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Fencer Intel
high 7.8

CVE-2026-21509 kev in the critical feed json

Microsoft Office Security Feature Bypass Vulnerability

published 9mo ago modified 3mo ago also EUVD-2026-4666

cvss
7.8
version 3.1, CNA
cisa kev
Exploited
added 2026-01-26
epss
70.8%
EPSS 70.8% chance of exploitation in 30 days, percentile 99.4%
public exploits
0
none known

Summary

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

Patch / fix

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.

  • Microsoft Microsoft Office 2016 16.0.5539.1001
  • Microsoft Microsoft Office 2019 16.0.10417.20095

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Timeline

  1. 0-day patched (Project Zero)
  2. CISA SSVC: exploitation active
  3. Added to CISA KEV
  4. Published
  5. CISA patch deadline

Known exploited

Added
9mo ago
Patch by
2026-02-16 (8mo ago)
Ransomware
Not known
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, product
Microsoft, Office
CISA name
Microsoft Office Security Feature Bypass Vulnerability

Zero-day

Detected in real attacks while still unknown to the vendor and the public, per Google Project Zero's 0day In-the-Wild tracker.

Patched
2026-01-26
Details
project zero tracker

CISA SSVC

Exploitation
Active, exploited in the wild
Automatable
no
Technical impact
total
Assessed
9mo ago

Exploit prediction

70.82% chance of exploitation in the next 30 days, higher than 99.4% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
CNA 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
CNA 3.1 7.8 CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Affected products

  • Microsoft Microsoft 365 Apps for Enterprise
  • Microsoft Microsoft Office 2016
  • Microsoft Microsoft Office 2019
  • Microsoft Microsoft Office LTSC 2021
  • Microsoft Microsoft Office LTSC 2024

Classification

Weakness
  • CWE-807 Reliance on Untrusted Inputs in a Security Decision

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.