CVE-2026-21509 | Microsoft Office Security Feature Bypass Vulnerability

Exp

Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.

Published: 2026-01-26 Last update: 2026-06-25 Assigner: [email protected] Source: [email protected]

NVD Status：Analyzed，CVE State: published

View at NVD, CVE.org, EUVD

Threat Intelligence & Risk Assessment for CVE-2026-21509

KEV EPSS CVSS CWE Affected

Conclusion & alert: CVE-2026-21509 is rated Critical Active Threat (96/100): CVSS High severity, with high exploitation likelihood (EPSS 72.15%, 99th percentile). Core evidence: CISA KEV confirms active exploitation (added 2026-01-26) affecting Microsoft / Office. a weakness (CWE-807) Unauthenticated remote administrative access may be possible. EPSS rose +64.21% over the last day, indicating growing attacker interest. Mandatory action: The CISA remediation deadline has passed—treat as an emergency patch priority.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

CISA KEV Record for CVE-2026-21509

Name: Microsoft Office Security Feature Bypass Vulnerability · CISA KEV detail

Exploit added: 2026-01-26

Action due: 2026-02-16

Required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Exploit prediction scoring system (EPSS) score for CVE-2026-21509

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	7.94%	72.15%	+64.21%
2	2026-06-11	12.05%	7.94%	-4.11%
3	2026-06-03	—	12.05%	—

Full EPSS history (26 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-21509

CVSS metrics for this CVE.

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
7.8	3.1	HIGH	`CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H` Click to expand Attack vector (AV:L) They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:N) No account or special rights needed—anonymous or random user is enough. User interaction (UI:R) A real person has to do something—click, install, enable—otherwise it doesn’t land. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:H) Serious risk that confidential data gets exposed in a big way. Integrity (I:H) They could widely tamper with or forge data—trust in the data is badly hurt. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	1.8	5.9	[email protected]

Weakness enumeration for CVE-2026-21509

CWE-807 MITRE ↗

Affected software / configurations for CVE-2026-21509

Vendor	Product	Version	Raw CPE
microsoft	365_apps	—	cpe:2.3:a:microsoft:365_apps:-::::enterprise::x64:*
microsoft	365_apps	—	cpe:2.3:a:microsoft:365_apps:-::::enterprise::x86:*
microsoft	office	2016	cpe:2.3:a:microsoft:office:2016::::::x64:
microsoft	office	2016	cpe:2.3:a:microsoft:office:2016::::::x86:
microsoft	office	2019	cpe:2.3:a:microsoft:office:2019::::::x64:
microsoft	office	2019	cpe:2.3:a:microsoft:office:2019::::::x86:
microsoft	office_long_term_servicing_channel	2021	cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:::::-:x64:*
microsoft	office_long_term_servicing_channel	2021	cpe:2.3:a:microsoft:office_long_term_servicing_channel:2021:::::-:x86:*
microsoft	office_long_term_servicing_channel	2024	cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:::::-:x64:*
microsoft	office_long_term_servicing_channel	2024	cpe:2.3:a:microsoft:office_long_term_servicing_channel:2024:::::-:x86:*

References for CVE-2026-21509

URL	Tags
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509	Vendor Advisory
https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability	Third Party Advisory
https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability	Mitigation Third Party Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509	US Government Resource

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