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ExploitedIn CISA KEV

CVE-2026-21509

Exploitation confirmed. First attested on 2026-01-26 by CISA-ADP, CISA KEV, GCVE · kevintel.

  • Published2026-01-26
  • WeaknessCWE-807

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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

Timeline

dates in UTC
  1. 2026-01-26Added to the CISA KEV catalogCISA KEV
  2. 2026-01-26Listed as exploited by GCVEGCVE
  3. 2026-01-26Exploitation first reportedCISA KEV
  4. 2026-01-2618:16CVE publishedNVD
  5. 2026-01-2713:34SSVC exploitation set to activeCISA-ADP

Observed activity

isMalicious corpus · updated 2026-11-09 15:37 UTC
Indicators tagged with this CVE, per day5 indicators over 7 days
0609
IndicatorSeen
7ccf7e8050c66eed69f35159042d8043032f8afe48ae1f51fce75ce2c51395f213 h ago
b2ba51b4491da8604ff9410d6e004971e3cd9a321390d0258e294ac42010b54614 h ago
fd3f13db41cd5b442fa26ba8bc0e9703ed243b3516374e3ef89be71cbf07436b14 h ago
9f4672c1374034ac4556264f0d4bf96ee242c0b5a9edaa4715b5e61fe8d55cc814 h ago
f36542b449e0b164bf0927d48bd934aa0e66bd2fab483f532cf2010f3fc9d02b4 days ago
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-21509?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-21509?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Microsoft office
BranchAffectedFixed in
2016.x= 2016no fix published
2019.x= 2019no fix published
Microsoft office long term servicing channel
BranchAffectedFixed in
2021.x= 2021no fix published
2024.x= 2024no fix published
Microsoft 365 apps
BranchAffectedFixed in
—all versionsno fix published

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Products declared by the CNA5 products

As microsoft, the CNA that assigned this CVE, declared them in the CVE record; versions as written.

  • Microsoft 365 Apps for Enterprise

    platforms: 32-bit Systems, x64-based Systems

    Affected
    • ≥ 16.0.1, < https://aka.ms/OfficeSecurityReleasesfixed in https://aka.ms/OfficeSecurityReleasescustom
  • Microsoft Office 2016

    platforms: 32-bit Systems, x64-based Systems

    Affected
    • ≥ 16.0.0, < 16.0.5539.1001fixed in 16.0.5539.1001custom
  • Microsoft Office 2019

    platforms: 32-bit Systems, x64-based Systems

    Affected
    • ≥ 19.0.0, < 16.0.10417.20095fixed in 16.0.10417.20095custom
  • Microsoft Office LTSC 2021

    platforms: 32-bit Systems, x64-based Systems

    Affected
    • ≥ 16.0.1, < https://aka.ms/OfficeSecurityReleasesfixed in https://aka.ms/OfficeSecurityReleasescustom
  • Microsoft Office LTSC 2024

    platforms: 32-bit Systems, x64-based Systems

    Affected
    • ≥ 16.0.0, < https://aka.ms/OfficeSecurityReleasesfixed in https://aka.ms/OfficeSecurityReleasescustom

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-04-01

Fix

  1. CISA’s 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.due 2026-02-16

Advisories and patches

Detection

read 2026-11-01 01:02 UTC
Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof of concept
Metasploit modules
none listed
Metasploit Framework

CISA Known Exploited Vulnerability

Date Added
2026-01-26
Patch Due Date
2026-02-16
Ransomware Use
Unknown
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.

Sources

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Published
2026-01-26
Last Modified
2026-06-25
NVD status
Analyzed
GitHub Advisory
GHSA-ch84-h92g-mw93

Frequently asked questions

What is CVE-2026-21509?

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

Is CVE-2026-21509 actively exploited?

Yes. CVE-2026-21509 is on the CISA Known Exploited Vulnerabilities (KEV) catalog, meaning it has been confirmed as actively exploited in the wild. CISA requires federal agencies to patch by 2026-02-16.

What is the CVSS score for CVE-2026-21509?

CVE-2026-21509 has a CVSS v3 base score of 7.8 (HIGH severity), with vector string CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

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Sources for this record