Report · SANS ISC Handlers DiaryAbout CVE-2026-21509Published 9 Feb 2026

Quick Howto: Extract URLs from RTF files, (Mon, Feb 9th)

Source SANS ISC Handlers Diary

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 12 Feb 2026, 07:49 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 2026-10-26.

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 implement advanced email filtering to detect and quarantine suspicious RTF documents, especially those with deceptive extensions like .doc. Employ endpoint detection and response (EDR) solutions capable of analyzing document behavior and extracting embedded URLs for threat intelligence correlation. Regularly update Microsoft Office and related software to apply any available patches addressing CVE-2026-21509. Conduct user awareness training focused on recognizing phishing attempts involving document attachments. Utilize sandboxing technologies to safely open and analyze suspicious documents before delivery to end users. Implement network monitoring to detect unusual WebDAV or UNC traffic patterns, particularly those involving non-standard port notations. Develop and deploy custom YARA or Sigma rules to identify malformed URLs and embedded objects consistent with this exploit. Finally, maintain threat intelligence sharing with European CERTs and industry groups to stay informed about emerging exploitation techniques related to this vulnerability.

Description

Malicious RTF (Rich Text Format) documents are back in the news with the exploitation of CVE-2026-21509 by APT28.

Impact

For European organizations, the exploitation of CVE-2026-21509 by APT28 poses a significant risk, especially for entities targeted by espionage campaigns such as government agencies, defense contractors, and critical infrastructure providers. Successful exploitation can lead to unauthorized code execution, enabling attackers to gain footholds within networks, steal sensitive information, or deploy additional malware. The use of obfuscated URLs and embedded WebDAV requests complicates detection, increasing the likelihood of successful compromise. Given the medium severity and the need for user interaction, phishing campaigns or spear-phishing emails remain the primary delivery method, which is a common attack vector in Europe. The impact on confidentiality and integrity is considerable, while availability impact is likely limited but possible if attackers deploy destructive payloads. Organizations with extensive use of Microsoft Office and RTF documents are particularly vulnerable. The threat also stresses the importance of monitoring document-based attacks and enhancing email security controls.

References

isc.sans.edu/diary/rss/32692 (opens in a new tab)

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Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 9 Feb 2026, 11:45 UTC

References

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