CVE-2026-21509
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
do firstApply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Can a WAF block it? No. The attack needs local access to the machine, so a WAF is not on its path.
Also consider: patch
Based on the attack vector (local or physical access). We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.
Is CVE-2026-21509 being exploited?
Yes. CVE-2026-21509 appears in CISA's Known Exploited Vulnerabilities catalog, which means exploitation in the wild has been confirmed and reported. Treat it as active rather than theoretical, and patch it ahead of higher-scoring vulnerabilities that carry no exploitation evidence.
How severe is CVE-2026-21509?
CVE-2026-21509 is rated High with a CVSS score of 7.8. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-21509?
Yes. A fix has been recorded for CVE-2026-21509. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-21509 affect?
CVE-2026-21509 affects microsoft:365_apps, microsoft:office, microsoft:office_long_term_servicing_channel. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
- affected>= 16.0.1 and < https://aka.ms/OfficeSecurityReleases
- affected>= 16.0.0 and < 16.0.5539.1001
- affected>= 19.0.0 and < 16.0.10417.20095
- affected>= 16.0.1 and < https://aka.ms/OfficeSecurityReleases
- affected>= 16.0.0 and < https://aka.ms/OfficeSecurityReleases
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.
public exploits
Unvetted proof-of-concept code on GitHub: 13 GitHub proofs of concept
- gavz/CVE-2026-21509-PoC · 42 ★
- kimstars/Ashwesker-CVE-2026-21509 · 12 ★
- suuhm/CVE-2026-21509-handler · 8 ★
- SimoesCTT/CTT-NFS-Vortex-RCE · 3 ★
Listed so you can judge urgency, not as a recommendation to run anything. GitHub “proofs of concept” are unvetted: some are fake or carry malware. Exploit-DB marks the entries its staff verified.
cisa ssvc
- exploitation
- active
- automatable
- no
- technical impact
- total
Assessed by CISA 26 Jan 2026 (SSVC 2.0.3).
timeline
- 26 Jan 2026Added to CISA KEV (exploited in the wild)
- 26 Jan 2026CISA SSVC assessment: exploitation active · same day later
- 26 Jan 2026Published · 1 day later
- 27 Jan 2026First proof-of-concept repository on GitHub (unvetted) · same day later
- 16 Feb 2026CISA remediation due date (US federal agencies) · 20 days later
- 30 Jul 2026CVE record last updated · 165 days (about 6 months) later
- 2 Nov 2026Analysed by vulneraibility.com · 95 days (about 3 months) later
- Patchmsrc.microsoft.com /update-guide/vulnerability/CVE-2026-21509
- Linkcisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509
- Linkvicarius.io /vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-featur
- Linkvicarius.io /vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-featu
sources
- CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
- NVD: CVSS and CPE data
- CISA KEV catalog: exploitation, date added, due date
- CISA Vulnrichment (ADP): SSVC decision points
- FIRST EPSS: exploit probability and percentile
- PoC-in-GitHub: GitHub repositories that claim a proof of concept
- Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.