Security Digest · Vulnerability record · Known exploited
CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability
Exploited in the wild (CISA KEV). Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
CISA KEV · added 2026-01-26 CVSS 7.8 High EPSS 70.8% · p99
Should you act?
Exploited: patch now if you run it
CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- 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.
CISA’s summary
Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.
Weakness type: CWE-807.
Product: Microsoft Office · Assigned by: microsoft · Published: 2026-01-26
Affected and fixed versions
| Vendor | Product | Affected versions (per 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 scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| microsoft | 3.1 | 7.8 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 |
- Attack vector: local access needed
- Attack complexity: low
- Privileges required: none (no login)
- User interaction: required (a user must click/open something)
- Scope: unchanged
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- 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.
Timeline
- — Added to CISA’s Known Exploited Vulnerabilities catalog
- — CVE record published by microsoft
- — CISA remediation deadline for US federal agencies
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- Microsoft Office Security Feature Bypass Vulnerability msrc.microsoft.com
Vulnerability records
- CVE.org record CVE-2026-21509 cve.org
- NVD entry CVE-2026-21509 nvd.nist.gov
Government
Other references from the CVE record and advisory
- vicarius.io vicarius.io
- vicarius.io vicarius.io
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