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 275 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)
- 70.7% probability of exploitation in the next 30 days, higher than 99.3% 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 2026-07-30
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. Weaponised exploit code for CVE-2026-21509 is publicly available. 11 public proof-of-concept repositories are tracked for this identifier. It currently carries a trending score of 58 in the Threadlinqs vulnerability feed.
- gavz/CVE-2026-21509-PoC (github)
- kimstars/Ashwesker-CVE-2026-21509 (github)
- suuhm/CVE-2026-21509-handler (github)
- SimoesCTT/CTT-NFS-Vortex-RCE (github)
- SimoesCTT/CTT-MICROSOFT-OFFICE-OLE-MANIFOLD-BYPASS-CVE-2026-21509 (github)
- decalage2/detect_CVE-2026-21509 (github)
- ksk-itdk/KSK-ITDK-CVE-2026-21509-Mitigation (github)
- SimoesCTT/SCTT-2026-33-0007-The-OLE-Vortex-Laminar-Bypass- (github)
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
13 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.
- APT28 PixyNetLoader Campaign Update: PNG-steganography loader delivering Covenant Grunt against Ukrainian military logistics — HIGH · 2026-10-24
- Weekly threat digest (week ending 2026-06-07): PAN-OS GlobalProtect CVE-2026-0257 exploitation, React2Shell CVE-2025-55182 abuse by APT-C-26, and npm/state-actor campaigns — HIGH · 2026-10-18
- 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 (Fancy Bear) Deploys BadPaw Loader and MeowMeow Backdoor Targeting Ukrainian Critical Infrastructure — HIGH · 2026-03-06
- Microsoft MSHTML Remote Code Execution Zero-Day (CVE-2026-21513) — CRITICAL · 2026-03-04
- 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
Enriched from CVE.org, FIRST EPSS, CISA KEV, public proof-of-concept repositories. Last verified by Threadlinqs on .