CVE-2026-21509 — Microsoft Office
CISA KEVAs of 2026-02-11, CVE-2026-21509 is a HIGH-severity vulnerability in Microsoft Office, CVSS v3.1 7.8, EPSS 7.4% (91.7th percentile). It is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-01-26), with a US federal remediation deadline of 2026-02-16. Threadlinqs Intelligence links 9 tracked threat campaigns to CVE-2026-21509, most recently “ASEC June 2026 APT Trend Report: Nation-State Actors Pivot to Cloud/OAuth Abuse, MaaS, and Supply-Chain Compromise”.
Last updated: 2026-02-11
What is CVE-2026-21509?
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
The record classifies CVE-2026-21509 under weakness class CWE-807. Its CVSS v3 base vector states that the flaw requires local access to the host, needs no prior authentication, needs a user to take an action first, and has high impact on confidentiality, integrity, availability. 10 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 230 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 7.8 — HIGH
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H - EPSS (FIRST)
- 7.4% probability of exploitation in the next 30 days, higher than 91.7% of all scored CVEs
- CISA KEV
- Listed since 2026-01-26, federal remediation deadline 2026-02-16
- Threadlinqs priority
- 9/10 — CISA KEV-listed, which Threadlinqs floors at 9
- Published
- 2026-01-26, last modified 2026-02-11
Is CVE-2026-21509 being exploited?
CISA added CVE-2026-21509 to the Known Exploited Vulnerabilities catalog on 2026-01-26, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2026-02-16 under BOD 22-01. It currently carries a trending score of 48 in the Threadlinqs vulnerability feed.
Affected products and versions
- Microsoft: 365 Apps, Office 2016, Office 2019, Office Long Term Servicing Channel 2021, Office Long Term Servicing Channel 2024
Showing 5 of 10 recorded product entries.
How to fix CVE-2026-21509
Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509. Because CVE-2026-21509 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2026-02-16. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.
Threat activity tracking CVE-2026-21509
9 tracked threats in the Threadlinqs corpus reference CVE-2026-21509, either in the campaign’s CVE list or as an indicator on the campaign record.
- ASEC June 2026 APT Trend Report: Nation-State Actors Pivot to Cloud/OAuth Abuse, MaaS, and Supply-Chain Compromise — MEDIUM · 2026-07-14
- APT28 PixyNetLoader — Loader Evolution 2024–2026 (Operation Neusploit, CVE-2026-21509) — HIGH · 2026-06-09
- Pawn Storm (APT28) Deploys PRISMEX Malware Suite via CVE-2026-21509 and CVE-2026-21513 Zero-Days Targeting Ukrainian Defense Supply Chain — CRITICAL · 2026-03-25
- APT28 (Fancy Bear) BEARDSHELL Backdoor & COVENANT C2 Framework — Long-term Ukrainian Military Espionage Campaign (CVE-2026-21509) — HIGH · 2026-03-10
- APT28 Microsoft Office Security Feature Bypass (CVE-2026-21509) — CISA KEV, Targeting Ukraine & EU via COREPER-Themed Spear-Phishing — HIGH · 2026-02-23
- APT28 Operation Neusploit: MS Office CVE-2026-21509 Espionage Campaign — CRITICAL · 2026-02-03
- CVE-2026-21509: Russian Hackers Exploit Microsoft Office Vulnerability Against Ukraine — CRITICAL · 2026-02-03
- CVE-2026-21509 - Microsoft Office Security Feature Bypass (CISA KEV) — HIGH · 2026-02-02
- Microsoft Office Zero-Day CVE-2026-21509: OLE Security Bypass Under Active Exploitation — HIGH · 2026-02-02
Sources
Seeded from nvd and not yet processed by the Threadlinqs enrichment pipeline, so blank CVSS, EPSS or KEV fields above mean NOT MEASURED rather than measured-absent.
← all vulnerabilities · Markdown version · Threadlinqs Intelligence