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Header image of ESET Research's WeLiveSecurity report on MythicRatatoskr, showing rows of server racks lit in teal and red in a darkened data center.
Image: ESET / WeLiveSecurity, header of MythicRatatoskr: An Yggdrasil-enabled backdoor deployed in a UAC-0099-Sandworm collaboration, Oct 22, 2026 (fair use).

Threat Intel

ESET Says a Russia-Aligned Group Handed Its Foothold in a Ukrainian Grain Firm to Sandworm for a Wiper Attack, After a New Backdoor, MythicRatatoskr, Hid Its Command Server Inside the Yggdrasil Mesh Network

ESET researchers say UAC-0099 phished its way into a Ukrainian grain logistics company in July 2025 and deployed a previously undocumented Go backdoor whose command server lives inside a decentralized IPv6 overlay network. Two months later Sandworm wiped the company's machines with SDelete pushed through Group Policy. The artifacts, the network trick and what defenders can block.

Priya Shah, Threat IntelSingapore5 min read

SINGAPORE - One Russia-aligned hacking group broke into a Ukrainian grain logistics company, and another came in behind it to destroy data, according to research ESET published Thursday. In between, a previously unknown backdoor kept the door open by talking to a command server that sits inside an experimental peer-to-peer network rather than on any ordinary host.

ESET's Anton Cherepanov and Peter Strýček write that UAC-0099 gained initial access on July 16, 2025, through a spearphishing email with a malicious attachment, and that Sandworm carried out a data-wiping attack on the same company on Sept. 17, 2025. Sandworm is attributed to GRU Unit 74455. ESET attributes each stage with high confidence and assesses with high confidence that UAC-0099 handed off access during the intrusion, though it could not determine which group primarily operated the new backdoor. The firm presented the findings at LABScon on Sept. 18, 2026, and at the Virus Bulletin conference on Oct. 16, 2026, before publishing them.

The handover fits a pattern ESET has seen before. Earlier in 2025 it documented Gamaredon working with Turla. A UAC-0099-to-Sandworm handoff, ESET wrote, is "unusual in the specific actors involved, but no longer surprising as a pattern."

From a scheduled task to a wiper

ESET telemetry shows a scheduled task named pdfopentask running at 06:59 on July 16, 2025, executing a PowerShell payload from %temp%\temporarydoc.txt. A minute later the MATCHBOIL downloader arrived and deployed the MATCHWOK backdoor, which used the command server uztickets[.]com and was fetched from egyptanimals[.]com. At 09:28 the next day the new backdoor, which ESET named MythicRatatoskr, landed on the same machine. More copies were later pushed to other machines through an OpenSSH SFTP server, and a standalone SOCKS5 server appeared on some hosts. About two months later, Sandworm deployed the SDelete utility through Active Directory Group Policy, a technique ESET says it has observed numerous times.

ESET Figure 1, a map of the Yggdrasil network's spanning tree generated by ESET on Oct. 11, 2025, with the MythicRatatoskr C&C server marked near the center and its address 201:b479:2771:8df4:b5eb:d007:63b0:2838 shown in the node panel.
Screenshot: ESET Research, Figure 1, Yggdrasil map with the C&C server, Oct 11, 2025 (fair use).

A command server with no fixed home

MythicRatatoskr is written in Go and built on yggstack, a project that lets a program join the Yggdrasil mesh in user space without creating a tunnel interface. Yggdrasil builds an encrypted IPv6 overlay in the 200::/7 range, a block the IETF deprecated in 2004, and has no central server to take down. ESET chose the name because the backdoor ferries messages across that network. Every sample carries the same base64-encoded list of 32 public peers, picks three at random, joins the public mesh and then talks to its command server at 201:b479:2771:8df4:b5eb:d007:63b0:2838 over HTTPS on port 443, using JSON POST messages for check-in, tasking, results and file retrieval.

"To our knowledge, this is the first documented case of Yggdrasil being abused in this way by a state-aligned threat actor," the researchers wrote. Yggdrasil is not built for anonymity, though, and ESET mapped the network itself to find the server near the center of the tree.

ESET Table 1 listing MythicRatatoskr installation paths such as C:\windows\system32\updater.exe, krbtman.exe, windowsdefendersvc.exe and OneDriveLiveUpdater.exe, configuration paths ending in .updatecache and .crond, and the service name ieupdate.
Screenshot: ESET Research, Table 1, host-based artifacts associated with MythicRatatoskr, Oct 22, 2026 (fair use).

The backdoor runs as a Windows service named ieupdate and masquerades under names such as updater.exe, krbtman.exe, WindowsDefenderSvc.exe and OneDriveLiveUpdater.exe. Its configuration is a JSON file encrypted with AES-256-CFB under a key hardcoded in the binary, and some samples wait 10 minutes after launch before joining the network. A configuration path ending in .crond, written with forward slashes, suggests that a Linux variant may also exist, ESET said.

ESET Table 3 listing MythicRatatoskr commands chtime, expose, upload, download, nodeinfo, screenshot, cd, ls, pwd, churl, proxy and shell, with the proxy command connecting to a hardcoded server on port 8080.
Screenshot: ESET Research, Table 3, MythicRatatoskr commands, Oct 22, 2026 (fair use).

Commands cover shell execution through cmd.exe, file transfer, directory listing, screenshots of every active display, exposing a local TCP port to the mesh and a reverse SOCKS5 proxy to the same server on port 8080. The separate SOCKS5 tool, internally named bindProxy, listens only on 127.0.0.1 port 33690; ESET says the operators may have exposed it through the backdoor or another tunnel.

What threat intel and detection teams should do now

  • Block or alert on connections to the 32 public Yggdrasil peers ESET lists. ESET says that although the C&C server is inside the mesh, "blocking the public peer addresses is a viable mitigation."
  • Hunt for a Windows service named ieupdate and for the installation paths in ESET's Table 1, especially executables named for Windows Defender, OneDrive or Internet Explorer under system32 or user Downloads folders.
  • Look for hidden-style configuration files named .updatecache under C:\Windows\Temp or C:\Windows\system32, and for .crond under the local Temp folder.
  • Alert on anything listening on 127.0.0.1:33690 and on unexpected OpenSSH SFTP servers used to move binaries between workstations.
  • Treat a confirmed MATCHBOIL or MATCHWOK infection as possible pre-positioning for a destructive operation, and watch for SDelete pushed through Group Policy. Look for the scheduled task pdfopentask and the Run value UpdateMonitor.
  • Pull ESET's file hashes, 12 samples in the published table, and the full IoC list from its GitHub repository.

Sources:


Priya Shah covers threat intelligence, intrusion analysis, and adversary tradecraft for SOCtember from Singapore.

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