Report · SecurityWeekAbout CVE-2026-21509Published 27 Jan 2026

Microsoft Patches Office Zero-Day Likely Exploited in Targeted Attacks

Source SecurityWeek

Authoritative record:CVE-2026-21509· High 7.8

Exploited in the wild · KEV due 16 Feb 2026 : upgrade to 16.0.5539.1001 now.

Data as of 27 Jan 2026, 20:30 UTC

CVSS base score

7.8High

How severe the flaw is on the 0–10 CVSS scale.

KEV deadline: 16 Feb 2026 US federal agencies must patch by this date; a sensible deadline for everyone else.
Patch: Available – fixed in 16.0.5539.1001, 16.0.10417.20095

Exploitation evidence

t = 0 is the CVE’s publication on cve.org

Disclosed: evidence found.
CVE published 26 Jan 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: evidence found.
Listed in CISA KEV the same day as disclosure
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

CVE ID reserved
by microsoft
2025-12-3018:10Z, 27 days before disclosure
CVE published
on cve.org
2026-01-2617:06Z, t = 0
In Threat Radar
record ingested
2026-01-2617:21Z, 15 minutes after disclosure
Added to CISA KEV (opens in a new tab)
exploitation confirmed
2026-01-26, the same day as disclosure
KEV remediation due
US federal deadline
2026-02-16, 21 days after disclosure

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CISA KEV (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-25.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade Microsoft Office to 16.0.5539.1001 (2016) or 16.0.10417.20095 (2019).

Affected software

Product	Affected	Fixed in
MicrosoftMicrosoft 365 Apps for Enterprise	Affected =16.0.1	Fixed in–
MicrosoftMicrosoft Office 2016	Affected >=16.0.0 <16.0.5539.1001	Fixed in 16.0.5539.1001
MicrosoftMicrosoft Office 2019	Affected >=19.0.0 <16.0.10417.20095	Fixed in 16.0.10417.20095
MicrosoftMicrosoft Office LTSC 2021	Affected =16.0.1	Fixed in–
MicrosoftMicrosoft Office LTSC 2024	Affected =16.0.0	Fixed in–

AI-suggested

European organizations should immediately apply the Microsoft patch for CVE-2026-21509 across all affected Office installations to close the vulnerability. Beyond patching, organizations should implement strict controls on Office document handling, including disabling macros by default and enabling Protected View for documents from untrusted sources. Email filtering should be enhanced to detect and block malicious attachments or links. Endpoint detection and response (EDR) solutions should be tuned to identify anomalous Office process behaviors indicative of exploitation attempts. User awareness training should emphasize the risks of opening unsolicited or unexpected Office documents. Network segmentation and least privilege principles can limit lateral movement if exploitation occurs. Regular audits of Office security configurations and logs can help detect early signs of compromise.

Description

The vulnerability is tracked as CVE-2026-21509 and it can be exploited to bypass security features. The post Microsoft Patches Office Zero-Day Likely Exploited in Targeted Attacks appeared first on SecurityWeek .

Impact

For European organizations, the impact of CVE-2026-21509 could be substantial due to the pervasive use of Microsoft Office across enterprises, government agencies, and critical infrastructure sectors. Successful exploitation could lead to unauthorized access, data exfiltration, or the deployment of further malware within corporate networks. Confidentiality could be compromised if sensitive documents or communications are accessed or altered. Integrity risks arise if attackers manipulate documents or system configurations. Availability impact is less direct but could occur if exploitation leads to system instability or ransomware deployment. The medium severity suggests that exploitation may not be trivial but remains a credible threat, especially in targeted attacks against high-value entities. European organizations with less mature patch management or those relying heavily on Office macros and document sharing are particularly vulnerable. The threat could disrupt business operations, damage reputations, and incur regulatory penalties under GDPR if personal data is exposed.

References

securityweek.com/microsoft-patches-office-zero-day-likely-exploited-in-targeted-attacks/ (opens in a new tab)

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Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Jan 2026, 08:50 UTC

References

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