---
id: CVE-2026-21509
title: >-
  Reliance on untrusted inputs in a security decision in Microsoft Office allows
  an unauthorized attacker to bypass a security feature locally.
summary: >-
  Reliance on untrusted inputs in a security decision in Microsoft Office allows
  an unauthorized attacker to bypass a security feature locally.
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H'
cwe:
  - CWE-807
vendor: microsoft
product: 365_apps
affected:
  - 365_apps
  - office = 2016
  - office = 2019
  - office_long_term_servicing_channel = 2021
  - office_long_term_servicing_channel = 2024
published: '2026-01-26'
updated: '2026-06-25'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-21509'
references:
  - url: 'https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509'
    label: secure@microsoft.com
  - url: >-
      https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: >-
      https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: >-
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - kev
  - in-the-wild
  - exploit-available
epss: 0.72872
epssPercentile: 0.99434
kev: true
kevDateAdded: '2026-01-26'
kevDueDate: '2026-02-16'
kevRansomware: false
exploited: true
zeroDay: true
ingestedAt: '2026-06-29T13:24:34.633Z'
exploits:
  github: 13
  githubRepos:
    - 'https://github.com/kimstars/Ashwesker-CVE-2026-21509'
    - 'https://github.com/gavz/CVE-2026-21509-PoC'
    - 'https://github.com/ksk-itdk/KSK-ITDK-CVE-2026-21509-Mitigation'
  checkedAt: '2026-09-25T08:20:52.886Z'
exploitAvailable: true
---

## Overview

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

## Affected

- `365_apps`
- `office = 2016`
- `office = 2019`
- `office_long_term_servicing_channel = 2021`
- `office_long_term_servicing_channel = 2024`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.