The National Security Agency and FBI exposed a hacking tool used by Russian military intelligence, issuing an advisory on Thursday warning that the previously undisclosed Russian malware posed a threat to U.S. government networks.

The NSA and the bureau jointly called out the Russian General Staff Main Intelligence Directorate, commonly known as the GRU, and its 85th Main Special Service Center (or GTsSS) military unit 26165, which has been dubbed Fancy Bear, Strontium, and Advanced Persistent Threat 28 by various groups in the private sector, alleging that the Russian hacking outfit is deploying malware called Drovorub designed to target Linux operating systems “as part of its cyber espionage operations.” The malware could give Russian intelligence hidden access to and control over a host of servers and networks, which the NSA and FBI said “represents a threat to National Security Systems, Department of Defense, and Defense Industrial Base customers” that use Linux.