CriticalCVE-2020-17087

    Windows Kernel Local Elevation of Privilege Vulnerability (CVE-2020-17087)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    Windows Kernel Local 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 1903 for 32-bit Systems gte 10.0.0..*
    • Microsoft Windows 10 Version 1903 for x64-based Systems gte 10.0.0..*
    • Microsoft Windows 10 Version 1903 for ARM64-based Systems gte 10.0.0..*
    • Microsoft Windows Server, version 1903 (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 1507 gte 10.0.0..*
    • Microsoft Windows 10 Version 1607 gte 10.0.0..*
    • Microsoft Windows Server 2016 gte 10.0.0..*
    • Microsoft Windows Server 2016 (Server Core installation) gte 10.0.0..*
    • Microsoft Windows 7 gte 6.1.0..*
    • Microsoft Windows 7 Service Pack 1 gte 6.1.0..*
    • Microsoft Windows 8.1 gte 6.3.0..*
    • Microsoft Windows Server 2008 Service Pack 2 gte 6.0.0..*
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) gte 6.0.0..*
    • Microsoft Windows Server 2008 R2 Service Pack 1 gte 6.1.0..*
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) gte 6.0.0..*
    • Microsoft Windows Server 2012 gte 6.2.0..*
    • Microsoft Windows Server 2012 (Server Core installation) gte 6.2.0..*
    • Microsoft Windows Server 2012 R2 gte 6.3.0..*
    • Microsoft Windows Server 2012 R2 (Server Core installation) gte 6.3.0..*

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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

    5.4 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 92.7

    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
    207.73 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
    5.4 %
    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
    92.7
    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 30 January 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-131

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw was used as a zero-day in an exploit chain with a browser renderer bug: the attacker first gains code execution inside the sandbox, then escapes to SYSTEM via the kernel. That is why it is on KEV despite being only locally triggerable with low privileges on its own. Exploitation likelihood is above average, and CISA reports no known ransomware use. For organisations with current patching, residual risk is low. It becomes high priority wherever unsupported Windows 10 builds linger, such as industrial PCs, OT HMIs or embedded systems in critical-infrastructure environments. There the hole is permanently open and must be closed through isolation and migration.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the November 2020 security update (Patch Tuesday, 10 Nov 2020) or any later cumulative update. Look up the build-specific KB number in the Microsoft advisory for CVE-2020-17087. The flaw sits in the kernel cryptography driver cng.sys, so any current cumulative rollup remediates it.
      • Inventory patch status: Use MECM, Intune or WSUS reporting to find every Windows 10 1507, 1607, 1803, 1809, 1903 and 1909 host without updates since November 2020. Verify Server SKUs and newer builds against the advisory.
      • Handle end-of-life builds: Most affected builds (1507, 1803, 1903, 1909) no longer receive updates outside LTSC/LTSB. Migrate these hosts to a supported build. Until then, disconnect them or move them into a quarantine VLAN.
      • Update browsers and document viewers now: CVE-2020-17087 was used as the sandbox escape in a chain with the Chrome/FreeType bug CVE-2020-15999. Bring Chrome, Edge and other Chromium browsers fully up to date to cut off the entry vector.
      • Triage suspected compromise: On endpoints with pre-patch drive-by or phishing alerts, rotate local admin and cached domain credentials. A SYSTEM-level compromise implies LSASS access.

    2. Runbook step 2

      Mitigation layers

      • Configuration/vendor: Microsoft lists no workaround, and cng.sys cannot be disabled without breaking the OS. Compensating controls must therefore target initial access and blast radius.
      • Endpoint hardening: Enable these ASR rules in block mode: "Block credential stealing from LSASS" (9e6c4e1f-7d60-472f-ba1a-a39ef669e4b2), "Block Office applications from creating child processes" and "Block executable content from email client and webmail". Turn on Credential Guard and LSA Protection (RunAsPPL=1).
      • Application control: Use WDAC or AppLocker policies to deny execution from user-writable paths (%TEMP%, %APPDATA%, Downloads). This makes it harder to drop the local exploit binary.
      • IAM/least privilege: Remove local admin rights and roll out Windows LAPS. Prevent Tier-0 accounts from logging on to workstations via the "Deny log on locally" GPO setting.
      • Network segmentation: Place unpatchable legacy hosts in an isolated segment. Block outbound internet there (no browsing). Also block SMB (445/TCP), RDP (3389/TCP) and WinRM (5985/5986/TCP) towards server and domain-controller networks to limit lateral movement after escalation.
      • Browser hardening: Enforce Site Isolation and restrict extensions to a policy allowlist, making renderer exploits harder to use as the first stage.

    3. Runbook step 3

      Detection rules

      • Process ancestry (Sysmon EID 1 / Security 4688 with command line): Look for a browser or Office process spawning a child running as NT AUTHORITY\SYSTEM. KQL: DeviceProcessEvents | where InitiatingProcessFileName in~ ("chrome.exe","msedge.exe","firefox.exe","winword.exe") and AccountSid == "S-1-5-18"
      • Integrity jump: Flag a Low or Medium integrity parent (Sysmon IntegrityLevel) with a direct System child. As a Sigma shape: ParentIntegrityLevel: Low|Medium AND IntegrityLevel: System AND ParentImage not a service host.
      • Failed exploit attempts: Watch for BSODs naming cng.sys as the faulting module (System log Event ID 1001 "BugCheck", Kernel-Power Event ID 41). Repeated crashes on a single endpoint strongly indicate pool-overflow attempts.
      • Post-exploitation: Check for LSASS access (Sysmon EID 10, TargetImage ending in lsass.exe, GrantedAccess 0x1010/0x1410). Also look for new services (System 7045) or local users (Security 4720) shortly after browser activity.
      • EDR telemetry: Handle opens or DeviceIoControl calls to \Device\CNG from browser renderer processes are highly suspicious, where your EDR captures them. Renderers should not talk to this driver directly.

    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 Local Elevation of Privilege Vulnerability

    Source: NVD (NIST), cvelistv5, CISA KEV Catalog

    Vendors and products

    Affected operating systems

    Affected operating systems
    WindowsMicrosoftWindows 10 Version 1507—
    WindowsMicrosoftWindows 10 Version 1607—
    WindowsMicrosoftWindows 10 Version 1803—
    WindowsMicrosoftWindows 10 Version 1809—
    WindowsMicrosoftWindows 10 Version 1903 for 32-bit Systems1903
    WindowsMicrosoftWindows 10 Version 1903 for ARM64-based Systems1903
    WindowsMicrosoftWindows 10 Version 1903 for x64-based Systems1903
    WindowsMicrosoftWindows 10 Version 1909—
    WindowsMicrosoftWindows 10 Version 2004—
    WindowsMicrosoftWindows 10 Version 20H2—
    WindowsMicrosoftWindows 7—
    WindowsMicrosoftWindows 7 Service Pack 1—
    WindowsMicrosoftWindows 8.1—
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 1—
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)—
    WindowsMicrosoftWindows Server 2008 Service Pack 2—
    WindowsMicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)—
    WindowsMicrosoftWindows Server 2012—
    WindowsMicrosoftWindows Server 2012 (Server Core installation)—
    WindowsMicrosoftWindows Server 2012 R2—
    WindowsMicrosoftWindows Server 2012 R2 (Server Core installation)—
    WindowsMicrosoftWindows Server 2016—
    WindowsMicrosoftWindows Server 2016 (Server Core installation)—
    WindowsMicrosoftWindows Server 2019—

    Weakness classes (CWE)

    • CWE-131Incorrect Calculation of Buffer Size

      Base

      The product does not correctly calculate the size to be used when allocating a buffer, which could lead to a buffer overflow.

      Entry at MITRE CWE-131

    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.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    References & sources

    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2020-17087x_refsource_MISC, Patch, Vendor advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2020-17087government-resource, US government resource
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