CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability
Advisory generated from the CVE record, CISA, Exploit Observer and VEDAS: what it is, whether it is exploited, who is affected, and how to fix and detect it.
Generated by VEDAS. Scores as of 2026-11-01 19:19 UTC. Rebuilt every few hours from cve-scores, CISA KEV and vedas-signatures. How this works and its limits.
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 quoted from the CVE record (assigned by microsoft, published 2026-01-26).
Weakness: CWE-807: Reliance on Untrusted Inputs in a Security Decision
How it can be exploited
Exploitable with local access with low attack complexity needing no privileges and a user must take an action. Impact: confidentiality high, integrity high, availability high. CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C
CISA assessment (SSVC): CISA assesses exploitation as active. Automatable: no. Technical impact: total.
- On CISA KEV since
- 2026-01-26
- US federal deadline
- 2026-02-16
- Ransomware use
- Not known
Public exploit evidence (Exploit Observer)
Also known as: BDU:2026-00828, CNNVD-202601-4359, CNVD-2026-16161, EUVD-2026-4666, GHSA-ch84-h92g-mw93
VEDAS cluster: VEDAS:GARNETGHAST
Exploit Observer links 15 pieces of public exploit code or detection modules that name this CVE or one of its aliases.
Exploit code and modules (15)
Code that names this CVE. A repository can be a working exploit, a scanner, a fake or malware; read it before running anything.
- Code repository: https://github.com/ashwesker/ashwesker-cve-2026-21509
- Code repository: https://github.com/damedode/cve-2026-21509-poc
- Code repository: https://github.com/decalage2/detect_cve-2026-21509
- Code repository: https://github.com/gavz/cve-2026-21509-poc
- Code repository: https://github.com/incursiohack/cve-2026-21509-poc
- Code repository: https://github.com/kaizensecurity/cve-2026-21509
- Code repository: https://github.com/kimstars/ashwesker-cve-2026-21509
- Code repository: https://github.com/ksk-itdk/ksk-itdk-cve-2026-21509-mitigation
- Code repository: https://github.com/nicole2ilodl/cve-2026-21509-poc
- Code repository: https://github.com/pinpin1140/cve-2026-21509
- Code repository: https://github.com/planetoid/cve-2026-21509-mitigation
- Code repository: https://github.com/razureink/cve-2026-21509-office_security_bypass_reproduction
- and 3 more
Research and write-ups (8)
- Research or write-up: https://blog.synapticsystems.de/apt28-geofencing-as-a-targeting-signal-cve-2026-21509
- Research or write-up: https://decalage.info/CVE-2026-21509
- Research or write-up: https://ptsecurity.com/research/trending-vulnerabilities/CVE-2026-21509
- Research or write-up: https://socprime.com/blog/detect-uac-0001-attacks-exploiting-cve-2026-21509
- Research or write-up: https://socprime.com/blog/latest-threats/cve-2026-21509-vulnerability
- Research or write-up: https://www.fdaytalk.com/microsoft-office-zero-day-cve-2026-21509
- Research or write-up: https://www.picussecurity.com/resource/blog/cve-2026-21509-apt28-exploits-microsoft-office-zero-day-vulnerability
- Research or write-up: https://www.zscaler.com/blogs/security-research/apt28-leverages-cve-2026-21509-operation-neusploit
Vulnerability databases and vendor advisories (11)
Catalogue entries, not exploit evidence.
- Vulnerability database or vendor advisory: https://euvd.enisa.europa.eu/enisa/EUVD-2026-4666
- Vulnerability database or vendor advisory: https://github.com/advisories/GHSA-ch84-h92g-mw93
- Vulnerability database or vendor advisory: https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2026-21509
- Vulnerability database or vendor advisory: https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2026-21509)
- Vulnerability database or vendor advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-21509
- Vulnerability database or vendor advisory: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509
- Vulnerability database or vendor advisory: https://www.cnnvd.org.cn/home/globalSearch?keyword=CNNVD-202601-4359
- Vulnerability database or vendor advisory: https://www.cnvd.org.cn/flaw/show/CNVD-2026-16161
- Vulnerability database or vendor advisory: https://www.tenable.com/cve/CVE-2026-21509
- Vulnerability database or vendor advisory: https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability
- Vulnerability database or vendor advisory: https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability
130 more references returned by Exploit Observer did not name this CVE or an alias, so they are left out.
VEDAS data as of 2026-11-01T17:12 UTC; fetched 2026-11-02.
Live scores
- EPSS
- 0.708
very likely to be attacked soon - VEDAS
- 0.977
strong exploit evidence - On CISA KEV
- Yes, since 2026-01-26
- Detection rule
- Out of scope (local)
Thin horizontal line: the “likely” (EPSS 0.10) or “strong” (VEDAS 0.90) threshold. Shaded “no data” areas are periods when cve-scores did not publish.
Who is affected
| Product | Affected | Not affected |
|---|---|---|
| Microsoft Microsoft 365 Apps for Enterprise | from 16.0.1 before https://aka.ms/OfficeSecurityReleases | Not stated |
| Microsoft Microsoft Office 2016 | from 16.0.0 before 16.0.5539.1001 | Not stated |
| Microsoft Microsoft Office 2019 | from 19.0.0 before 16.0.10417.20095 | Not stated |
| Microsoft Microsoft Office LTSC 2021 | from 16.0.1 before https://aka.ms/OfficeSecurityReleases | Not stated |
| Microsoft Microsoft Office LTSC 2024 | from 16.0.0 before https://aka.ms/OfficeSecurityReleases | Not stated |
Version ranges exactly as the CVE record states them. “Before X” means X is the first fixed version.
Exploit Observer also maps it to: microsoft 365_apps, microsoft office, microsoft office_long_term_servicing_channel, microsoft_corp microsoft_365_apps_for_enterprise, microsoft_corp microsoft_office_2016, microsoft_corp microsoft_office_2019, microsoft_corp microsoft_office_ltsc_2021, microsoft_corp microsoft_office_ltsc_2024.
How to fix it
Fixed versions stated in the CVE record: https://aka.ms/OfficeSecurityReleases, 16.0.5539.1001, 16.0.10417.20095.
CISA required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Vendor guidance:
- Microsoft Office Security Feature Bypass Vulnerability (vendor-advisory, patch)
How to detect it
This bug needs local access to exploit. VEDAS signatures cover remotely exploitable bugs for now, so there is no network rule or remote check for it. Patch it and watch your endpoint telemetry for unusual local activity.
Advisories on this CVE
References
Vendor advisories and patches
- Microsoft Office Security Feature Bypass Vulnerability (vendor-advisory, patch)
Government sources
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509 (government-resource)
Other references
- https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability
- https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability
Other databases
Where this comes from
This advisory is generated automatically by VEDAS from: CVE.org record, CISA KEV, cve-scores (EPSS and VEDAS), Exploit Observer. Text in quotes is reproduced from the source; everything else is a fixed template filled with source fields. Nothing here is written by a language model. Check the vendor advisory before acting, and read how VEDAS and Exploit Observer work, and their limits.