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Cover of the ESET APT Activity Report for April 2026 to September 2026, titled From Espionage to Destruction: Belarus-Aligned APT Activity Converges With Russia's War Priorities.
Image: ESET Research (fair use).

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ESET Says Russia-Aligned Hackers Hijacked Ukrainian MDaemon Mail Servers With No Click Needed, Using a WinRAR Flaw Left in MDaemon's Bundled ClamAV Library Until Version 26.0.4

The finding, in ESET's APT Activity Report for April through September 2026, comes with a second warning for Windows shops: Belarus-aligned FrostyNeighbor pushed a VBScript wiper through Group Policy at a Ukrainian manufacturer, the first destructive attack ESET has documented from a Belarus-aligned group. Mail administrators should confirm every MDaemon server is on version 26.0.4 or later.

Noah Park, ResponseNew York5 min read

NEW YORK - Russia-aligned hackers have been taking over MDaemon mail servers in Ukraine without anyone opening a message, by mailing them a booby-trapped RAR archive that the server's own antivirus scanner unpacked, ESET said Wednesday. The flaw they used had been fixed in the open-source ClamAV scanner more than a year earlier, but MDaemon kept shipping the old library until August.

The finding is one of the most directly actionable items in ESET's APT Activity Report covering April through September 2026, which also documents the first destructive attack ESET has attributed to a Belarus-aligned group: a wiper pushed through Group Policy at a Ukrainian manufacturer.

A zero-click path through the mail scanner

According to ESET, MDaemon relies on ClamAV to scan email attachments, and old versions of ClamAV were vulnerable to CVE-2025-8088, a path traversal flaw in WinRAR that ESET itself uncovered in 2025. ClamAV fixed the flaw on July 27, 2025, but MDaemon continued to ship an old version of the libclamunrar.dll library. MDaemon fixed the vulnerability on August 25, 2026, in version 26.0.4.

Page from the ESET APT Activity Report describing MDaemon mail server exploitation: attackers craft a RAR archive exploiting CVE-2025-8088, email it to any address on a vulnerable MDaemon server, and MDaemon's ClamAV scan writes a malicious DLL to the server's file system.
Image: ESET Research (fair use).ESET's three-step description of the MDaemon exploitation chain.

The exploitation, as ESET describes it, takes three steps. The attackers prepare a RAR archive that exploits CVE-2025-8088, send it as an attachment to any email address on a vulnerable MDaemon server, and wait for MDaemon to hand the archive to ClamAV for scanning, which triggers the exploit and writes a malicious DLL to the server's file system. "Because no user interaction is needed, the attackers didn't bother to add any convincing lure," ESET wrote.

The intruders then deployed two backdoor families that CERT-UA has named HELLAGE and HELLTAG. HELLAGE steals email messages and other data, talks to a hardcoded command server and can deploy more malware. HELLTAG is controlled through a hardcoded Telegram bot and supports remote command execution, file transfer and system reconnaissance. ESET called the operation an extensive campaign in Ukraine and said it cannot yet attribute it to any known Russia-aligned group.

For mail administrators, the response is narrow and concrete. Confirm every MDaemon server is on version 26.0.4 or later, check that the libclamunrar.dll on disk is the updated library, and review the server's file system for DLLs written by the scanning process. Any server that received RAR attachments while unpatched should be treated as possibly compromised, with outbound connections to unfamiliar hosts and to the Telegram API checked for signs of HELLAGE or HELLTAG.

A wiper through Group Policy

The report's headline finding concerns FrostyNeighbor, the Belarus-aligned group also tracked as UNC1151, PUSHCHA, Storm-0257 and TA445. In late May, ESET said, the group deployed Chisel, UltraVNC, Cobalt Strike and finally a VBScript wiper via Group Policy against a Ukrainian manufacturing company that appears to play a significant role in the country's transportation sector. "To the best of our knowledge, this marks the very first documented instance of a Belarus-aligned threat actor conducting a destructive cyberattack," the report says.

Page from the ESET APT Activity Report on Belarus-aligned groups, with charts showing that government and public sector organizations were 56.5% of their targets and that spearphishing attachments were 87.4% of their initial access techniques.
Image: ESET Research (fair use).ESET's charts of sectors targeted and initial access techniques used by Belarus-aligned groups.

ESET said the wiper may have been meant only to erase traces of an espionage operation, but that the targeting sits closer to Moscow's priorities than Minsk's and suggests FrostyNeighbor may have been "primarily guided by some Russian intelligence or operational requirements, acting as a proxy of sorts." The report warns that the attack "sets a precedent, which opens the possibility of other Belarus-aligned APT groups conducting wiper attacks in the near future." ESET's charts show government and public sector organizations made up 56.5% of the targets of Belarus-aligned groups in the period, and spearphishing attachments accounted for 87.4% of their initial access.

Pushing destructive scripts through Group Policy turns a domain's own management plane into the delivery system, which is why defenders treat changes to Group Policy Objects, especially new startup, logon or scheduled-task scripts in SYSVOL, as high-value telemetry. The tools ESET lists ahead of the wiper, a Chisel tunnel, UltraVNC and Cobalt Strike, are also the stage at which an intrusion can still be stopped.

Chart from the ESET APT Activity Report showing the share of attack sources by alignment, led by China-aligned and Russia-aligned APT groups.
Image: ESET Research (fair use).ESET's breakdown of the attack sources it documented from April through September 2026.

The report also flags a small but telling use of AI by attackers. "In one notable case, Russia-aligned UAC-0099 inserted a decoy request for instructions on building a nuclear weapon into a script comment, apparently attempting to trigger the safety guardrails of LLM-based security analysis and prevent the inspection of the malicious code that followed," said Jean-Ian Boutin, ESET's director of threat research. Overall, ESET said, it has not seen AI substantially reshaping the APT landscape over the past six months.

Sources:


Noah Park covers incident response and blue-team operations for SOCtember from New York.

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