CVE-2021-33771: Microsoft Windows Kernel Privilege Escalation Vulnerability

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 3 Nov 2021, 112 days after the CVE record was published.

How likely is exploitation in the next 30 days?

10.2%

EPSS score, percentile 95.6 of all scored records

How severe?

7.8 High

CVSS v3.1 base score, given by the CNA

What should I do?

Required action (CISA), due 17 Nov 2021: Apply updates per vendor instructions.

Advisories and patches (1)

Which versions are affected?

Affected: 10.0.10240.0 up to, not including, 10.0.10240.19003; 10.0.14393.0 up to, not including, 10.0.14393.4530; 10.0.17763.0 up to, not including, 10.0.17763.2061; 10.0.0 up to, not including, 10.0.17763.2061; 10.0.0 up to, not including, 10.0.18363.1679; 10.0.0 up to, not including, 10.0.19041.1110; 10.0.0 up to, not including, 10.0.19042.1110; 10.0.0 up to, not including, 10.0.19043.1110; 6.3.0 up to, not including, 6.3.9600.20069; 6.3.9600.0 up to, not including, 6.3.9600.20069; 6.3.9600.0 up to, not including, 6.3.9600.20069; 10.0.14393.0 up to, not including, 10.0.14393.4530; 10.0.14393.0 up to, not including, 10.0.14393.4530; 10.0.17763.0 up to, not including, 10.0.17763.2061; 10.0.17763.0 up to, not including, 10.0.17763.2061; 10.0.0 up to, not including, 10.0.19041.1110; 10.0.0 up to, not including, 10.0.19042.1110

Versions by product

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

CISA due date for this action: 17 November 2021.

Advisories and patches

• Windows Kernel Elevation of Privilege Vulnerability (Vendor advisory, Patch)

Linked from CISA's catalog entry

• https://nvd.nist.gov/vuln/detail/CVE-2021-33771

Affected products and versions

As listed in the CVE record.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

Microsoft Windows kernel contains an unspecified vulnerability that allows for privilege escalation.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

• CISA added CVE-2021-33771 to its Known Exploited Vulnerabilities (KEV) catalog on 3 Nov 2021.
• CISA's ransomware flag for this entry is "Unknown": no evidence of ransomware use in the catalog, which is not proof of none.
• The due date CISA sets for the required action is 17 Nov 2021.
• The CVE record was published on 14 Jul 2021, 112 days before it was added to KEV.
• The CVE record carries a CVSS v3.1 base score of 7.8 (High), given by its CNA.
• FIRST's EPSS model puts the probability of exploitation in the next 30 days at 10.2% (percentile 95.6), as of 28 Oct 2026.
• CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact total.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

Description

Windows Kernel Elevation of Privilege Vulnerability

Windows Kernel Elevation of Privilege Vulnerability

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Weakness

• CWE-119

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

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