Windows Kernel Information Disclosure Vulnerability (CVE-2021-31955)
Published In CISA KEV since Sources: CVE.org
- Actively exploited
- Source: ANSSI / CERT-FR How we classify
What is affected, what to do
Windows Kernel Information Disclosure Vulnerability
The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 1809 to version 10.0.17763.1999, Microsoft Windows Server 2019 to version 10.0.17763.1999, Microsoft Windows Server 2019 (Server Core installation) to version 10.0.17763.1999. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Windows 10 Version 1809
lt 10.0.0..10.0.17763.1999 - Microsoft Windows Server 2019
lt 10.0.0..10.0.17763.1999 - Microsoft Windows Server 2019 (Server Core installation)
lt 10.0.0..10.0.17763.1999 - 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 1809
- Fixed in
- Microsoft Windows 10 Version 1809
10.0.17763.1999 - Microsoft Windows Server 2019
10.0.17763.1999 - Microsoft Windows Server 2019 (Server Core installation)
10.0.17763.1999 - 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 1809
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Metrics
5.5
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
5.5
CVSS v3
Severity Medium on the scale from 0 to 10.
Source: CVE.org
81.1 %
EPSS score
High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
Source: FIRST EPSS, percentile 99.63
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- 206.91 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- 5.5 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- 5.5
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- 81.1 %
- High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
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- 99.63
- 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: partial
- Source: CISA Vulnrichment, as of 4 February 2025
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-497
Response and mitigation
Why act now?
Prioritisation rationale
On its own, this is a local, low-privilege, confidentiality-only bug. It is on KEV because it has served as a KASLR bypass in real exploit chains: leaked kernel addresses make a separate kernel privilege escalation (e.g. CVE-2021-31956 from the same Patch Tuesday) reliable, typically after a browser sandbox escape. Exploitation likelihood is very high, though CISA does not flag known ransomware use. In a well-maintained estate this is a medium-priority compliance gap. Treat any host still below the June 2021 builds as high priority, especially browsing clients, RDS servers and end-of-life releases, because such hosts almost certainly carry other, more severe unpatched vulnerabilities as well.
Runbook step 1
Immediate response (0-24 h)
- Verify build levels and deploy the June 2021 security update: Minimum fixed builds are: Windows 10 1809 / Server 2019 (incl. Server Core) ≥ 10.0.17763.1999, Windows 10 1909 ≥ 10.0.18363.1621, Windows 10 2004 / Server 2004 ≥ 10.0.19041.1052, 20H2 ≥ 10.0.19042.1052, 21H1 ≥ 10.0.19043.1052. Look up the KB number per build in the Microsoft Security Update Guide. Any later cumulative update includes the fix.
- Hunt for non-compliant builds: Query MDE, Intune or SCCM, e.g.
DeviceTvmSoftwareVulnerabilities | where CveId == "CVE-2021-31955" | summarize count() by DeviceName, OSVersion. Anything still below these builds signals broken patch hygiene and should be treated as high-risk. - Prioritise out-of-support releases: Windows 10 1809 (non-LTSC), 1909, 2004, 20H2 and 21H1, as well as Server 2004, no longer receive security fixes. Migrate them to a supported release, or isolate them until you can.
- Check browser and Office patching in the same pass: Exploitation requires local code execution, which usually starts with a browser or document exploit. Make sure Chrome, Edge and Office are current on every affected host.
- Triage suspicious hosts: If a host is on a stale build and shows suspicious process activity (see Detection), isolate it via EDR and capture memory before rebooting. Kernel exploit artefacts rarely survive a restart.
Runbook step 2
Mitigation layers
- No vendor workaround: Microsoft lists no configuration-based mitigation. Only the update fully fixes the issue; the controls below only reduce exploitability.
- Application control (endpoint hardening): Enforce WDAC or AppLocker and block unsigned binaries from
%TEMP%,%APPDATA%,Downloadsand browser cache paths. This stops second-stage exploit loaders from running after a browser compromise. - ASR rules: Set the following rules to block mode: "Block executable files from running unless they meet a prevalence, age, or trusted list criterion", "Block Office applications from creating child processes" and "Block credential stealing from LSASS". The last one limits the blast radius after privilege escalation.
- IAM / least privilege: Restrict interactive logons on servers, especially Server 2019, to admin jump hosts. On terminal and RDS servers, every logged-on user is a potential attacker.
- Network segmentation for legacy hosts: Move unpatchable systems into a dedicated VLAN with no web browsing and no mail client. Allow egress only via an authenticating proxy with an allowlist. Without initial access, this bug cannot be reached.
- Strengthen browser sandboxing: Enforce Site Isolation via Edge/Chrome policy. Enable hardware-enforced stack protection (including kernel-mode) on capable hardware, and turn on HVCI/Memory Integrity where supported.
Runbook step 3
Detection rules
- Unusual browser child processes (Sysmon EID 1 / Security 4688): Sigma shape:
ParentImage|endswith: ['\chrome.exe','\msedge.exe','\firefox.exe'] AND Image|endswith: ['\cmd.exe','\powershell.exe','\rundll32.exe','\regsvr32.exe']. A sandbox escape is usually the first link in the chain. - SYSTEM-token processes with an unexpected parent: KQL:
DeviceProcessEvents | where AccountSid == "S-1-5-18" and InitiatingProcessAccountSid != "S-1-5-18" and InitiatingProcessFileName !in~ ("services.exe","svchost.exe","wininit.exe"). This points to token theft following a kernel leak and privilege escalation. - Event ID 4672 (special privileges) on unexpected processes: Flag SeDebugPrivilege or SeTcbPrivilege assigned to user accounts or processes outside the admin baseline, and correlate with 4688 events from the same logon ID.
- EDR kernel-exploit telemetry: Treat Defender alerts such as "Exploit:Win32/…", "Suspicious token modification" or "Possible exploitation of a kernel vulnerability" as P1. Unsigned DLLs or EXEs dropped in user directories just before the alert (Sysmon EID 11/7) are strong corroborating evidence.
- Caveat: The kernel memory read itself (e.g. via NtQuerySystemInformation information classes) is effectively invisible to standard logging. Build detections around the stage before it (initial access) and the stage after it (privilege escalation).
- Unusual browser child processes (Sysmon EID 1 / Security 4688): Sigma shape:
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
Windows Kernel Information Disclosure 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 Microsoft Windows Server 2004
- Microsoft Microsoft Windows Server 2019
Affected operating systems
| Windows | Microsoft | Windows 10 Version 1809 | 10.0.17763.1999 |
| 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 2019 | 10.0.17763.1999 |
| Windows | Microsoft | Windows Server 2019 (Server Core installation) | 10.0.17763.1999 |
| 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 | 10.0.17763.1999 |
| Windows | microsoft | windows_10_1809 | 10.0.18363.1621 |
| Windows | microsoft | windows_10_1809 | 10.0.19041.1052 |
| Windows | microsoft | windows_10_1809 | — |
| 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_2019 | 10.0.17763.1999 |
| Windows | microsoft | windows_server_2019 | — |
Weakness classes (CWE)
CWE-497Exposure of Sensitive System Information to an Unauthorized Control Sphere
Base
The product does not properly prevent sensitive system-level information from being accessed by unauthorized actors who do not have the same level of access to the underlying system as the product does.
Entry at MITRE CWE-497
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-2021-31955x_refsource_MISC, Patch, Vendor advisory
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31955government-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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