CVE-2026-21509: Microsoft Office (High, CVSS 7.8)
- CVSS
- 7.8 High (v3.1)
- EPSS
- 70.8% chance of exploitation in the next 30 days (higher than 99.3% of CVEs)
- Exploited in the wild
- Yes, on CISA’s KEV list since Jan 26, 2026
- Public exploit
- Yes, reported in 13 GitHub repositories
- Weakness
- CWE-807
- Vector
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H- Published
- Jan 26, 2026
- CISA fix-by date
- Feb 16, 2026 (for US federal agencies)
What it is
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Description from the National Vulnerability Database.
What to do
CISA: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Check whether you run Microsoft Office, then apply the vendor’s fix or mitigation from the advisories below. Because this flaw is exploited or has public exploit code, treat it as urgent and check for signs of compromise, not just the patch level.
Advisories and fixes
ThreatRoundup coverage
APT28 Turned a Fresh Office Bug on Ukraine Within Days, CERT-UA Says
APT28 Hides Its Implants in PNGs and Rebuilds the Loader Every Few Weeks
Sources: NVD, FIRST EPSS, CISA KEV, and public exploit databases (availability only). ThreatRoundup reports whether public exploit code exists but never links to it: fake proof-of-concepts that infect security researchers are common. This page updates automatically.