Win32k Elevation of Privilege Vulnerability (CVE-2021-40450)
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 1809 to version 10.0.17763.2237, Microsoft Windows Server 2019 to version 10.0.17763.2237, Microsoft Windows Server 2019 (Server Core installation) to version 10.0.17763.2237. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Windows 10 Version 1809
lt 10.0.0..10.0.17763.2237 - Microsoft Windows Server 2019
lt 10.0.0..10.0.17763.2237 - Microsoft Windows Server 2019 (Server Core installation)
lt 10.0.0..10.0.17763.2237 - Microsoft Windows 10 Version 1909
lt 10.0.0..10.0.18363.1854 - 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 1809
- Fixed in
- Microsoft Windows 10 Version 1809
10.0.17763.2237 - Microsoft Windows Server 2019
10.0.17763.2237 - Microsoft Windows Server 2019 (Server Core installation)
10.0.17763.2237 - Microsoft Windows 10 Version 1909
10.0.18363.1854 - 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 1809
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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 75.4
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- 184.54 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- 75.4
- 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 February 2025
Response and mitigation
Why act now?
Prioritisation rationale
This is a local privilege escalation in the Win32k kernel component that needs only low privileges and no user interaction. It is not an initial-access vector but a second-stage building block: attackers chain it after phishing, macro or browser execution to go from user to SYSTEM, tamper with EDR and harvest credentials. CISA does not flag known ransomware use, and exploitation likelihood is moderate, but Win32k bugs are a staple of post-exploitation toolkits. Rank it high if you run multi-user terminal servers, VDI or stale legacy builds. If your fleet is fully patched (October 2021 updates or later), verifying compliance through routine vulnerability management is sufficient.
Runbook step 1
Immediate response (0-24 h)
- Deploy the October 2021 security update or any later cumulative update. The flaw is fixed from these builds onwards: Windows 10 1809 / Server 2019 (incl. Server Core) 10.0.17763.2237, 1909 10.0.18363.1854, 2004 10.0.19041.1288, 20H2 10.0.19042.1288, 21H1 10.0.19043.1288, Windows 11 21H2 10.0.22000.258. Per-build KB numbers are listed in the Microsoft advisory (MSRC Update Guide).
- Reconcile the inventory: Pull OS build plus UBR from SCCM/Intune/EDR, or locally via
Get-CimInstance Win32_OperatingSystem | select Version,BuildNumber. Flag every host below the fixed builds. - Patch multi-user systems first: Prioritise RDS/terminal servers, Citrix VDAs, jump hosts and build servers, where many non-admin users run code. Service-account-heavy servers and internet-facing clients come next.
- Find out-of-support builds (e.g. 1909, 2004). Isolate or segment them until they are migrated. Fixes only exist for builds that were serviced when the update shipped.
- On suspected compromise: Isolate the host via EDR and rotate credentials for every account that logged on there. Assume LSASS was accessed once the attacker reached SYSTEM.
Runbook step 2
Mitigation layers
- No vendor workaround exists. Microsoft lists none, so compensating controls focus on preventing the preceding code execution.
- Application control (endpoint hardening): Run WDAC or AppLocker in enforce mode. Block execution from
%TEMP%,%APPDATA%,DownloadsandC:\Users\Public, the usual drop locations for unsigned LPE binaries and DLLs. - Shrink the Win32k attack surface: Use Exploit Protection to enable "Disable Win32k system calls" for processes that do not need a GUI. Enable ASR rules, notably "Block Office applications from creating child processes" and "Block executable files unless they meet prevalence, age or trusted list criterion".
- IAM / least privilege: Restrict interactive and RDP logon to servers via GPO user-rights assignments. Enforce admin tiering so privileged accounts never sign in to Tier-2 endpoints.
- Contain the blast radius: Enable Credential Guard and LSA Protection (
RunAsPPL=1). Deploy LAPS so each host has a unique local admin password, which limits lateral movement after escalation. - Network segmentation: Move unpatched legacy hosts into a dedicated VLAN. Allow SMB (445), RDP (3389) and WinRM (5985/5986) into it only from designated admin workstations.
Runbook step 3
Detection rules
- SYSTEM processes spawned from user context (Sysmon EID 1 / Security 4688): Look for processes running as
NT AUTHORITY\SYSTEMwhose parent runs as a standard user. KQL:DeviceProcessEvents | where AccountSid == "S-1-5-18" and InitiatingProcessAccountSid != "S-1-5-18" and InitiatingProcessFileName !in~ ("services.exe","svchost.exe","wininit.exe") - Post-escalation shells:
cmd.exe,powershell.exeorrundll32.exerunning as SYSTEM with a parent launched from\Users\or\Temp\. Sigma shape:ParentImage|contains: '\Users\'combined withUser|contains: 'SYSTEM'. - Kernel crashes from failed exploit attempts: System log EID 1001 (BugCheck) or EID 41 (Kernel-Power) where the minidump references
win32k*.sys. Correlate repeated crashes with recent file drops on the same host. - Unknown binary drops: Sysmon EID 11 for
.exe/.dllwritten to user-writable paths, followed by EID 1 for the same hash within minutes. Filter on low-prevalence files in EDR. - Follow-on activity: Sysmon EID 10 with
TargetImagelsass.exeandGrantedAccess0x1010/0x1410. Also watch for Security EID 4720/4732 (new account, or addition to local Administrators) shortly after a suspicious SYSTEM process.
- SYSTEM processes spawned from user context (Sysmon EID 1 / Security 4688): Look for processes running as
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 Microsoft Windows 10 1809
- Microsoft Windows 10 1909
- Microsoft Windows 10 2004
- Microsoft Windows 10 20h2
- Microsoft Windows 10 21h1
- Microsoft Windows 10 Version 1809
- Microsoft Windows 10 Version 1909
- Microsoft Windows 10 Version 2004
- Microsoft Windows 10 Version 20h2
- Microsoft Windows 10 Version 21h1
- Microsoft Windows 11 21h2
- Microsoft Windows 11 Version 21h2
Affected operating systems
| Windows | Microsoft | Windows 10 Version 1809 | 10.0.17763.2237 |
| Windows | Microsoft | Windows 10 Version 1909 | 10.0.18363.1854 |
| 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 2019 | 10.0.17763.2237 |
| Windows | Microsoft | Windows Server 2019 (Server Core installation) | 10.0.17763.2237 |
| 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 | 10.0.17763.2237 |
| Windows | microsoft | windows_10_1809 | 10.0.18363.1854 |
| Windows | microsoft | windows_10_1809 | 10.0.19041.1288 |
| Windows | microsoft | windows_10_1809 | — |
| Windows | microsoft | windows_10_1909 | 10.0.18363.1854 |
| Windows | microsoft | windows_10_1909 | — |
| 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 | — |
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-2021-40450x_refsource_MISC, Patch, Vendor advisory
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40450government-resource, US government resource
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<a href="https://intel.neosec.eu/en/cve/CVE-2021-40450"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-40450.svg?lang=en" alt="CVE-2021-40450 – NEOSEC Intel" height="20"></a>