Company Solarwinds has been compromised. Hackers were able to add malware to their network software update. Thanks to that - all their users has been infected by malware called SUNBURST which can create a backdoor to their own network systems.
This discover has been made on December 2020 however attack was already ongoing during May 2020. All sources agrees that posible attacker is APT Russia.
Current US goverement statement: https://www.cisa.gov/news/2021/01/05/joint-statement-federal-bureau-investigation-fbi-cybersecurity-and-infrastructure