Russian APT Deploys ‘StockStay’ Backdoor Against Ukrainian Targets

Russia-linked APT Turla has been targeting government and military organizations in Ukraine with a new backdoor specifically designed for espionage, Google Threat Intelligence Group (GTIG) reports. Also known as Krypton, Snake, Summit, UAC-0194, Venomous Bear, and Waterbug, Turla has been active since at least 2004. The US officially linked the APT to Russia’s Federal Security Service (FSB) in 2023. According to a fresh GTIG report, Turla has been developing a .NET backdoor tracked as StockStay since 2022, and has been using it in attacks against Ukraine’s government and military, as well as against entities with an interest in Italian foreign policy. Designed for ongoing cyber espionage, the backdoor shows code and functionality overlap with Kazuar, a known Turla implant that has been around since at least 2015. A multi-component backdoor written in .NET, StockStay initially masqueraded as a stock market data viewing tool, but recent iterations pose as PDF viewers and calculator utilities. The backdoor relies on a secure WebSocket connection, via the open source websocket-sharp library, for command-and-control (C&C) communication. Its components use an inter-process communication (IPC) channel to communicate with one another. StockStay payloads are fetched from a remote server using a proxy-aware downloader named StockStay.MarketMaker, which runs in the background and sets up autorun entries to execute core backdoor components. Network communication is provided through StockStay.StockBroker, a proxy-aware tunneler, while the implant’s configurability is enabled through the StockStay.StockMarket orchestrator. An encrypted on-disk configuration file contains various options regarding malware execution. The backdoor component, named StockStay.StockTrader, supports various command execution capabilities, including file download/exfiltration/modification, folder tampering, screen capture, task processing, registry modification, process execution, and system information harvesting. Most of the observed StockStay activity has been targeting Ukrainian government and military entities, in line with Russian interests in the region. In-country compromised infrastructure, including government services, has been used for malware deployment, GTIG says. Some of the early StockStay activity, however, targeted European entities in Italy, the Netherlands, Poland, and Germany, including a foreign affairs ministry, but the intended victims for most of these infections have not been confirmed. StockStay operations rely on academia and diplomacy themes: phishing emails sent from a compromised Ukrainian university email account and diplomatic education platform, filenames containing academic institution names, phishing domains containing ‘education’ and ‘diplo’ in their names, and backdoor MSI files named ‘DiplomacyEduAI’. GTIG also observed Turla deploying the backdoor via malicious RDP configuration files delivered via phishing emails. Some of these files were hosted on a compromised diplomatic-themed education platform. Additionally, GTIG noticed that the cyberespionage group deployed StockStay at different stages of its attacks, either for initial access, for reconnaissance, or at later stages, likely through existing access to the victim’s environment. In one attack in November 2025, Turla sent phishing emails to 20 Ukraine-based targets, linking to a malicious RAR archive exploiting CVE-2025-8088 for the execution of StockStay. In January, GTIG warned that multiple Russian APTs and cybercrime groups had been targeting the WinRAR vulnerability. Related: Russian Initial Access Broker Behind FortiBleed Campaign Related: Russian Spies Are Aggressively Seeking Western Technology as Sanctions Bite, Officials Say Related: Russia-Linked ‘GreyVibe’ Attackers Use AI to Supercharge Cyberattacks Related: UK Cyberspying Chief Calls AI ‘an Unstoppable Force’ and Warns About Russia WRITTEN BY Ionut Arghire Ionut Arghire is an international correspondent for SecurityWeek. 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