APT28 Exploitation of Microsoft Office Zero-Day CVE-2026-21509 Against Ukraine and EU Targets
Ukraine’s CERT-UA reported active exploitation of a Microsoft Office zero-day, CVE-2026-21509 (a security feature bypass), attributed to Russia-linked UAC-0001 / APT28 (Fancy Bear) and used to target Ukrainian government bodies and organizations across the EU. Microsoft disclosed the flaw with a warning that it was already being exploited in the wild, and CERT-UA observed rapid weaponization: a lure document titled Consultation_Topics_Ukraine(Final).doc appeared shortly after disclosure and was themed around EU discussions on Ukraine, suggesting the exploit chain was prepared in advance.
CERT-UA also described a parallel phishing campaign impersonating the Ukrhydrometeorological Center, sent to 60+ recipients largely in Ukrainian central executive bodies. The attack chain described includes opening a malicious DOC that triggers a WebDAV connection to attacker infrastructure, downloads a shortcut (.lnk) used to stage additional payloads, and deploys components including a DLL masquerading as a legitimate Windows component (e.g., EhStoreShell.dll) with shellcode hidden in a decoy file (e.g., SplashScreen.png), alongside persistence techniques such as COM hijacking (registry modification) and scheduled task creation.
Timeline
Feb 9, 2026
SANS analysis extracts additional IOCs from disguised RTF lure documents
A SANS ISC diary showed that reported .doc lure files were actually RTF documents and demonstrated how to extract embedded URLs and other indicators from them. The analysis surfaced domains, UNC paths, and malformed WebDAV-style references that could aid defenders investigating the campaign.
Feb 4, 2026
Trellix identifies broader sector targeting and additional implants
Subsequent reporting expanded the victimology beyond government agencies to maritime, transport, military, and NATO-aligned organizations in countries including Poland, Ukraine, and Turkey. Trellix said the campaign used additional implants such as BEARDSHELL and NotDoor, with command-and-control traffic routed through Filen cloud storage.
Feb 3, 2026
CISA adds CVE-2026-21509 to the Known Exploited Vulnerabilities catalog
Following confirmation of in-the-wild exploitation, CISA added CVE-2026-21509 to its Known Exploited Vulnerabilities list. The move underscored the urgency for organizations to patch affected Microsoft Office installations.
Feb 2, 2026
CERT-UA publishes exploit-chain details and defensive guidance
On 2026-02-02, CERT-UA described the attack chain using WebDAV retrieval, a malicious shortcut, EhStoreShell.dll, shellcode hidden in SplashScreen.png, COM hijacking, and a scheduled task named OneDriveHealth. It warned attacks were likely to increase due to slow patching and advised blocking indicators and Filen-related traffic where appropriate.
Feb 2, 2026
CERT-UA attributes Ukrainian and EU targeting to UAC-0001/APT28
CERT-UA publicly linked the exploitation of CVE-2026-21509 against Ukrainian government agencies and EU organizations to UAC-0001, also known as APT28 or Fancy Bear. The attribution connected the activity to Russia's GRU-linked espionage operations.
Jan 29, 2026
Researchers document dual malware chains: MiniDoor and PixyNetLoader
Technical analysis revealed two main post-exploitation paths in the campaign: one deploying the Outlook-focused MiniDoor implant for email theft and forwarding, and another using PixyNetLoader to stage follow-on payloads. Reports also linked related tooling including NotDoor, BEARDSHELL, and Covenant Grunt to the activity.
Jan 29, 2026
Operation Neusploit targets Central and Eastern Europe
Zscaler ThreatLabz reported a broader APT28 espionage campaign, dubbed Operation Neusploit, targeting organizations in Ukraine, Slovakia, Romania, and other Central and Eastern European countries. The campaign used localized RTF or Word phishing lures to exploit CVE-2026-21509.
Jan 29, 2026
CERT-UA observes phishing campaign targeting Ukrainian government entities
CERT-UA identified malicious Office lures themed around EU COREPER consultations on Ukraine and fake bulletins from the Ukrainian hydrometeorological center. The emails were sent to more than 60 addresses, mostly tied to Ukrainian state authorities and government-related organizations.
Jan 29, 2026
APT28 begins exploiting CVE-2026-21509 within days of disclosure
Researchers reported that Russia-linked APT28 rapidly weaponized CVE-2026-21509 within 24 to 72 hours of Microsoft's patch and disclosure. Early exploitation was observed around 2026-01-28 to 2026-01-29, showing the group quickly adapted the flaw for phishing-based intrusion campaigns.
Jan 28, 2026
Trellix reports a 72-hour spearphishing wave across nine countries
Trellix disclosed a concentrated 72-hour APT28 spearphishing operation beginning on 2026-01-28 that used at least 29 lures sent from compromised government email accounts. The campaign targeted diplomatic, defense, maritime, transport, and logistics organizations in nine countries, primarily in Eastern Europe.
Jan 26, 2026
Microsoft discloses and patches CVE-2026-21509 as an exploited Office zero-day
On 2026-01-26, Microsoft released an out-of-band security update for CVE-2026-21509, a Microsoft Office security feature bypass affecting multiple Office versions. Microsoft warned that the flaw was already being actively exploited in the wild.
Sep 1, 2025
Trend Micro links APT28 PRISMEX campaign to Ukraine and allied targets
Trend Micro uncovered an APT28 spear-phishing campaign active since September 2025 targeting Ukraine’s defense supply chain, aid infrastructure, and government entities in Central and Eastern Europe. The operation used military- and aid-themed RTF lures exploiting CVE-2026-21509 and CVE-2026-21513 and deployed the PRISMEX toolkit, including a dropper, loader, and Covenant-based implant using cloud services such as Filen.io for encrypted command-and-control.
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1 months ago