CVE-2026-21509

high

Description

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

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509

https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability

https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability

https://therecord.media/russian-hackers-microsoft-office-europe

https://arstechnica.com/security/2026/02/russian-state-hackers-exploit-office-vulnerability-to-infect-computers/

https://www.securityweek.com/russias-apt28-rapidly-weaponizes-newly-patched-office-vulnerability/

https://www.helpnetsecurity.com/2026/02/03/russian-hackers-are-exploiting-recently-patched-microsoft-office-vulnerability-cve-2026-21509/

https://www.darkreading.com/cyberattacks-data-breaches/russian-hackers-weaponize-office-bug-within-days

https://therecord.media/russian-state-hackers-exploit-new-microsoft-flaw

https://thehackernews.com/2026/02/apt28-uses-microsoft-office-cve-2026.html

https://securityaffairs.com/187581/apt/apt28-exploits-microsoft-office-flaw-in-operation-neusploit.html

https://hackread.com/op-neusploit-russia-apt28-microsoft-office-malware/

https://www.theregister.com/2026/02/02/russialinked_apt28_microsoft_office_bug/

https://www.infosecurity-magazine.com/news/fancy-bear-exploits-office-flaw/

https://www.bleepingcomputer.com/news/security/russian-hackers-exploit-recently-patched-microsoft-office-bug-in-attacks/

https://www.malwarebytes.com/blog/news/2026/01/microsoft-office-zero-day-lets-malicious-documents-slip-past-security-checks

https://blog.talosintelligence.com/microsoft-oob-update-january-2026/

https://www.theregister.com/2026/01/27/office_zeroday_exploited_in_the/

https://www.sophos.com/en-us/blog/microsoft-office-vulnerability-cve-2026-21509-in-active-exploitation

https://www.securityweek.com/microsoft-patches-office-zero-day-likely-exploited-in-targeted-attacks/

https://www.infosecurity-magazine.com/news/microsoft-patch-office-zero-day/

https://www.hipaajournal.com/microsoft-issues-emergency-patch-for-actively-exploited-office-vulnerability/

https://www.helpnetsecurity.com/2026/01/27/microsoft-reveals-actively-exploited-office-zero-day-provides-emergency-fix-cve-2026-21509/

https://thehackernews.com/2026/01/microsoft-issues-emergency-patch-for.html

https://securityaffairs.com/187375/security/u-s-cisa-adds-microsoft-office-gnu-inetutils-smartertools-smartermail-and-linux-kernel-flaws-to-its-known-exploited-vulnerabilities-catalog.html

https://www.cisa.gov/news-events/alerts/2026/01/26/cisa-adds-five-known-exploited-vulnerabilities-catalog

https://www.bleepingcomputer.com/news/microsoft/microsoft-patches-actively-exploited-office-zero-day-vulnerability/

https://securityaffairs.com/187349/hacking/emergency-microsoft-update-fixes-in-the-wild-office-zero-day.html

https://krebsonsecurity.com/2026/02/patch-tuesday-february-2026-edition/

https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509

Details

Source: Mitre, NVD

Published: 2026-01-26

Updated: 2026-02-11

Known Exploited Vulnerability (KEV)

Risk Information

CVSS v2

Base Score: 7.2

Vector: CVSS2#AV:L/AC:L/Au:N/C:C/I:C/A:C

Severity: High

CVSS v3

Base Score: 7.8

Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Severity: High

EPSS

EPSS: 0.13006

Vulnerability Watch

Tenable Research has classified this CVE under the following Vulnerability Watch classification, which includes active and historical (inactive) classifications. You can learn more about these classifications on our blog.

Vulnerability of Interest