CVE-2026-21509highunder attackCWE-807

CVE-2026-21509: high-severity vulnerability in Microsoft 365 Apps for Enterprise

Microsoft Office Security Feature Bypass Vulnerability

Published · Updated

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E-mail alert when it lands in KEV, gets a public PoC or its score goes up.

93Vexday Risk Score

Prioritize patching. It under exploitation confirmed by CISA, has an official Brazilian government alert and has a public proof of concept.

ssvc Actcvss 7.8epss 71%
from disclosure to weapon1 day
Published on NVDJan 26
1st PoC+1d
CISA KEVJan 26
Official alert in Brazil

Brazil's federal government published an alert, recommendation or bulletin that cites this vulnerability.

Source: CTIR Gov (GSI) and CISC Gov.br, under the CC BY-ND 3.0 license.
Action required by CISAfederal deadline: 2026-02-16

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

In short

Microsoft Office accepts untrusted data when making security decisions, allowing someone on your computer to bypass built-in security protections. This could let malicious content run when it should be blocked.

Technical detail

CWE-807 vulnerability in Microsoft Office's security decision logic trusts user-controllable input to determine security policy enforcement. Local attacker can manipulate inputs to circumvent access controls or content restrictions; requires local access but bypasses intended security boundaries.

Summary generated and translated by AI from the official description.

The full analysis of this CVE is available in Portuguese →

Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
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
Affected products
Microsoft · Microsoft 365 Apps for EnterpriseMicrosoft · Microsoft Office 2016Microsoft · Microsoft Office 2019Microsoft · Microsoft Office LTSC 2021Microsoft · Microsoft Office LTSC 2024
⚠ Public resources, to assess the exposure of systems you control or are authorized to test. Test only with authorization.
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