Win32k Elevation of Privilege Vulnerability (CVE-2021-28310)
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. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Windows 10 Version 1803
gte 10.0.0..* - Microsoft Windows 10 Version 1809
gte 10.0.0..* - Microsoft Windows Server 2019
gte 10.0.0..* - Microsoft Windows Server 2019 (Server Core installation)
gte 10.0.0..* - Microsoft Windows 10 Version 1909
gte 10.0.0..* - Microsoft Windows Server, version 1909 (Server Core installation)
gte 10.0.0..* - Microsoft Windows 10 Version 2004
gte 10.0.0..* - Microsoft Windows Server version 2004
gte 10.0.0..* - Microsoft Windows 10 Version 20H2
gte 10.0.0..* - Microsoft Windows Server version 20H2
gte 10.0.0..*
- Microsoft Windows 10 Version 1803
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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
8.4 %
EPSS score
Moderate — model estimates 1-10% exploitation likelihood.
Source: FIRST EPSS, percentile 95.0
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- 211.17 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- 8.4 %
- 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- 95.0
- 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 7 March 2024
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-787
Response and mitigation
Why act now?
Prioritisation rationale
CVE-2021-28310 was exploited as a zero-day at disclosure and serves attackers as a second-stage privilege escalation. After a low-privileged foothold, for example via a browser exploit or phishing, it gives full SYSTEM control with high confidentiality, integrity and availability impact and needs no user interaction. Because it requires local access, it cannot serve as a network entry point, but its exploitation likelihood is well above average, and CISA records no known ransomware use. Rate it high if you run Windows Server 2019 RDS/VDI hosts or unsupported Windows 10 builds. For fully updated, supported estates, patch-state verification is sufficient.
Runbook step 1
Immediate response (0-24 h)
- Apply the security update: Microsoft fixed this in the April 2021 security update (Patch Tuesday, 13 April 2021). Every later cumulative update also contains the fix. Look up the KB number per build in the MSRC entry for CVE-2021-28310. Verify patch state via WSUS/Intune/SCCM against OS build numbers (
Get-ComputerInfo | select OsBuildNumber). - Flag end-of-service builds: Windows 10 1803, 1909, 2004 and 20H2, plus Windows Server 1909 (Server Core), no longer receive updates. Tag these as high-risk assets and fast-track migration to a supported release. Apply the compensating controls below until then.
- Patch multi-user hosts first: Prioritise Windows Server 2019 systems acting as RDS/terminal servers, Citrix VDAs or jump hosts. On these hosts, any logged-on standard user can escalate to SYSTEM and harvest other users' sessions and credentials.
- Treat suspicious unpatched endpoints as compromised: If an unpatched endpoint shows suspicious activity (phishing hits, browser crashes, EDR alerts), assume possible SYSTEM-level compromise. Isolate the host, rotate local admin passwords (LAPS) and reset cached domain credentials for accounts that logged on there.
- Confirm EDR/AV coverage: Make sure behavioural protection against Win32k/kernel exploits is enabled on all affected builds. Microsoft Defender and mainstream EDRs detect known exploit samples for this CVE.
- Apply the security update: Microsoft fixed this in the April 2021 security update (Patch Tuesday, 13 April 2021). Every later cumulative update also contains the fix. Look up the KB number per build in the MSRC entry for CVE-2021-28310. Verify patch state via WSUS/Intune/SCCM against OS build numbers (
Runbook step 2
Mitigation layers
- IAM / least privilege: Exploitation requires local code execution. Restrict interactive and RDP logon rights on servers via GPO ("Allow log on locally", "Allow log on through Remote Desktop Services"). Enforce admin tiering so domain-admin tokens never land on workstations.
- Application control: Use WDAC or AppLocker to block execution from user-writable paths (
%TEMP%,%APPDATA%,Downloads,C:\Users\Public). This removes the usual staging location for the second-stage exploit loader. - Endpoint hardening: Enable these Defender ASR rules: "Block executable files unless they meet a prevalence, age, or trusted list criterion", "Block Office applications from creating child processes" and "Block credential stealing from LSASS". Turn on LSA Protection (RunAsPPL) and Credential Guard to limit post-escalation damage.
- Shrink initial access: This bug is typically chained after an initial foothold such as a browser exploit, a macro or a phishing attachment. Enforce browser auto-update, block internet-sourced Office macros via GPO and sandbox email attachments.
- Network segmentation: Move unpatched EoS systems into a dedicated VLAN. Allow egress only via proxy and block lateral SMB/RDP/WinRM (445/3389/5985/5986) towards other client segments. This contains a gained SYSTEM foothold.
Runbook step 3
Detection rules
- Sysmon EID 1 / Security 4688 – integrity jump: Look for processes running at System integrity whose parent runs at Medium integrity in user context and is not a service host. KQL:
DeviceProcessEvents | where ProcessIntegrityLevel == "System" and InitiatingProcessIntegrityLevel == "Medium" and InitiatingProcessFileName !in~ ("services.exe","svchost.exe","consent.exe") - SYSTEM shell from user paths: Look for
cmd.exe,powershell.exeorrundll32.exerunning asNT AUTHORITY\SYSTEMwith a parent image under\AppData\,\Temp\or\Downloads\. Sigma shape:ParentImage|contains: ['\AppData\','\Temp\']ANDUser|contains: 'SYSTEM'. - Crash artefacts: Application log Event ID 1000/1001 (Application Error/WER) for
dwm.exe,csrss.exeor graphics/Win32k modules, and unexpected Desktop Window Manager restarts. Failed exploit attempts often leave these crashes behind. - Post-escalation activity: Sysmon EID 10 showing access to
lsass.exe(GrantedAccess0x1010/0x1FFFFF), Security 4697/7045 for new services and 4720/4732 for new local accounts or admin-group additions. Correlate these within minutes of a suspicious process launch. - EDR ancestry chain: Look for the chain browser/Office → rare, unsigned binary → child process as SYSTEM. Correlate it with proxy telemetry (Zeek
http.log/ssl.log) to rare domains.
- Sysmon EID 1 / Security 4688 – integrity jump: Look for processes running at System integrity whose parent runs at Medium integrity in user context and is not a service host. 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), CISA KEV Catalog, CVE.org (MITRE), cvelistv5
Vendors and products
- Microsoft Windows 10 1803
- Microsoft Microsoft Windows 10 1809
- Microsoft Windows 10 1909
- Microsoft Windows 10 2004
- Microsoft Windows 10 20h2
- Microsoft Windows 10 Version 1803
- Microsoft Windows 10 Version 1809
- Microsoft Windows 10 Version 1909
- Microsoft Windows 10 Version 2004
- Microsoft Windows 10 Version 20h2
- Microsoft Windows Server 1909
- Microsoft Microsoft Windows Server 2004
Affected operating systems
| Windows | Microsoft | Windows 10 Version 1803 | — |
| Windows | Microsoft | Windows 10 Version 1809 | — |
| Windows | Microsoft | Windows 10 Version 1909 | — |
| Windows | Microsoft | Windows 10 Version 2004 | — |
| Windows | Microsoft | Windows 10 Version 20H2 | — |
| Windows | Microsoft | Windows Server 2019 | — |
| Windows | Microsoft | Windows Server 2019 (Server Core installation) | — |
| Windows | Microsoft | Windows Server version 2004 | — |
| Windows | Microsoft | Windows Server version 20H2 | — |
| Windows | Microsoft | Windows Server, version 1909 (Server Core installation) | — |
| Windows | microsoft | windows_10_1803 | — |
| Windows | microsoft | windows_10_1809 | — |
| Windows | microsoft | windows_10_1909 | — |
| Windows | microsoft | windows_10_2004 | — |
| Windows | microsoft | windows_10_20h2 | — |
| Windows | microsoft | windows_server_1909 | — |
| Windows | microsoft | windows_server_2004 | — |
| Windows | microsoft | windows_server_2019 | — |
| Windows | microsoft | windows_server_20h2 | — |
Weakness classes (CWE)
CWE-787Out-of-bounds Write
Base
The product writes data past the end, or before the beginning, of the intended buffer.
Entry at MITRE CWE-787
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-2021-28310x_refsource_MISC, Patch, Vendor advisory
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-28310government-resource, US government resource
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<a href="https://intel.neosec.eu/en/cve/CVE-2021-28310"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-28310.svg?lang=en" alt="CVE-2021-28310 – NEOSEC Intel" height="20"></a>