Evidence used
- CISA confirms exploitation in the wild.
- Exploitation requires an existing local or physical foothold with privileges.
- EPSS is 10.17% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2021
CVE-2021-33771High confidence
Microsoft Windows Kernel Privilege Escalation Vulnerability
Microsoft · Windows
7.8HighCVSS 3.1
Patch now
Patch availableCISA confirms exploitation in the wild and lists 2021-11-17 as the remediation due date.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityHigh→Operational priority:Critical, raised one band.upgradedsince 3 Nov 2021
Compensating controls
- Validate the affected product branch and deploy the verified fixed release.
- Restrict local access and enforce least privilege on affected hosts.
- Increase monitoring for the attack path and post-exploitation behaviour described in the report.
Verification
- Confirm that the asset runs Microsoft Windows and falls inside the recorded affected range.
- Verify the installed build against the product-specific fixed version after deployment.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
CISA confirms exploitation in the wild and lists 2021-11-17 as the remediation due date.
Patch available01
What, why and how
Microsoft Windows kernel contains an unspecified vulnerability that allows for privilege escalation.
What
Microsoft Windows kernel contains an unspecified vulnerability that allows for privilege escalation.
Why
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.
How
An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may gain additional privileges.
What
Microsoft Windows kernel contains an unspecified vulnerability that allows for privilege escalation.
Why
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.
How
An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may gain additional privileges.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation Confirmed in the wild
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2021-11-03.
Public PoC / exploit material None recorded
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.
- Likely attack path
- local access → Improper Restriction of Operations within the Bounds of a Memory Buffer → gain additional privileges
- Attack surface
- Local
- Privileges required
- Low: a basic authenticated account is required
- User interaction
- None
- Attack complexity
- Low: no specialised conditions are recorded
- Security boundary
- Unchanged: impact remains within the vulnerable component's security authority
- Weakness
?
CWE means Common Weakness Enumeration: a standard category for the underlying weakness. - CWE-119 ↗
CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer. The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.
- CVSS vector
?
CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity. CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:CCommon Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.
AVLocalAttack vector: The attacker needs local access to the vulnerable system.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRLowPrivileges required: The attacker needs basic user-level privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.- Post-exploitation / living off the land
- The issue can support a local privilege or sandbox boundary transition; normal system utilities may then be available in the gained context.
03
Patch and workaround
Operational remediation based on structured source evidence.
- Status
?
Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change. - Patch available
- Affected
- Windows Server 2016: 10.0.14393.0 < 10.0.14393.4530
- Windows 10 Version 20H2: 10.0.0 < 10.0.19042.1110
- Windows 10 Version 1607: 10.0.14393.0 < 10.0.14393.4530
- Windows 10 Version 21H1: 10.0.0 < 10.0.19043.1110
- Windows Server 2019: 10.0.17763.0 < 10.0.17763.2061
- Windows 10 Version 1507: 10.0.10240.0 < 10.0.10240.19003
- Windows Server version 20H2: 10.0.0 < 10.0.19042.1110
- Windows 10 Version 1909: 10.0.0 < 10.0.18363.1679
- Windows Server 2012 R2: 6.3.9600.0 < 6.3.9600.20069
- Windows 10 Version 2004: 10.0.0 < 10.0.19041.1110
- Windows Server 2019 (Server Core installation): 10.0.17763.0 < 10.0.17763.2061
- Windows Server 2012 R2 (Server Core installation): 6.3.9600.0 < 6.3.9600.20069
- Windows 8.1: 6.3.0 < 6.3.9600.20069
- Windows Server 2016 (Server Core installation): 10.0.14393.0 < 10.0.14393.4530
- Windows Server version 2004: 10.0.0 < 10.0.19041.1110
- Windows 10 Version 1809: 10.0.0 < 10.0.17763.2061, 10.0.17763.0 < 10.0.17763.2061
- Action
- Review the linked authoritative reference and apply the recorded fixed release appropriate to the affected product branch.
- Workaround
- No verified workaround is recorded. Limit untrusted access and use least privilege until authoritative guidance is available.
04
Evidence and provenance
Published 14 Jul 2021 · Last source change 10 Aug 2026, 15:11 UTC · CWE-119 · Improper Restriction of Operations within the Bounds of a Memory Buffer
CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source FIRST · tracked since 2026-08-14
?
The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.European sourceENISA EUVD · EUVD-2021-20448
Product sourceCISA KEV
Remediation sourceCVE/CNA references
CWE sourceCISA KEV
NVD statusNVD enriched
Core structured fields are present and their contributing authorities are shown above.
Material change intelligence
View recent updates ↗What changed after publication
- Affected versionsThe structured affected or fixed version information changed.CNA ↗
- Before
- 10.0.10240.0 < 10.0.10240.19003; 10.0.14393.0 < 10.0.14393.4530; 10.0.17763.0 < 10.0.17763.2061; 10.0.0 < 10.0.17763.2061; 10.0.0 < 10.0.18363.1679; 10.0.0 < 10.0.19041.1110; 10.0.0 < 10.0.19042.1110; 10.0.0 < 10.0.19043.1110 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
- After
- Windows 10 Version 1507: 10.0.10240.0 < 10.0.10240.19003; Windows 10 Version 1607: 10.0.14393.0 < 10.0.14393.4530; Windows 10 Version 1809: 10.0.17763.0 < 10.0.17763.2061, 10.0.0 < 10.0.17763.2061; Windows 10 Version 1909: 10.0.0 < 10.0.18363.1679; Windows 10 Version 2004: 10.0.0 < 10.0.19041.1110; Windows 10 Version 20H2: 10.0.0 < 10.0.19042.1110; Windows 10 Version 21H1: 10.0.0 < 10.0.19043.1110; Windows 8.1: 6.3.0 < 6.3.9600.20069; Windows Server 2012 R2: 6.3.9600.0 < 6.3.9600.20069; Windows Server 2012 R2 (Server Core installation): 6.3.9600.0 < 6.3.9600.20069; Windows Server 2016: 10.0.14393.0 < 10.0.14393.4530; Windows Server 2016 (Server Core installation): 10.0.14393.0 < 10.0.14393.4530; Windows Server 2019: 10.0.17763.0 < 10.0.17763.2061; Windows Server 2019 (Server Core installation): 10.0.17763.0 < 10.0.17763.2061; Windows Server version 2004: 10.0.0 < 10.0.19041.1110; Windows Server version 20H2: 10.0.0 < 10.0.19042.1110 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
CVE published
Added to CISA KEV
Fixed release or patch reference recorded
Technical terms and abbreviations used in this report
- CVE
- Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
- CVSS
- Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
- EPSS
- Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
- CWE
- Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
- CNA
- CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
- CISA ADP
- Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
- NVD
- National Vulnerability Database: NIST's enrichment service for CVE records.
- CERT / CSIRT
- A computer security incident response team that publishes warnings or coordinates incident response.
- PoC
- Proof of concept: public material that demonstrates or helps reproduce exploitation.
- CSAF
- Common Security Advisory Framework: a machine-readable format for security advisories.
- LoTL
- Living off the land: abuse of legitimate tools or system functions during an attack.