---
title: "CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability"
url: https://erroneo.us/security/cve/CVE-2026-21509/
description: "CVE-2026-21509 (Microsoft Office): Exploited in the wild (CISA KEV). Reliance on untrusted inputs in a security decision in Microsoft Office allows an…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-01-26
date_modified: 2026-07-30
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-21509"
kev: true
cvss: 7.8
epss: 0.70795
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability

**Signal:** Exploited: patch now if you run it. CISA has confirmed exploitation in the wild. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Yes, if you run Microsoft Office. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-01-26, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-02-16 ; treat that as the outer limit, not the target.
- EPSS 70.8% , percentile 99: FIRST’s model estimates a 70.8% chance of exploitation activity in the next 30 days, higher than 99% of all scored CVEs (very high). EPSS is a probability, not a verdict.
- Needs local access needed. Lower urgency for most teams unless an attacker could already run code on the machine.

## What it is

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

## Affected products (CVE record)

- 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

## Severity

- microsoft CVSS 3.1: 7.8 `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`

## Exploitation

- CISA KEV: added 2026-01-26; federal due date 2026-02-16. Known ransomware use: Unknown.
- 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.
- CISA SSVC assessment: exploitation active , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 70.80% probability of exploitation activity in the next 30 days; percentile 99.

## Sources

- [Microsoft Office Security Feature Bypass Vulnerability](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509)
- [CVE.org record CVE-2026-21509](https://www.cve.org/CVERecord?id=CVE-2026-21509)
- [NVD entry CVE-2026-21509](https://nvd.nist.gov/vuln/detail/CVE-2026-21509)
- [CISA Known Exploited Vulnerabilities catalog entry](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509)
- [vicarius.io](https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability)
- [vicarius.io](https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability)

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Source page: https://erroneo.us/security/cve/CVE-2026-21509/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt