microsoft Windows Server version 2004
10.0.19041.1288 - Microsoft Windows 10 Version 20H2
10.0.19042.1288 - Microsoft Windows Server version 20H2
10.0.19042.1288 - Microsoft Windows 11 version 21H2
10.0.22000.258
- Microsoft Windows 10 Version 21H1
Not sure whether and how you are affected? Ask a question about this vulnerability
Metrics
7.8
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
7.8
CVSS v3
Severity High on the scale from 0 to 10.
Source: CVE.org
1.6 %
EPSS score
Moderate — model estimates 1-10% exploitation likelihood.
Source: FIRST EPSS, percentile 75.5
Show all metrics
- Severity
- Critical
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 184.58 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- actively exploited
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 7.8 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 7.8
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 1.6 %
- Moderate — model estimates 1-10% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 75.5
- High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- Actively exploited
- Published
In CISA KEV since
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- CISA assessment (SSVC)
- Exploitation: active · not automatable · Technical impact: total
- Source: CISA Vulnrichment, as of 4 June 2024
Response and mitigation
Why act now?
Prioritisation rationale
This is a local kernel privilege escalation that requires low-privileged code execution beforehand. Attackers use it as a second-stage step after phishing, a malware dropper or compromised RDP access, to gain SYSTEM, disable EDR and harvest credentials. Exploitation likelihood is moderate and CISA does not flag ransomware use, but Win32k bugs are routinely chained into reliable exploit kits, which is why this one made KEV. Rank shared session hosts and out-of-support builds at the top. Fully updated endpoints on a current cumulative update only need verification, not emergency action.
Runbook step 1
Immediate response (0-24 h)
- Inventory build levels: Check every Windows 10, Windows 11 and Windows Server host against the fixed builds: 10.0.19041.1288 (Windows 10 2004 / Server 2004), 10.0.19042.1288 (Windows 10 20H2 / Server 20H2), 10.0.19043.1288 (Windows 10 21H1), 10.0.22000.258 (Windows 11 21H2) and 10.0.20348.288 (Server 2022). In MDE:
DeviceTvmSoftwareVulnerabilities | where CveId == "CVE-2021-41357". Otherwise read the build number and UBR fromHKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion. - Deploy a cumulative update: Use any cumulative security update that brings hosts to at least those builds. Patch available from the vendor — look up the KB number in the Microsoft Security Update Guide entry for CVE-2021-41357. Any later cumulative update also contains the fix. Hosts still below these builds are missing many other security fixes too.
- Patch shared hosts first: Prioritise RDS/Citrix session hosts, VDI pools, jump hosts and build agents. These are where many low-privileged users run code locally, and exploitation only requires a local session.
- Flag out-of-support releases: Windows 10 2004/20H2/21H1 and Server 2004/20H2 no longer receive updates. Upgrade them to a supported release, or isolate them from production networks until you can.
- Trigger compromise assessment: On unpatched hosts with a known initial-access event (phishing, malware detection), assume SYSTEM-level escalation may have occurred. Reset local accounts and cached credentials. If privileged domain accounts were logged on to the host, rotate those as well.
- Inventory build levels: Check every Windows 10, Windows 11 and Windows Server host against the fixed builds: 10.0.19041.1288 (Windows 10 2004 / Server 2004), 10.0.19042.1288 (Windows 10 20H2 / Server 20H2), 10.0.19043.1288 (Windows 10 21H1), 10.0.22000.258 (Windows 11 21H2) and 10.0.20348.288 (Server 2022). In MDE:
Runbook step 2
Mitigation layers
- Application control (endpoint): Enforce WDAC or AppLocker for user-writable paths (
%TEMP%,%APPDATA%,Downloads,C:\Users\Public). A Win32k exploit needs attacker code running on the host first, and this makes that prerequisite much harder to meet. - Win32k syscall filtering: Use Exploit Protection (
Set-ProcessMitigation -Name <app>.exe -Enable DisableWin32kSystemCalls) to strip the Win32k attack surface from exposed non-GUI processes. Validate in audit mode first, because GUI applications break under this setting. - Restrict interactive logon (IAM): On servers, limit "Allow log on locally" and "Allow log on through Remote Desktop Services" to admin groups via GPO. Apply "Deny log on locally" to service accounts.
- Contain post-escalation impact: Enable Credential Guard and LSA protection (
RunAsPPL=1) so that SYSTEM access does not translate directly into stolen LSASS secrets. Deploy LAPS for unique local admin passwords and enforce admin tiering, with no Tier-0 logons on workstations. - Kernel hardening: Turn on HVCI / Memory Integrity and confirm EDR tamper protection is enabled. Kernel-level escalation is commonly used to blind security tooling.
- Application control (endpoint): Enforce WDAC or AppLocker for user-writable paths (
Runbook step 3
Detection rules
- Integrity-level jump (Sysmon EID 1 / MDE): Alert when a System-integrity process is spawned by a Medium/Low-integrity parent. KQL:
DeviceProcessEvents | where ProcessIntegrityLevel == "System" and InitiatingProcessIntegrityLevel in ("Medium","Low") and InitiatingProcessAccountSid != "S-1-5-18". Allowlist legitimate brokers such asconsent.exe. - SYSTEM shell from a user context (Security EID 4688): Look for
cmd.exe,powershell.exeorwhoami.exerunning asS-1-5-18where the ancestry traces back to a user session (explorer.exe→ unknown binary → SYSTEM shell). Sigma shape:User|contains: 'SYSTEM'ANDParentImage|contains: '\Users\'. - Failed exploit attempts: Watch for BugCheck events (System EID 1001) whose dumps reference
win32kfull.sysorwin32kbase.sys. Repeated WER crashes (Application EID 1000) of unknown binaries in user profiles also point to unstable exploit attempts. - Post-exploitation chain: Correlate the integrity jump with any of the following within a 10-minute window on the same host: LSASS access (Sysmon EID 10,
TargetImageending inlsass.exe,GrantedAccess 0x1010/0x1410), new service installs (System EID 7045), or EDR services being stopped or reconfigured (EID 7036/7040). - Unsigned binaries dropped and executed: Alert on Sysmon EID 11/29 for unsigned EXE/DLL files in user profiles that are subsequently executed. Combined with the first rule, this is a high-confidence signal.
- Integrity-level jump (Sysmon EID 1 / MDE): Alert when a System-integrity process is spawned by a Medium/Low-integrity parent. KQL:
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
Win32k Elevation of Privilege Vulnerability
Source: NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)
Vendors and products
- Microsoft Windows 10 2004
- Microsoft Windows 10 20h2
- Microsoft Windows 10 21h1
- Microsoft Windows 10 Version 2004
- Microsoft Windows 10 Version 20h2
- Microsoft Windows 10 Version 21h1
- Microsoft Windows 11 21h2
- Microsoft Windows 11 Version 21h2
- Microsoft Microsoft Windows Server 2004
- Microsoft Windows Server 2022
- Microsoft Windows Server 20h2
- Microsoft Windows Server Version 2004
Affected operating systems
| Windows | Microsoft | Windows 10 Version 2004 | 10.0.19041.1288 |
| Windows | Microsoft | Windows 10 Version 20H2 | 10.0.19042.1288 |
| Windows | Microsoft | Windows 10 Version 21H1 | 10.0.19043.1288 |
| Windows | Microsoft | Windows 11 version 21H2 | 10.0.22000.258 |
| Windows | Microsoft | Windows Server 2022 | 10.0.20348.288 |
| Windows | Microsoft | Windows Server version 2004 | 10.0.19041.1288 |
| Windows | Microsoft | Windows Server version 20H2 | 10.0.19042.1288 |
| Windows | microsoft | windows_10_1809 | 10.0.19041.1288 |
| Windows | microsoft | windows_10_2004 | — |
| Windows | microsoft | windows_10_20H2 | 10.0.19042.1288 |
| Windows | microsoft | windows_10_20h2 | — |
| Windows | microsoft | windows_10_21H1 | 10.0.19043.1288 |
| Windows | microsoft | windows_10_21h1 | — |
| Windows | microsoft | windows_11_21H2 | 10.0.22000.258 |
| Windows | microsoft | windows_11_21h2 | — |
| Windows | microsoft | windows_server_2004 | 10.0.19041.1288 |
| Windows | microsoft | windows_server_2004 | — |
| Windows | microsoft | windows_server_2022 | 10.0.20348.288 |
| Windows | microsoft | windows_server_2022 | — |
| Windows | microsoft | windows_server_20H2 | 10.0.19042.1288 |
| Windows | microsoft | windows_server_20h2 | — |
Public exploit references
Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited |
References & sources
- portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-41357x_refsource_MISC, Patch, Vendor advisory
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-41357government-resource, US government resource
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<a href="https://intel.neosec.eu/en/cve/CVE-2021-41357"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-41357.svg?lang=en" alt="CVE-2021-41357 – NEOSEC Intel" height="20"></a>