CVE-2026-21509: listed in the CISA KEV catalog

CVE-2026-21509 · CVSS 7.8 HIGH · EPSS 71% · KEV 2026-01-26

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

Is CVE-2026-21509 exploited?

• Listed in the CISA KEV catalog on 2026-01-26.
• Federal remediation due 2026-02-16.
• Past that date by 226 days.
• EPSS puts exploitation in the next 30 days at 71%.
• Public exploit code: none found in monitored sources.

Which products and versions are affected?

• Microsoft · Microsoft 365 Apps for Enterprise · >= 16.0.1, < https://aka.ms/OfficeSecurityReleases
• Microsoft · Microsoft Office 2016 · >= 16.0.0, < 16.0.5539.1001
• Microsoft · Microsoft Office 2019 · >= 19.0.0, < 16.0.10417.20095
• Microsoft · Microsoft Office LTSC 2021 · >= 16.0.1, < https://aka.ms/OfficeSecurityReleases
• Microsoft · Microsoft Office LTSC 2024 · >= 16.0.0, < https://aka.ms/OfficeSecurityReleases
• microsoft · office long term servicing channel · 2021, 2024

Is there a patch?

No patch identifier recorded here yet.

What PlainSec published about CVE-2026-21509

• APT28 Expands Supply-Chain Attacks Using Fresh Office Zero-Days (2026-04-09)
• APT28 Exploits CVE-2026-21509; Notepad++, OpenClaw, Chrome Targeted (2026-03-03)

Primary sources

• NVD record
• CISA KEV catalog

What this record does not say

• No patch identifier.

KEV and EPSS are re-checked daily. Record last updated 2026-09-27.