CriticalCVE-2021-36934

Windows Elevation of Privilege Vulnerability (CVE-2021-36934)

Published 22.07.2021Last modified 10.08.2026In CISA KEV since 10.02.2022Sources: SingCERT, CVE.org

What is affected, what to do

The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 1809 to version 10.0.17763.2114, Microsoft Windows 10 Version 1909 to version 10.0.18363.1734, Microsoft Windows 10 Version 2004 to version 10.0.19041.1165. Immediate actions, mitigation and detection rules are in the runbook below.

Affected: Microsoft Windows 10 Version 1809 lt 10.0.17763.0..10.0.17763.2114
Microsoft Windows 10 Version 1909 lt 10.0.0..10.0.18363.1734
Microsoft Windows 10 Version 2004 lt 10.0.0..10.0.19041.1165
Microsoft Windows 10 Version 20H2 lt 10.0.0..10.0.19042.1165
Microsoft Windows 10 Version 21H1 lt 10.0.0..10.0.19043.1165
Fixed in: Microsoft Windows 10 Version 1809 10.0.17763.2114
Microsoft Windows 10 Version 1909 10.0.18363.1734
Microsoft Windows 10 Version 2004 10.0.19041.1165
Microsoft Windows 10 Version 20H2 10.0.19042.1165
Microsoft Windows 10 Version 21H1 10.0.19043.1165

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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: SingCERT, CVE.org

67.3 %

EPSS score

High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

Source: FIRST EPSS, percentile 99.30

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: 217.88 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: 67.3 %; 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: 99.30; 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: 22.07.2021
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: 10.02.2022
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

HiveNightmare/SeriousSAM requires local code execution, but once that exists it is trivial and reliable. A standard user reads SAM, SYSTEM and SECURITY from a shadow copy and obtains local admin hashes, LSA secrets and cached domain credentials. Mature public exploits and a very high exploitation likelihood make it a routine step in post-exploitation chains after phishing or other initial access. CISA does not flag known ransomware use, but its value lies in the lateral movement that typically precedes ransomware deployment. Rank it high for estates with large Windows 10 fleets, shared or multi-user hosts, or no LAPS. Treat a host as remediated only once it is patched, its shadow copies are purged and exposed credentials have been rotated.

01
Runbook step 1
Immediate response (0-24 h)
- Check exposure: Run icacls %windir%\system32\config\SAM on every Windows 10 client. If the output contains BUILTIN\Users:(I)(RX), the host is vulnerable. Sweep the fleet via Intune, SCCM or an EDR live query.
- Patch: Deploy cumulative updates to at least the fixed builds: 1809 → 10.0.17763.2114, 1909 → 10.0.18363.1734, 2004 → 10.0.19041.1165, 20H2 → 10.0.19042.1165, 21H1 → 10.0.19043.1165. Look up the exact KB number for each build in the Microsoft advisory.
- Delete shadow copies (mandatory after patching): The update does not fix ACLs inside existing shadow copies. Follow KB5005357: run icacls %windir%\system32\config\*.* /inheritance:e, then vssadmin delete shadows /for=c: /quiet. Verify with vssadmin list shadows and create a fresh restore point if required.
- Rotate credentials: Hashes captured from old shadow copies stay valid until the passwords change. Rotate local admin passwords (via LAPS), service accounts with stored LSA secrets, and the passwords of privileged users whose domain logons are cached (DCC2) on affected clients.
- Triage order: Handle multi-user systems first: RDS/VDI hosts, jump hosts, admin workstations and kiosks. On these, the precondition of "local code execution as a standard user" is easiest to meet.
02
Runbook step 2
Mitigation layers
- Configuration: If patching is delayed, push the KB5005357 steps (re-enable ACL inheritance, delete shadow copies) on their own via a GPO startup script or an Intune remediation script. Confirm the result with a compliance query.
- Endpoint hardening: Use WDAC/AppLocker to block unsigned binaries running from user-writable paths such as %USERPROFILE%, %TEMP% and Downloads. This stops off-the-shelf HiveNightmare-style tools. Add EDR prevention rules for non-system reads of HarddiskVolumeShadowCopy*\Windows\System32\config\.
- IAM/least privilege: Enforce LAPS (Windows LAPS or legacy) so a stolen local admin hash cannot be reused across hosts. Reduce cached logons via GPO ("Interactive logon: Number of previous logons to cache") and keep Tier-0 accounts off workstations.
- Network segmentation: Block workstation-to-workstation SMB (TCP 445) and RPC/WinRM (TCP 135, 5985/5986) with the host firewall. This prevents pass-the-hash using harvested hashes.
- Backup compensation: Deleting shadow copies removes restore points. Confirm that central client backups are current before mass deletion.
03
Runbook step 3
Detection rules
- Sysmon EID 1 / EDR process events: Flag command lines that reference HarddiskVolumeShadowCopy together with config\SAM, config\SYSTEM or config\SECURITY. KQL: DeviceProcessEvents | where ProcessCommandLine has "HarddiskVolumeShadowCopy" and ProcessCommandLine has_any ("SAM","SECURITY","SYSTEM").
- Sysmon EID 11 (FileCreate): Flag files named SAM*, SYSTEM* or SECURITY* written to user profiles by non-SYSTEM processes (default PoC output looks like SAM-2021-07-19). Sigma shape: TargetFilename|re: '\\Users\\.*\\(SAM|SYSTEM|SECURITY)(-\d{4}-\d{2}-\d{2})?$'.
- Windows Event 4663 (object access): Set a SACL on %windir%\System32\config\SAM and alert on read access from processes other than lsass.exe, svchost.exe and backup agents. Also hunt for tool artefacts such as lsadump::sam, secretsdump and HiveNightmare.exe.
- Follow-on activity, Events 4624/4625: Look for logon type 3 with NTLM using the local Administrator account across several hosts in a short window, which indicates pass-the-hash. Correlate with 4720/4732 (local account created or added to Administrators) originating from standard-user sessions.
- Compliance telemetry: Run a daily EDR query for icacls output still showing BUILTIN\Users:(I)(RX) on config\SAM, or for stale entries in vssadmin list shadows. This is not an exploitation signal, but it closes blind spots.

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

An elevation of privilege vulnerability exists because of overly permissive Access Control Lists (ACLs) on multiple system files, including the Security Accounts Manager (SAM) database. An attacker who successfully exploited this vulnerability could run arbitrary code with SYSTEM privileges. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. An attacker must have the ability to execute code on a victim system to exploit this vulnerability. After installing this security update, you must manually delete all shadow copies of system files, including the SAM database, to fully mitigate this vulnerabilty. Simply installing this security update will not fully mitigate this vulnerability. See KB5005357- Delete Volume Shadow Copies.

Source: CSA/SingCERT, CISA KEV Catalog, NVD (NIST), cvelistv5, CVE.org (MITRE)

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
Microsoft	Windows 10 Version 1809	`10.0.17763.0 – < 10.0.17763.2114`	`10.0.17763.2114`
Microsoft	Windows 10 Version 1909	`10.0.0 – < 10.0.18363.1734`	`10.0.18363.1734`
Microsoft	Windows 10 Version 2004	`10.0.0 – < 10.0.19041.1165`	`10.0.19041.1165`
Microsoft	Windows 10 Version 20H2	`10.0.0 – < 10.0.19042.1165`	`10.0.19042.1165`
Microsoft	Windows 10 Version 21H1	`10.0.0 – < 10.0.19043.1165`	`10.0.19043.1165`

Vendors and products

Affected operating systems

Affected operating systems

Windows	microsoft	windows_10_1809	`10.0.17763.2114`
Windows	microsoft	windows_10_1809	`10.0.18363.1734`
Windows	microsoft	windows_10_1809	`10.0.19041.1165`
Windows	microsoft	windows_10_1809	—
Windows	microsoft	windows_10_1909	`10.0.18363.1734`
Windows	microsoft	windows_10_1909	—
Windows	microsoft	windows_10_2004	—
Windows	microsoft	windows_10_20H2	`10.0.19042.1165`
Windows	microsoft	windows_10_20h2	—
Windows	microsoft	windows_10_21H1	`10.0.19043.1165`
Windows	microsoft	windows_10_21h1	—

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 Catalog	CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov	actively exploited	10.02.2022
Packet Storm	HiveNightmare AKA SeriousSAM (opens in a new tab)packetstormsecurity.com	PoC (publicly reported)reported, not verified	—

References & sources

portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-36934x_refsource_MISC, x_transferred
packetstormsecurity.comfiles/164006/HiveNightmare-AKA-SeriousSAM.htmlx_refsource_MISC, x_transferred
Microsoft Security Update GuideWindows Elevation of Privilege VulnerabilityVendor advisory, Patch
kb.cert.orgvuls/id/506989
cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-36934government-resource

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

10.08.2026NVD analysis modifiedNVD

New references:

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36934

Show the original change

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference type	added	—	Microsoft Corporation: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36934 Types: Patch, Vendor Advisory

Source identifier: nvd@nist.gov

10.08.2026Record modifiedMicrosoft

Description added or changed

New references:

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36934

Show 7 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Affected versions (publisher)	changed	Windows 10 Version 1809, Windows 10 Version 1909, Windows 10 Version 21H1 (+2)	Windows 10 Version 1809, Windows 10 Version 1809, Windows 10 Version 1909 (+3)
Description	changed	<p>An elevation of privilege vulnerability exists because of overly permissive Access Control Lists (ACLs) on multiple system files, including the Security Accounts Manager (SAM) database. An attacker who successfully exploited this vulnerability could run arbitrary code with SYSTEM privileges. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.</p> <p>An attacker must have the ability to execute code on a victim system to exploit this vulnerability.</p> <p>After installing this security update, you <em>must</em> manually delete all shadow copies of system files, including the SAM database, to fully mitigate this vulnerabilty. <strong>Simply installing this security update will not fully mitigate this vulnerability.</strong> See <a href="https://support.microsoft.com/topic/1ceaa637-aaa3-4b58-a48b-baf72a2fa9e7">KB5005357- Delete Volume Shadow Copies</a>.</p>	An elevation of privilege vulnerability exists because of overly permissive Access Control Lists (ACLs) on multiple system files, including the Security Accounts Manager (SAM) database. An attacker who successfully exploited this vulnerability could run arbitrary code with SYSTEM privileges. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. An attacker must have the ability to execute code on a victim system to exploit this vulnerability. After installing this security update, you must manually delete all shadow copies of system files, including the SAM database, to fully mitigate this vulnerabilty. Simply installing this security update will not fully mitigate this vulnerability. See KB5005357- Delete Volume Shadow Copies.
Reference	added	—	https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36934
Reference	removed	http://packetstormsecurity.com/files/164006/HiveNightmare-AKA-SeriousSAM.html	—
Reference	removed	https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-36934	—
Reference type	removed	http://packetstormsecurity.com/files/164006/HiveNightmare-AKA-SeriousSAM.html Types: Third Party Advisory, VDB Entry	—
Reference type	removed	https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-36934 Types: Patch, Vendor Advisory	—

Source identifier: secure@microsoft.com

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What is affected, what to do

Metrics

Response and mitigation

Prioritisation rationale

Immediate response (0-24 h)

Mitigation layers

Detection rules

Detection rules

Classification under NIS2 and CRA

Description

Affected products

Vendors and products

Affected operating systems

Public exploit references

References & sources

Changes to this CVE