Google Threat Intelligence Group (GTIG) has published a detailed analysis of STOCKSTAY, a multi-component .NET backdoor attributed to the Russian state-linked threat actor Turla (FSB Center 16). Active since at least December 2022, STOCKSTAY targets Ukrainian government and military organizations as well as European entities with foreign affairs interests. The backdoor uses a modular architecture (STOCKBROKER for C2 tunneling, STOCKMARKET for orchestration, STOCKTRADER for execution) communicating over encrypted WebSockets. It masquerades as benign applications like stock market tools, PDF viewers, and calculators. STOCKSTAY shares significant code overlaps with Turla's established KAZUAR toolkit, including a shared string obfuscation mechanism called K1MORPHER based on the Squirrel3 PRNG. Deployment methods include malicious RDP files, HTA files, WinRAR CVE-2025-8088 exploitation, and GitHub-hosted MSI installers. The post includes a detailed operational timeline from 2022–2025, YARA detection rules, and indicators of compromise.
