Exploited vulnerability · CISA KEV

Is CVE-2026-21509 exploited? Yes: in CISA KEV since 26 Jan 2026

Gociux · Microsoft Office · data: CISA KEV, CVE record, FIRST EPSS

Yes. CVE-2026-21509, Microsoft Office Security Feature Bypass Vulnerability, is being exploited in the wild. CISA added it to its Known Exploited Vulnerabilities catalog on 26 Jan 2026, the day it was published. It affects Microsoft Office. If you run it, CISA's required action is: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

What it is

Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.

What to do

1. Check whether you run it. Look for Office in your asset inventory, including appliances and cloud images. Stack check lists every exploited Microsoft CVE at once.
2. Apply the fix. Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
3. If you can't patch today, take the affected service off the internet or restrict who can reach it, and check its logs for signs of compromise since 26 Jan 2026.
4. Get told next time. Email alerts when CISA adds a CVE for Microsoft.

Other exploited CVEs in Office

• CVE-2026-124194: Microsoft Office Outlook Missing User Interaction for Potentially Unsafe Action Vulnerability (10 Nov 2026)
• CVE-2009-0238: Microsoft Office Remote Code Execution (14 Apr 2026)
• CVE-2026-21514: Microsoft Office Word Reliance on Untrusted Inputs in a Security Decision Vulnerability (10 Feb 2026)
• CVE-2009-0556: Microsoft Office PowerPoint Code Injection Vulnerability (7 Jan 2026)
• CVE-2007-0671: Microsoft Office Excel Remote Code Execution Vulnerability (12 Aug 2025)
• CVE-2023-23397: Microsoft Office Outlook Privilege Escalation Vulnerability (14 Mar 2023)

References

• nvd.nist.gov/vuln/detail/CVE-2026-21509

Sources: CISA KEV (exploitation, ransomware use, required action, due date), the CVE record (publication date, CVSS, CNA), NVD and FIRST EPSS (live score). More: all KEV additions in January 2026 · Microsoft scorecard · act-now radar.