FireEye and Microsoft on Thursday said they discovered three more malware strains in connection with the SolarWinds supply-chain attack, including a “sophisticated second-stage backdoor,” as the investigation into the sprawling espionage campaign continues to yield fresh clues about the threat actor’s tactics and techniques.

Dubbed GoldMax (aka SUNSHUTTLE), GoldFinder, and Sibot, the new set of malware adds to a growing list of malicious tools such as Sunspot, Sunburst (or Solorigate), Teardrop, and Raindrop that were stealthily delivered to enterprise networks by alleged Russian operatives.