Microsoft DWM Core Library Elevation of Privilege Vulnerability (CVE-2021-33739)
Published In CISA KEV since Sources: CVE.org
- Actively exploited
- Source: ANSSI / CERT-FR How we classify
What is affected, what to do
Microsoft DWM Core Library Elevation of Privilege Vulnerability
The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 1909 to version 10.0.18363.1621, Microsoft Windows 10 Version 21H1 to version 10.0.19043.1052, Microsoft Windows 10 Version 2004 to version 10.0.19041.1052. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Windows 10 Version 1909
lt 10.0.0..10.0.18363.1621 - Microsoft Windows 10 Version 21H1
lt 10.0.0..10.0.19043.1052 - Microsoft Windows 10 Version 2004
lt 10.0.0..10.0.19041.1052 - Microsoft Windows Server version 2004
lt 10.0.0..10.0.19041.1052 - Microsoft Windows 10 Version 20H2
lt 10.0.0..10.0.19042.1052 - Microsoft Windows Server version 20H2
lt 10.0.0..10.0.19042.1052
- Microsoft Windows 10 Version 1909
- Fixed in
- Microsoft Windows 10 Version 1909
10.0.18363.1621 - Microsoft Windows 10 Version 21H1
10.0.19043.1052 - Microsoft Windows 10 Version 2004
10.0.19041.1052 - Microsoft Windows Server version 2004
10.0.19041.1052 - Microsoft Windows 10 Version 20H2
10.0.19042.1052 - Microsoft Windows Server version 20H2
10.0.19042.1052
- Microsoft Windows 10 Version 1909
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Metrics
8.4
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
8.4
CVSS v3
Severity High on the scale from 0 to 10.
Source: CVE.org
6.6 %
EPSS score
Moderate — model estimates 1-10% exploitation likelihood.
Source: FIRST EPSS, percentile 93.8
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- 212.37 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 exploitedSource: ANSSI / CERT-FR How we classify
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- 8.4 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- 8.4
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- 6.6 %
- 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- 93.8
- Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
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 20 March 2024
Response and mitigation
Why act now?
Prioritisation rationale
This flaw was exploited as a zero-day local privilege escalation. It is the classic second link in a chain, lifting an attacker from a phishing- or browser-delivered foothold to SYSTEM. It is not network-reachable, but it requires no privileges and no user interaction, and its exploitation likelihood sits well above average. CISA does not flag known ransomware use. For estates running supported, current Windows builds the risk is effectively closed. It becomes urgent wherever outdated Windows 10 SAC builds, terminal servers or unmanaged legacy endpoints persist — and under NIS2 those systems are also a finding in vulnerability and lifecycle management.
Runbook step 1
Immediate response (0-24 h)
- Inventory vulnerable builds: Identify all Windows 10 and Windows Server SAC hosts (1909, 2004, 20H2, 21H1) below build 10.0.18363.1621 (1909) or 10.0.19041/19042/19043.1052 (2004/20H2/21H1). Check the patch level locally via
(Get-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion').UBR, or at scale through Intune, ConfigMgr or Defender TVM. - Deploy the cumulative update: The update must bring each host to at least those builds. A patch is available from the vendor (Microsoft security update for CVE-2021-33739) — look up the KB number for each version in the vendor advisory. Any later cumulative update also covers the fix.
- Treat end-of-service systems as high-risk: Windows 10 1909, 2004 and 20H2 and Windows Server SAC 2004/20H2 no longer receive updates. Any survivors should be migrated to a supported release as a priority, or isolated until then.
- Patch multi-user hosts first: RDS/Citrix hosts, jump servers, kiosks and shared workstations come first. On these systems, a local SYSTEM escalation from any user session exposes many identities at once.
- Triage suspected compromise: On unpatched hosts with signs of initial access (phishing payload, browser exploit), take a full forensic capture. Rotate local admin passwords (LAPS) and any cached credentials.
- Inventory vulnerable builds: Identify all Windows 10 and Windows Server SAC hosts (1909, 2004, 20H2, 21H1) below build 10.0.18363.1621 (1909) or 10.0.19041/19042/19043.1052 (2004/20H2/21H1). Check the patch level locally via
Runbook step 2
Mitigation layers
- Application control: Use WDAC or AppLocker to block unsigned binaries and scripts from user-writable paths (
%APPDATA%,%TEMP%,Downloads). This bug needs prior code execution, so breaking the chain earlier removes the opportunity. - Endpoint hardening (ASR): Enable Defender ASR rules, notably "Block Office applications from creating child processes", "Block executable files unless they meet prevalence, age or trusted list criteria" and "Block credential stealing from LSASS". The last one limits what an attacker gains after escalating.
- IAM/least privilege: Restrict interactive and RDP logon on servers to tiered admin accounts. Standard users should never get a server desktop session.
- Network segmentation: Move unpatchable legacy hosts into a dedicated VLAN with proxy-only, allowlisted egress. Block SMB/RDP/WinRM from that segment toward domain controllers and server networks.
- EDR tamper protection: Enforce tamper protection. Disabling Defender/EDR is the standard first move once SYSTEM is obtained.
- Microsoft provides no configuration workaround. Only the update fully remediates the issue.
- Application control: Use WDAC or AppLocker to block unsigned binaries and scripts from user-writable paths (
Runbook step 3
Detection rules
- Unusual children of dwm.exe (Sysmon EID 1 / Security 4688):
dwm.exeessentially never spawns child processes. Sigma shape:ParentImage|endswith: '\dwm.exe'withImage|endswith: ['\cmd.exe','\powershell.exe','\rundll32.exe','\conhost.exe']. KQL:DeviceProcessEvents | where InitiatingProcessFileName =~ "dwm.exe" and FileName !in~ ("WerFault.exe") - DWM crashes as failed exploit attempts: Look for Application log Event ID 1000 (faulting application
dwm.exe, moduledwmcore.dll) and Event ID 1001 (WER). Clusters of these on a single host within a short window warrant investigation. - SYSTEM tokens arising from user context: Flag processes running at
IntegrityLevel=Systemwhose ancestry traces back to a standard user's medium-integrity process, withoutservices.exein between. Use EDR token-change telemetry or unexpected Security 4672 events. - Suspicious modules or access to dwm.exe: Watch Sysmon EID 7 (ImageLoad) where
Image|endswith: '\dwm.exe'loads unsigned DLLs or DLLs from user paths. Also watch Sysmon EID 8/10 (CreateRemoteThread/ProcessAccess) targetingdwm.exefrom non-system processes. - Follow-on activity: Shortly after escalation, expect LSASS access (Sysmon EID 10,
TargetImagelsass.exe), new local admin group members (Security 4732) and new service installs (System 7045).
- Unusual children of dwm.exe (Sysmon EID 1 / Security 4688):
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
Microsoft DWM Core Library Elevation of Privilege Vulnerability
Source: NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)
Vendors and products
- Microsoft Windows 10 1909
- Microsoft Windows 10 2004
- Microsoft Windows 10 20h2
- Microsoft Windows 10 21h1
- Microsoft Windows 10 Version 1909
- Microsoft Windows 10 Version 2004
- Microsoft Windows 10 Version 20h2
- Microsoft Windows 10 Version 21h1
- Microsoft Microsoft Windows Server 2004
- Microsoft Windows Server 20h2
- Microsoft Windows Server Version 2004
- Microsoft Windows Server Version 20h2
Affected operating systems
| Windows | Microsoft | Windows 10 Version 1909 | 10.0.18363.1621 |
| Windows | Microsoft | Windows 10 Version 2004 | 10.0.19041.1052 |
| Windows | Microsoft | Windows 10 Version 20H2 | 10.0.19042.1052 |
| Windows | Microsoft | Windows 10 Version 21H1 | 10.0.19043.1052 |
| Windows | Microsoft | Windows Server version 2004 | 10.0.19041.1052 |
| Windows | Microsoft | Windows Server version 20H2 | 10.0.19042.1052 |
| Windows | microsoft | windows_10_1809 | 10.0.18363.1621 |
| Windows | microsoft | windows_10_1809 | 10.0.19041.1052 |
| Windows | microsoft | windows_10_1909 | 10.0.18363.1621 |
| Windows | microsoft | windows_10_1909 | — |
| Windows | microsoft | windows_10_2004 | — |
| Windows | microsoft | windows_10_20H2 | 10.0.19042.1052 |
| Windows | microsoft | windows_10_20h2 | — |
| Windows | microsoft | windows_10_21H1 | 10.0.19043.1052 |
| Windows | microsoft | windows_10_21h1 | — |
| Windows | microsoft | windows_server_2004 | 10.0.19041.1052 |
| Windows | microsoft | windows_server_2004 | — |
| Windows | microsoft | windows_server_20H2 | 10.0.19042.1052 |
| 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 | |
| ANSSI / CERT-FRLicence Ouverte 2.0, as of | CERTFR-2021-ALE-012 – Multiples vulnérabilités dans Microsoft Windows (clôturée 2021-10-19) (opens in a new tab)cert.cyber.gouv.fr | actively exploited |
References & sources
- portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-33739x_refsource_MISC, Patch, Vendor advisory
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33739government-resource, US government resource
- cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-012 – Multiples vulnérabilités dans Microsoft Windows (clôturée 2021-10-19)cert-fr
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<a href="https://intel.neosec.eu/en/cve/CVE-2021-33739"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-33739.svg?lang=en" alt="CVE-2021-33739 – NEOSEC Intel" height="20"></a>