Solarwinds Orion was compromised via a supply chain attack. Fireeye and Microsoft have good write ups and details and are the source for most of the news sources. There is also a CISA notice.
https://msrc-blog.microsoft.com/2020/12/13/customer-guidance-on-recent-nation-state-cyber-attacks/
Tweets from Kyle @ Huntress https://twitter.com/KyleHanslovan/status/1338358831722749953?s=20
